232210 - EverTag Ext LTE EU¶


Article Number	232210
Name	EverTag Extension PCB LTE EU
Base PCB	Connects to 232203 Base Bat+WiFi via board-to-board connector
LTE Modem	SIMCom SIM7672E (LTE Cat-1bis, Qualcomm QCX216)
Region	EU (CE)
Status	Not included in the first PCB iteration

Not in First PCB Iteration

This extension PCB is fully specified but is not included in the first PCB production run. The base platform (232203 with B2B connector) and enclosure (232506) are designed to accommodate this board in a future iteration.

1. Overview¶

The 232210 is a small extension PCB that stacks on top of the 232203 base PCB via a board-to-board connector. It carries the LTE modem (SIMCom SIM7672E) for EU market cellular connectivity. The LTE modem enables cloud connectivity as a backup or alternative to WiFi, using Cat-1bis LTE.

Key Features¶

SIMCom SIM7672E LTE Cat-1bis modem (EU bands, including B28 / 700 MHz for rural coverage)
Two 1.27 mm pitch board-to-board connectors (Samtec TMM/SFM, 2×8 = 16 pins each, 32 total) flanking the ESP32-C5 WiFi module on the 232203 base PCB
Nano-SIM (4FF) card holder (on this extension board, accessible from underneath)
On-board SMD chip antenna (Quectel YC0001CA) with matching network -- eliminates cable assembly
U.FL test/fallback connector for RF testing and optional external FPC antenna
3.8V LDO to power modem from 5V B2B rail
1.8V↔3.3V level shifters for UART and control signals (modem I/O is 1.8V)
NETLIGHT development LED (DNP in production)
UART interface to ESP32 on base PCB (AT commands via B2B connector)
Power from 232203 5V rail via B2B connector (2A burst capable)
All components on top side only

2. Block Diagram¶

graph TB
    subgraph base [232203 Base PCB]
        ESP32["ESP32-C5 WROOM<br/>(3.3V logic)"]
        PWR_5V["5V / 2A Rail"]
        B2B_BASE_L["B2B-L Header (left of WiFi)"]
        B2B_BASE_R["B2B-R Header (right of WiFi)"]
    end

    subgraph ext [232210 Extension PCB - all components top side]
        B2B_EXT_L["B2B-L Socket (power+GND)"]
        B2B_EXT_R["B2B-R Socket (signals+GND)"]
        LDO38["3.8V LDO<br/>(from 5V B2B)"]
        LVLSHIFT["1.8V↔3.3V<br/>Level Shifters"]
        MODEM["SIMCom SIM7672E<br/>(1.8V I/O)"]
        SIM["Nano-SIM 4FF Holder"]
        CHIP_ANT["On-Board Chip Antenna<br/>Quectel YC0001CA"]
        UFL["U.FL (test/fallback)"]
        LED_NET["NETLIGHT LED (DNP in prod)"]
    end

    PWR_5V --> B2B_BASE_L
    ESP32 <-->|"LTE UART + control<br/>(3.3V)"| B2B_BASE_R
    B2B_BASE_L <-->|"16-pin B2B-L"| B2B_EXT_L
    B2B_BASE_R <-->|"16-pin B2B-R"| B2B_EXT_R

    B2B_EXT_L -->|"5V"| LDO38
    LDO38 -->|"3.8V VBAT"| MODEM
    B2B_EXT_R <-->|"3.3V signals"| LVLSHIFT
    LVLSHIFT <-->|"1.8V signals"| MODEM

    MODEM <--> SIM
    MODEM -->|"RF"| CHIP_ANT
    MODEM -.->|"RF (alt)"| UFL
    MODEM -.->|"NETLIGHT<br/>(1.8V)"| LVLSHIFT
    MODEM -.->|"NETLIGHT"| LED_NET

3. Schematics¶

3.1 LTE Modem -- SIMCom SIM7672E¶

Parameter	Value
Manufacturer	SIMCom
Part Number	SIM7672E
Chipset	Qualcomm QCX216
Technology	LTE Cat-1bis (single RX antenna)
Region	EU (CE-RED)
Bands	B1, B3, B7, B8, B20, B28 (EU coverage incl. 700 MHz rural)
Fallback	None (no GSM -- 2G networks sunsetting across EU)
Interface	UART (AT commands), all I/O at 1.8V logic level
Supply	3.4--4.2V (VBAT pins), typical 3.8V
Peak Current	Up to 2A during TX burst
Sleep Current	<1 mA (SLEEP1 mode, DTR high, USB disconnected)
Package	LCC+LGA, 24 x 24 x 2.4 mm

1.8V Logic Level

All SIM7672E UART, GPIO, STATUS, and NETLIGHT pins operate at 1.8V logic (the module provides a VDD_1V8 reference output on pin 15). The ESP32 on the base PCB uses 3.3V logic. Bidirectional level shifters are required on all signals crossing the B2B connector. See section 3.7.

Band 28 (700 MHz) for Rural EU Coverage

The SIM7672E adds Band 28 (700 MHz) compared to the A7672E it replaces. B28 is actively being deployed by Nordic/EU operators (Telia, Tele2/Telenor Net4mobility) for rural and countryside coverage. This is critical for installations in agricultural, forestry, and remote industrial sites. The on-board chip antenna supports 698--960 MHz, covering B28.

Design notes:

Module has internal PMIC -- provide clean 3.8V input (see section 3.6)
Follow SIMCom SIM7672X hardware design guide for decoupling and layout
Keep RF trace to chip antenna short and impedance-controlled (50 ohm)
PWRKEY and RESET pins operate in VBAT voltage domain (3.4--4.2V), not 1.8V -- use open-drain drive from level shifter or NPN transistor from 3.3V ESP32 GPIO
USB_BOOT (pin 6) must not be pulled LOW during power-up (strapping pin, internal pull-up)

SIM7672E Modem Pin Assignment (Used Pins)¶

The SIM7672E has 80 LCC + 44 LGA = 124 total pins. Only the pins used in this design are listed below. All unused pins should be left unconnected (NC) unless the SIMCom hardware design guide specifies otherwise.

Pin #	Name	I/O	Voltage Domain	Connection	Notes
1	PWRKEY	DI	VBAT (3.8V)	NPN driver ← B2B pin 6 (LTE_POWER_KEY)	Active LOW, >500 ms pulse to toggle power. Internal pull-up.
9	MAIN_UART_TXD	DO	1.8V	Level shifter → B2B pin 5 (LTE_UART_RX)	AT response data from modem
10	MAIN_UART_RXD	DI	1.8V	Level shifter ← B2B pin 4 (LTE_UART_TX)	AT command data to modem
15	VDD_EXT (VDD_1V8)	PO	1.8V	Level shifter VCCA supply	1.8V reference output from modem. Decoupling cap close to pin.
38	NRST	DI	VBAT (3.8V)	NPN driver ← B2B pin 7 (LTE_RESET)	Active LOW, >100 ms pulse for hard reset. Internal pull-up.
52	NETLIGHT	DO	1.8V	Dev LED (local) + level shifter → B2B pin 9	Blink pattern: 64/800 ms = searching, 64/3000 ms = registered, 64/300 ms = data
55	VBAT	PI	3.8V	3.8V LDO output	Power supply #1 (with bulk caps)
56	VBAT	PI	3.8V	3.8V LDO output	Power supply #2
57	VBAT	PI	3.8V	3.8V LDO output	Power supply #3
60	RF_ANT	RF	RF	Matching network → chip antenna / U.FL	50 Ω RF output
66	STATUS	DO	1.8V	Level shifter → B2B pin 8 (LTE_STATUS)	HIGH = module active, retained in sleep
6	USB_BOOT	DI	1.8V	NC (or 10k pull-up to VDD_1V8)	Strapping pin. Must NOT be LOW at power-up.
SIM pins	SIM_CLK, SIM_DATA, SIM_RST, SIM_VCC	--	1.8/3V	SIM card holder (local routing)	See section 3.3
GND	Multiple	--	--	Ground plane	Connect all GND and exposed pad to ground

DI = Digital Input, DO = Digital Output, PI = Power Input, PO = Power Output, RF = Radio Frequency.

Pin Numbering Source

Pin numbers are from the SIMCom SIM7672X / A7672X hardware design guide, which covers the pin-compatible SIM7672E. See A7672 Series Spec and Compatible Design Guide V1.02.

Complete Signal Path: ESP32 → B2B → Level Shift → Modem¶

The table below shows the end-to-end signal chain for every LTE signal, from the ESP32-C5 GPIO on the 232203 base PCB, through the B2B connector, through level shifting on this extension board, to the SIM7672E modem pin.

Signal	ESP32 GPIO	ESP32 Dir	B2B Pin	B2B Voltage	Level Shift	Modem Pin	Modem Name	Modem Voltage
LTE_UART_TX	GPIO4	Output	4	3.3V	TXB0106 3.3V→1.8V	10	MAIN_UART_RXD	1.8V
LTE_UART_RX	GPIO5	Input	5	3.3V	TXB0106 1.8V→3.3V	9	MAIN_UART_TXD	1.8V
LTE_POWER_KEY	GPIO1	Output	6	3.3V	NPN open-drain → VBAT	1	PWRKEY	VBAT (3.8V)
LTE_RESET	GPIO0	Output	7	3.3V	NPN open-drain → VBAT	38	NRST	VBAT (3.8V)
LTE_STATUS	GPIO3	Input	8	3.3V	TXB0106 1.8V→3.3V	66	STATUS	1.8V
NETLIGHT	GPIO15	Input	9	3.3V	TXB0106 1.8V→3.3V	52	NETLIGHT	1.8V
LTE_TX_BLANK	GPIO25	Output	10	3.3V	TXB0106 3.3V→1.8V	TBD (GPIO)	Modem GPIO	1.8V

TX/RX Naming Convention

Note the TX/RX crossover: the ESP32's LTE_UART_TX (GPIO4, output) connects to the modem's MAIN_UART_RXD (pin 10, input), and vice versa. This is standard UART wiring (TX→RX). The B2B connector signal names (LTE_UART_TX, LTE_UART_RX) follow the ESP32 perspective -- TX means "data flowing from ESP32 toward modem".

LTE_TX_BLANK Modem Pin — Resolve During Prototype Bring-Up

The LTE_TX_BLANK signal is used for WiFi/LTE coexistence blanking. The specific SIM7672E GPIO pin depends on modem firmware configuration via AT+CGFUNC. The pin is not hardcoded — SIMCom allows remapping GPIO functions at runtime. Action items for bring-up:

Contact SIMCom support (support@simcom.com) with the exact firmware version (AT+CGMR) and request the available GPIO pins for coexistence blanking output.
Confirm using AT+CGFUNC=? to list available GPIO function assignments.
Assign using AT+CGFUNC=<gpio>,<function> and verify the TX blanking pulse appears on the selected pin during LTE transmission.
Update this table with the resolved pin number once confirmed.

If the modem does not support hardware TX blanking on any GPIO, implement software TX blanking in ESP32 firmware: the ESP32 monitors the modem's TX activity via AT status commands and gates WiFi transmissions in software. The B2B pin 10 (LTE_TX_BLANK) would then be unused (NC).

3.2 Board-to-Board Connectors (Dual Split -- 1.27 mm pitch)¶

Two 1.27 mm pitch connectors flank the ESP32-C5 WiFi module on the 232203 base PCB, one on each side. This replaces the previous single Harwin M20 (2.54 mm) connector.

Rationale for dual split connectors:

Symmetric mechanical support. Two mounting points eliminate cantilever rocking under vibration -- critical for industrial wall/ceiling-mounted installations.
Better LTE antenna ground plane. GND pins on both sides of the extension board provide low-inductance, symmetric ground current return to the base board, directly improving the LTE chip antenna's effective ground plane.
Compact footprint. 1.27 mm pitch fits within the ~9 mm clearance on each side of the ESP32-C5 module (18 mm wide on a ~36 mm board).
Clean signal/power separation. Power and GND are concentrated on the left connector (near the LTE antenna); signals on the right connector.

Parameter	Value
Connector Family	Samtec TMM / SFM series, 1.27 mm (0.05") pitch
Header (base)	TMM-108-01-x-D (2×8 = 16-pin, vertical SMT, male)
Socket (ext)	SFM-108-01-x-D (2×8 = 16-pin, vertical SMT, female)
Total Pins	2 × 16 = 32 pins (16 per connector)
Stacking Height	~8--10 mm PCB-to-PCB (selectable via post length)
Current Rating	3.7A per pin (2 pins powered); derate for multiple power pins
Pitch	1.27 mm (0.05")

Connector Alternatives

If Samtec TMM/SFM is not available, pin-compatible alternatives at 1.27 mm pitch include Samtec FTSH/SFSD, Harwin M50 series, or Amphenol FCI Minitek 1.27 mm. The key requirement is 2×8 vertical SMT at 1.27 mm pitch with ≥2A per-pin rating on power pins.

B2B-L (Left Connector -- Power + GND, near LTE antenna)¶

Located on the left side of the ESP32-C5 module (LTE antenna side). Concentrates power and ground for low-impedance supply to the LTE modem and symmetric ground plane coupling.

Pin	Signal	Direction (ext→base)	Voltage	Notes
L1	VIN_5V	Power from base	5V	2A burst for LTE modem TX
L2	VIN_5V	Power from base	5V	Doubled for 2A burst capability
L3	GND	Power	--	Power return #1
L4	GND	Power	--	Power return #2
L5	GND	Power	--	Power return #3 (antenna GND symmetry)
L6	GND	Power	--	Power return #4 (antenna GND symmetry)
L7	GND	Power	--	Power return #5 (antenna GND symmetry)
L8	GND	Power	--	Power return #6 (antenna GND symmetry)
L9	GND	Power	--	Power return #7 (antenna GND symmetry)
L10	GND	Power	--	Power return #8 (antenna GND symmetry)
L11	GND	Power	--	Power return #9 (antenna GND symmetry)
L12	GND	Power	--	Power return #10 (antenna GND symmetry)
L13	GND	Power	--	Power return #11
L14	GND	Power	--	Power return #12
L15	GND	Power	--	Power return #13
L16	GND	Power	--	Power return #14

GND-Heavy Left Connector

14 of 16 pins on B2B-L are GND. This is intentional: the left connector is closest to the LTE chip antenna. Maximising GND pins here creates a low-inductance, wide ground current return path between the extension board and the base board, directly extending the base board's ground plane into the extension board's antenna counterpoise. This improves sub-GHz antenna efficiency (B20/B28 700--900 MHz) where ground plane length is the primary limiting factor on a small PCB.

B2B-R (Right Connector -- Signals + GND, opposite LTE antenna)¶

Located on the right side of the ESP32-C5 module (WiFi antenna side). Carries all control and data signals plus additional GND pins for symmetry.

Pin	Signal	Direction (ext→base)	Voltage	Notes
R1	GND	Power	--	Signal return / shield
R2	LTE_UART_TX	ESP32 → modem	3.3V	AT commands from ESP32 (level-shifted to 1.8V on ext board)
R3	LTE_UART_RX	modem → ESP32	3.3V	AT responses to ESP32 (level-shifted from 1.8V on ext board)
R4	GND	Power	--	Signal return between UART and control
R5	LTE_POWER_KEY	ESP32 → modem	3.3V	Modem power on/off (level-shifted to VBAT domain)
R6	LTE_RESET	ESP32 → modem	3.3V	Hardware reset (level-shifted to VBAT domain)
R7	LTE_STATUS	modem → ESP32	3.3V	Network registration indicator (level-shifted from 1.8V)
R8	GND	Power	--	Signal return between control and coex
R9	NETLIGHT	modem → ESP32	3.3V	Blink pattern decoded by ESP32 for LED2 status (level-shifted from 1.8V)
R10	LTE_TX_BLANK	ESP32 → modem	3.3V	WiFi/LTE TX coexistence blanking
R11	EXT_DETECT	ext → base (P1.12)	Analog	Extension variant ADC divider
R12	GND	Power	--	Signal return
R13	GND	Power	--	Symmetry GND
R14	GND	Power	--	Symmetry GND
R15	GND	Power	--	Symmetry GND
R16	GND	Power	--	Symmetry GND

Signal Integrity: GND Between Signal Groups

Signal pins on B2B-R are interleaved with GND pins (R1, R4, R8, R12) to provide local return paths and reduce crosstalk between UART, control, and coexistence signals. This is especially important at 1.27 mm pitch where adjacent signal coupling is higher than at 2.54 mm.

All B2B Signals are 3.3V

Level shifting happens on the extension board, close to the modem. All signals crossing the B2B connectors are at 3.3V logic (matching the ESP32 on the base PCB). The base PCB (232203) does not need any level shifting circuitry for the LTE interface.

No SIM or RF Signals on B2B Connectors

SIM card and RF coax are both local to this extension board. SIM signals run from the modem directly to the SIM holder (short traces, same PCB). The on-board chip antenna is soldered directly to the extension PCB. Nothing crosses the B2B connectors except power, UART, and control/status signals.

GND pin summary: 14 GND (B2B-L) + 9 GND (B2B-R) = 23 GND pins out of 32 total. This high GND ratio is by design: the primary constraint on LTE antenna performance at this board size (~36 × 50 mm) is ground plane inductance between the extension and base boards.

3.3 SIM Card Holder¶

Parameter	Value
Type	Nano-SIM (4FF) push-push holder
Location	On this extension PCB (232210), top side, card slot facing downward
Access	SIM card accessible from underneath the extension board
Routing	SIM signals routed locally from modem SIM pins (short traces)

Design notes:

SIM holder mounted on top side of extension board, with the card insertion slot facing downward -- the SIM card is inserted from underneath (same access side as the USB-C connector on the base PCB)
All SIM traces (SIM_CLK, SIM_DATA, SIM_RST, SIM_VCC) are routed locally on the extension PCB between modem and SIM holder -- no B2B routing
ESD protection on all SIM lines (TVS diode array close to SIM holder)
Controlled impedance per ISO/IEC 7816 SIM specification
SIM signals operate at 1.8V or 3.0V (module-controlled) -- no external level shifting needed

3.4 On-Board Chip Antenna -- Placement and Coexistence¶

Parameter	Value
Manufacturer	Quectel
Part Number	YC0001CA
Type	SMD chip monopole antenna (IFA), tape & reel
Frequency	698--960 MHz / 1710--2700 MHz
Peak Efficiency	Up to 72.9%
Dimensions	35 x 8.5 x 3 mm
Mounting	SMD reflow (no cable, no manual assembly)
Impedance	50 ohm (with matching network)
Compliance	RoHS, REACH
Est. Price	~$0.19 @ 500+ (JLCPCB/LCSC)
Datasheet	YC0001CA Datasheet

Assembly Simplification

The on-board chip antenna eliminates the FPC antenna, U.FL coax cable, and manual cable connection step. All antenna assembly is done during standard SMD reflow -- no manual labor required. This is significant for European EMS assembly where labor cost is high.

Antenna Placement: Left Edge (Opposite WiFi)¶

The LTE chip antenna is placed at the left edge of the extension board, with the ESP32-C5 WiFi module antenna on the right side of the base board above. This provides maximum diagonal separation between the two antennas.

    Top view (extension board, ~36 × 50 mm):

         ← ~36 mm →
    ┌────────────────────────────────┐
    │  B2B-L        B2B-R           │  ← toward base board
    │  (power+GND)  (signals+GND)   │
    │                                │
    │      ┌────────────┐            │
    │      │  SIM7672E   │  [SIM]    │
    │      │  24×24 mm   │           │
    │      └────────────┘  [LDO]    │
    │  [match]                       │
    │                                │
    │▓▓▓▓▓▓▓▓▓▓│                    │
    │▓ YC0001CA ▓│  (5 mm keep-out)  │
    │▓ LTE ANT  ▓│  (no GND pour)   │
    │▓▓▓▓▓▓▓▓▓▓│                    │
    └────────────────────────────────┘
      ↑ left edge         right edge ↑
      LTE antenna         WiFi antenna is
      (bottom-left)       top-right on base

Why NOT under the WiFi antenna (vertical stacking):

The LTE antenna's high band (1710--2700 MHz) overlaps the WiFi 2.4 GHz band (2400--2484 MHz). LTE Band 7 (EU) is 2620--2690 MHz, immediately adjacent to WiFi. Stacking the LTE antenna directly below the ESP32-C5 WiFi antenna with only ~5--10 mm vertical gap would cause:

Problem	Impact
Near-field coupling	Both antennas in each other's reactive near field; mutual detuning shifts resonant frequencies, destroys matching networks
Desensitization	LTE TX at +23 dBm (200 mW) at 5--10 mm from WiFi receiver overwhelms WiFi front-end, even with TX blanking
Reduced efficiency	Mutual coupling absorbs 2--5 dB from both antennas; likely fails TRP (Total Radiated Power) in CE-RED certification
Spurious emissions	Coupled energy between antennas causes spurious radiation outside allowed bands

Why left edge (diagonal separation) works:

Factor	Benefit
40--60 mm diagonal separation	Out of each antenna's reactive near field; coupling drops dramatically vs. 5--10 mm vertical
Orthogonal polarization	LTE antenna element runs vertically (left edge); WiFi antenna runs horizontally (right edge of ESP32 module). Adds ~10--15 dB isolation.
Independent ground planes	Each antenna has its own section of ground plane without the other antenna's return currents flowing through it
GND-heavy B2B-L connector	14 GND pins on the left connector create a low-inductance ground bridge between boards, extending the base board's ground plane to the LTE antenna's counterpoise

Ground Plane Budget¶

The YC0001CA is a monopole -- it uses the PCB ground plane as its counterpoise. Sub-GHz efficiency (B20 at 800 MHz, B28 at 700 MHz) depends on ground plane length. The minimum for CE-RED certification is ~80 mm; the Quectel reference design uses 121 mm.

Section	Length	Notes
Antenna body (no GND under this)	8.5 mm	Keep-out: antenna radiating element
Keep-out zone (no GND pour)	5 mm	Per Quectel design note
Usable extension board GND	~36.5 mm	Remaining board length
Base board GND (via B2B-L GND pins)	+50 mm (typ.)	Connected through 14 GND pins in B2B-L
Effective total	~86 mm	Above 80 mm floor; marginal for B28, comfortable for B20 and higher

B28 (700 MHz) Marginal

At 86 mm effective ground plane, B28 performance is marginal. Tune the matching network aggressively and measure TRP with VNA. If B28 fails certification, options: (1) drop B28 and certify B1/B3/B7/B8/B20 only, or (2) fall back to external FPC antenna via U.FL.

Design Notes¶

Place at left PCB board edge with the antenna element extending beyond the ground plane. Antenna 35 mm width runs along the edge; 8.5 mm depth extends inward.
Keep-out zone: No ground plane or copper pour within ~5 mm of the antenna radiating edge. No components within the keep-out zone.
Matching network: 2--3 component pi-network (0402 L/C pads) between modem RF pad and antenna input. Tuned during prototype phase with VNA to compensate for PCB-specific detuning.
Ground plane: Extension board GND, connected to base board via B2B-L (14 GND pins), provides the antenna counterpoise. Maximise ground plane area, especially in the axis perpendicular to the chip antenna.
The antenna covers all SIM7672E bands (B1, B3, B7, B8, B20, B28) plus Cat-M/NB-IoT bands for future module flexibility.

3.5 U.FL Test / Fallback Connector¶

Parameter	Value
Type	U.FL or equivalent
Location	On this extension PCB, top side, near modem RF pad
Purpose	RF testing (VNA, spectrum analyser) and optional external FPC antenna fallback
Impedance	50 ohm

Design notes:

Place adjacent to the modem RF pad, between the matching network and the chip antenna
Use a 0 ohm resistor selector (two 0402 pads) to route RF to either the chip antenna (default, populated) or the U.FL connector (DNP by default)
During development: populate U.FL path, connect VNA or spectrum analyser for RF characterisation. Measure antenna efficiency and return loss on all bands.
In production: populate chip antenna path (0R to chip antenna), DNP the U.FL connector
If field testing shows the chip antenna has insufficient rural B20/B28 performance, the design can fall back to an external FPC antenna (Quectel YPCA004AA, $0.50--0.80) via U.FL + cable -- same antenna article 232251 originally planned

3.6 Modem Power Supply (3.8V LDO)¶

The SIM7672E requires a 3.4--4.2V supply (VBAT pins 55--57) with up to 2A burst capability during LTE TX. The B2B connector provides 5V from the base PCB. A dedicated LDO on the extension board converts 5V to 3.8V.

Parameter	Value
Input	5V from B2B connector (VIN_5V, pin 1)
Output	3.8V (typical VBAT for SIM7672E)
Max Current	2A continuous, 2A peak
Dropout	<500 mV at 2A
Suggested IC	TPS7A2020 (TI), or RT9013-38 (Richtek), or similar 3.8V fixed LDO with ≥2A rating

Bulk Capacitance Required

SIMCom requires ≥100 µF (recommended 300 µF) bulk capacitance on VBAT, placed close to the modem power pins. Use a combination of ceramic (22 µF × 2, X5R, 6.3V) and electrolytic/tantalum (100--220 µF, 6.3V) capacitors. VBAT trace width: minimum 3 mm per SIMCom hardware design guide.

Design notes:

Place LDO close to B2B power pins to minimise input trace length
Place bulk capacitors (ceramic + tantalum) close to modem VBAT pins
The LDO must handle 2A transient current during LTE TX bursts -- verify transient response with adequate output capacitance
The SIM7672E VDD_1V8 output (pin 15) provides the 1.8V reference for level shifters -- no external 1.8V regulator needed

3.7 Level Shifters (1.8V ↔ 3.3V)¶

All SIM7672E digital I/O operates at 1.8V logic (power domain: VDD_1V8, pin 15). The ESP32 on the base PCB operates at 3.3V. Bidirectional level shifters are required for all signals crossing the B2B connector.

Signal Group	Signals	Direction	Level Shift
UART data	LTE_UART_TX, LTE_UART_RX	Bidirectional	3.3V ↔ 1.8V
Control	LTE_POWER_KEY, LTE_RESET	3.3V → VBAT (3.8V)	See note below
Status	LTE_STATUS, NETLIGHT	1.8V → 3.3V	Unidirectional
Coex	LTE_TX_BLANK	3.3V → 1.8V	Unidirectional

Suggested level shifter IC: TXB0106 (TI, 6-channel bidirectional, auto-direction-sensing) or TXS0108E (8-channel). Both support 1.2--3.6V on each side. Use VDD_1V8 from modem pin 15 as the low-side supply, and 3.3V from B2B as the high-side supply.

PWRKEY and RESET: VBAT Domain

The SIM7672E PWRKEY (pin 1) and RESET (pin 16) pins operate in the VBAT voltage domain (3.4--4.2V), not 1.8V. These cannot use a standard 1.8V↔3.3V level shifter. Options:

NPN open-drain driver: 3.3V ESP32 GPIO → 10k base resistor → NPN transistor → collector to modem pin with pull-up to VBAT. Active-low drive (ESP32 HIGH = modem pin LOW).
N-channel MOSFET level shifter: BSS138 or equivalent, source to 3.3V side, drain to VBAT side with pull-up.

The PWRKEY requires a >500 ms LOW pulse to toggle power. RESET requires a >100 ms LOW pulse for hard reset. Both are infrequent operations -- simple NPN/MOSFET circuits are sufficient.

3.8 NETLIGHT Development LED¶

Parameter	Value
LED	Green 0402 or 0603 LED + series resistor (1k)
Signal	Taps SIM7672E NETLIGHT pin (push-pull output, 1.8V)
Purpose	Visual LTE registration status during development
Production	DNP (Do Not Populate) once firmware is validated

The NETLIGHT pin outputs a hardware blink pattern indicating network registration state:

Pattern	Meaning
64ms on / 800ms off	Not registered, searching
64ms on / 3000ms off	Registered to network
64ms on / 300ms off	GPRS/data transfer active

The LED taps the NETLIGHT signal before level shifting, directly on the 1.8V side. A 1k series resistor limits current for the 1.8V drive. The same signal is also level-shifted to 3.3V and routed to B2B pin 9 for ESP32 blink-pattern decoding.

Development Aid -- DNP in Production

The NETLIGHT LED and its series resistor are populated during development and prototype builds for visual LTE status verification without requiring firmware. Mark these two components as DNP in the production BOM once LTE status reporting via LED2 is validated in firmware.

3.9 USB_BOOT Strapping (Pin 6)¶

Parameter	Value
Pin	6 (USB_BOOT)
Domain	1.8V
Requirement	Must not be pulled LOW during power-up

The SIM7672E pin 6 (USB_BOOT) is a strapping pin. If pulled LOW during power-up, the module enters USB firmware download mode instead of normal operation. The pin has an internal pull-up. Do not route any signal to this pin -- leave unconnected or add a 10k pull-up to VDD_1V8 for extra safety.

4. Pin-Out¶

Signals are split between the B2B connector (to/from base PCB) and local connections (modem ↔ SIM, modem → antenna).

4.1 B2B Connector Signals¶

See section 3.2 for the full 14-pin assignment. All B2B signals are at 3.3V logic (level shifting to 1.8V/VBAT happens on this extension board). Signals connect to the ESP32 on the 232203 base PCB (not nRF54 directly).

4.2 Local Signals (Extension Board Only)¶

These signals do not cross the B2B connector -- they are routed locally on this extension PCB:

Signal	From	To	Voltage	Notes
SIM_CLK	SIM7672E	SIM holder	1.8/3V	SIM clock (up to 5 MHz)
SIM_DATA	SIM7672E	SIM holder	1.8/3V	SIM data (bidirectional)
SIM_RST	SIM7672E	SIM holder	1.8/3V	SIM reset
SIM_VCC	SIM7672E	SIM holder	1.8/3V	SIM power (from modem internal regulator)
RF_OUT	SIM7672E	Matching network	RF	50 ohm to chip antenna or U.FL (via 0R selector)
NETLIGHT	SIM7672E	LED + level shifter	1.8V	Tapped locally for dev LED, then level-shifted to 3.3V for B2B
VDD_1V8	SIM7672E	Level shifters	1.8V	Module 1.8V reference output (pin 15), supplies low-side of level shifters
VBAT	3.8V LDO	SIM7672E	3.8V	Module power supply (pins 55--57)

5. Component Selection¶

5.1 LTE Modem¶

Parameter	Value
Manufacturer	SIMCom
Part Number	SIM7672E
Chipset	Qualcomm QCX216
Technology	LTE Cat-1bis (no GSM fallback)
Region	EU (CE-RED certified)
Bands	LTE-FDD: B1, B3, B7, B8, B20, B28
Package	LCC+LGA, 24 x 24 x 2.4 mm
Est. Price	~$7--10 @ 1k volume (LCSC/Western distributor)
Datasheet	A7672 Series Spec, Compatible Design Guide V1.02, Sleep Mode App Note

Rationale (see also ADR #17):

Band 28 (700 MHz): Adds critical rural EU coverage missing from A7672E (B1/B3/B5/B7/B8/B20 only). B28 is actively deployed by Nordic operators for forests, mountains, and countryside.
Cat-1bis: Single RX antenna (no MIMO diversity required). Sufficient throughput (10/5 Mbps) for MQTT gateway data. Simpler RF design than full Cat-1.
Qualcomm QCX216 chipset: Lower sleep current (<1 mA vs ~3 mA for A7672E's ASR1603). Important for battery-backed gateway operation.
Footprint-compatible: Same 24x24mm LCC+LGA as A7672E. Same AT command set. Pin-compatible for UART, SIM, power, NETLIGHT, STATUS, PWRKEY, RESET.
No GSM fallback: Acceptable trade-off -- EU 2G networks are being shut down (Telia SE 2025, Telenor NO 2025--2026, Vodafone DE 2025).

Alternative modules (evaluate before production):

Module	Category	Key Difference	Footprint
SIM7672G	Cat-1bis	Global bands (incl. B28), LTE-TDD	Same 24x24mm
A7672E (FASE)	Cat-1	ASR1603, no B28, cheaper	Same 24x24mm
A7673E	Cat-1bis	ASR1606 chipset, may be cheaper	Same 24x24mm
A7682E	Cat-1	Smaller (19.6x19.6mm)	Different footprint

Supplier Links:

Source	Part Number / Link	Notes
SIMCom	SIM7672 Series	Manufacturer direct
Texim Europe	SIM7672E-LNGV	EU distributor
DigiKey	A7672E(FASE) (3781-A7672E(FASE)-ND)	Pin-compatible A7672E fallback; SIM7672E via SIMCom direct or Texim
LCSC	A7672E	Pin-compatible A7672E available; SIM7672E check availability

5.2 Board-to-Board Connectors (Dual -- 1.27 mm pitch)¶

Two identical connector pairs (header on base PCB, socket on extension) flank the ESP32-C5 WiFi module. Qty 2 of each part per assembly.

Parameter	Value
Manufacturer	Samtec
Socket (ext)	SFM-108-01-x-D (2×8, 16-pin, vertical SMT, female)
Header (base)	TMM-108-01-x-D (2×8, 16-pin, vertical SMT, male)
Configuration	2×8 = 16-pin per connector, 2 connectors = 32 pins total
Pitch	1.27 mm (0.05")
Stacking Height	~8--10 mm PCB-to-PCB (selectable via post length option)
Current Rating	3.7A per pin (2 pins powered)
Qty per Board	2 sockets (extension), 2 headers (base)
Est. Price	~$0.50--0.80 per connector pair

Source	Part Number	Notes
Samtec	SFM-108-01-S-D	Socket (female), standard post
Samtec	TMM-108-01-S-D	Header (male), standard post
DigiKey	Search "Samtec SFM-108"	Multiple post length options
Mouser	Search "Samtec TMM-108"	Multiple post length options

Connector Alternatives at 1.27 mm Pitch

Pin-compatible alternatives: Harwin M50-3530842 (2×8 socket, 1.27 mm), Amphenol FCI Minitek 1.27 mm (2×8), or Samtec FTSH/SFSD series. Key requirements: 2×8 vertical SMT, 1.27 mm pitch, ≥2A per-pin, stacking height 8--10 mm.

5.3 SIM Card Holder¶

Parameter	Value
Type	Nano-SIM (4FF) push-push, SMT, with detect switch
Mounting	Top side, card slot facing downward
Pins	6+1 (6 SIM contacts + 1 detect switch)
Est. Price	~$0.30--0.50 USD

Source	Part Number	Notes
DigiKey	GCT SIM8075-6-1-12-00-A	Nano-SIM, push-push, 6P+SW, SMT, 1.35mm height
Mouser	GCT SIM8075-6-1-12-00-A	Same
LCSC	SHOU HAN NANO SIM 7P H1.37 (C7529384)	Nano-SIM, push-push, SMT, low-cost alternative
LCSC (alt)	ATTEND 115S-ACA0	Nano-SIM, bar-push, SMT, with detect

5.4 On-Board Chip Antenna¶

Parameter	Value
Manufacturer	Quectel
Part Number	YC0001CA
Type	SMD chip monopole (tape & reel)
Frequency	698--960 MHz / 1710--2700 MHz
Dimensions	35 x 8.5 x 3 mm
Efficiency	Up to 72.9%
Est. Price	~$0.19 @ 500+ (JLCPCB/LCSC)

Supplier Links:

5.5 U.FL Connector (Test / Fallback)¶

Parameter	Value
Type	U.FL (IPEX MHF1) SMT receptacle
Impedance	50 ohm
Height	~1.2 mm
Est. Price	~$0.10--0.25 USD
BOM Status	DNP in production (test/fallback only)

Source	Part Number	Notes
DigiKey	Hirose U.FL-R-SMT-1(10)	Original U.FL, gold contacts
Mouser	Hirose U.FL-R-SMT-1(10)	Same
LCSC	IPEX 20279-001E-03 (C69316)	Low-cost IPEX Gen1, widely used
LCSC (alt)	Various MHF4 connectors	Search "U.FL SMT" -- many generic options available

5.6 3.8V LDO (Modem VBAT Supply)¶

Parameter	Value
Output	3.8V (adjustable, set with R-divider)
Max Current	≥3A (must sustain 2A TX bursts)
Input	5V (from B2B)
Dropout	<500 mV @ 2A
Package	SOT-223-3 or D-PAK (good thermal)
Est. Price	~$0.30--0.80 USD

Previous Suggestions Were Wrong

AP2112K (max 600 mA) and RT9013 (max 500 mA) are far too low for the 2A TX burst current of the SIM7672E. A 3A-rated adjustable LDO is required.

Source	Part Number	Notes
DigiKey	Microchip MIC29302WU-TR	3A adj LDO, SOT-223-3, dropout 450 mV @ 3A. Set Vout = 3.8V with external divider. Confirmed in SIMCom reference designs.
Mouser	Microchip MIC29302WU-TR	Same
LCSC	Microchip MIC29302WU (C47053)	Low-cost, widely stocked
Alternative	Analog Devices LT1764AEQ-3.3#PBF (adj)	3A, SOT-223. Ultra-low noise. Higher cost (~$3). Use adjustable version for 3.8V.

Resistor divider for 3.8V output (MIC29302):

R1 (ADJ → GND) = 1.0 kΩ
R2 (OUT → ADJ) = 2.05 kΩ (use 2.0 kΩ + 47Ω series, or nearest E96 value)
Vout = 1.24V × (1 + R2/R1) = 1.24 × 3.05 ≈ 3.78V

Place 1% resistors close to the LDO ADJ pin. Add 10 µF ceramic + 100 µF tantalum on output for stability.

5.7 Level Shifter IC¶

Parameter	Value
Manufacturer	Texas Instruments
Part Number	TXB0106PWR (primary) or TXS0108EPWR (8-ch alternative)
Channels	6 bidirectional (auto-direction-sensing)
Low side	1.8V (from modem VDD_1V8, pin 15)
High side	3.3V (from B2B)
Package	TSSOP-16
Max Speed	100 Mbps (TXB0106)
Est. Price	~$0.20--0.50 USD

Channel Usage

6 channels handle: LTE_UART_TX, LTE_UART_RX, LTE_STATUS, NETLIGHT, LTE_TX_BLANK + 1 spare. PWRKEY and RESET use separate NPN drivers (section 5.10) because they operate in the VBAT domain, not 1.8V.

Source	Part Number	Notes
DigiKey	TI TXB0106PWR	6-ch bidirectional, TSSOP-16
Mouser	TI TXB0106PWR	Same
LCSC	TI TXB0106PWR (C38253)	Low-cost, widely stocked
Alternative	TI TXS0108EPWR	8-ch, if more channels needed

5.8 NETLIGHT Development LED¶

Parameter	Value
LED	Green 0402 or 0603
Resistor	1k series (0402), sized for 1.8V drive (I ≈ 0.6 mA)
Est. Price	~$0.02 total
BOM Status	DNP in production (dev aid only)

Source	Part Number	Notes
LCSC	0402 Green LED (C2297)	Everlight 19-217/GHC-YR1S⅔T, ~$0.01
LCSC	0402 1kΩ resistor (C11702)	UniOhm 0402WGF1001TCE, ~$0.001

5.9 Bulk Capacitors (Modem VBAT)¶

Parameter	Value
Ceramic	2x 22 µF, X5R/X7R, 6.3V, 1206
Tantalum	1x 100--220 µF, 6.3V, low-ESR, size D
Placement	Close to modem VBAT pins (55--57)

Source	Part Number	Notes
LCSC	22 µF 6.3V 1206 X5R (C59461)	Samsung CL31A226MPHNNNE, ~$0.02 ea
LCSC	100 µF 6.3V tantalum (C7171)	AVX TAJD107K006RNJ, ~$0.15
DigiKey	Same Samsung / AVX parts	Available from Western distributors

5.10 NPN Transistors (PWRKEY / RESET Drivers)¶

PWRKEY (modem pin 1) and NRST (modem pin 38) operate in the VBAT domain (3.4--4.2V), not 1.8V. They cannot use the TXB0106 level shifter. Two N-channel MOSFETs (or NPN transistors) provide open-drain drive from the 3.3V ESP32 GPIO. The modem pins have internal pull-ups to VBAT.

Parameter	Value
Transistor	BSS138 N-channel MOSFET, SOT-23
VDS max	50V
ID max	220 mA
Gate threshold	0.8--1.5V (turns on reliably with 3.3V)
Quantity	2 (one for PWRKEY, one for RESET)
External parts	10 kΩ gate pull-down resistor (0402) per MOSFET
Est. Price	~$0.02 ea

Circuit (per signal):

ESP32 GPIO → 10 kΩ gate resistor → BSS138 gate
BSS138 source → GND
BSS138 drain → modem pin (PWRKEY or NRST); modem internal pull-up to VBAT
ESP32 HIGH → MOSFET ON → modem pin pulled LOW (active)
ESP32 LOW → MOSFET OFF → modem pin pulled HIGH by internal pull-up (inactive)

Source	Part Number	Notes
DigiKey	Onsemi BSS138	SOT-23, N-channel MOSFET
Mouser	Onsemi BSS138	Same
LCSC	Onsemi BSS138 (C52895)	~$0.02, very widely stocked

5.11 SIM ESD Protection (TVS Diode Array)¶

ESD protection on all SIM card lines. The SIM holder is the primary user-accessible interface on this board -- SIM insertion/removal is a common ESD event.

Parameter	Value
Part Number	PRTR5V0U2X (Nexperia) or equivalent
Protection	ESD ±8 kV contact, ±15 kV air
Lines	2 per package (use 2 packages for CLK, DATA, RST, VCC)
Package	SOT-143B
Clamping	<6V @ 1A (IEC 61000-4-2)
Est. Price	~$0.05 ea

Source	Part Number	Notes
DigiKey	Nexperia PRTR5V0U2X,215	Dual-line TVS, SOT-143B
Mouser	Nexperia PRTR5V0U2X,215	Same
LCSC	Nexperia PRTR5V0U2X (C12333)	~$0.05, widely used

6. PCB Design Notes¶

6.1 Assembly Principle -- Top Side Components Only¶

All components are placed on the top side only of the extension PCB. The bottom side faces the 232203 base PCB across the B2B connector gap (~10--11mm). The B2B socket pins protrude from the bottom side but no other components are placed there.

Layer	Contents
Top	SIM7672E modem, chip antenna, SIM holder, level shifters, LDO, U.FL (DNP), B2B socket body, dev LED, passives
Bottom	B2B socket solder pads only (no components)

The SIM card holder is mounted on the top side with the card insertion slot facing downward -- the SIM card is inserted from underneath the extension board. This provides SIM access from the same side as the USB-C connector on the base PCB.

6.2 Board Dimensions¶

Extension PCB: ~36 × 50 mm (constrained by enclosure internal geometry)
Board outline constrained by enclosure volume above the 232203 base PCB
Stack-up: base PCB (~1.6 mm) + B2B gap (~8--10 mm) + extension PCB (~1.6 mm) = ~11--13 mm total

6.3 RF Layout and Antenna Placement¶

    Cross-section (side view):

    ┌──────────────────────────────────────────────┐
    │  BASE BOARD (top PCB, ~36 × 50 mm)           │
    │                              ┌──────┐        │
    │                              │ESP32 │ ← WiFi antenna
    │                              │ C5   │   (right edge)
    │                              └──────┘        │
    └──B2B-L──┬──────────┬──B2B-R──────────────────┘
              │          │
    ┌──B2B-L──┴──────────┴──B2B-R──────────────────┐
    │  EXTENSION BOARD (bottom PCB, ~36 × 50 mm)   │
    │ ┌─────┐  ┌────────────┐                      │
    │ │ LTE │  │  SIM7672E   │     [SIM] [LDO]     │
    │ │ ANT │  │  24×24 mm   │                     │
    │ │     │  └────────────┘                      │
    │ └─────┘                                      │
    │ ← left edge                    right edge →  │
    └──────────────────────────────────────────────┘

Chip antenna placement: Left edge of extension board, opposite the WiFi antenna on the base board above. This provides maximum diagonal separation (~40--60 mm) between LTE and WiFi antennas, avoiding near-field coupling and frequency-overlap interference (LTE 1710--2700 MHz overlaps WiFi 2.4 GHz). See §3.4 for full rationale.
Antenna orientation: The 35 mm antenna element runs along the left board edge; the 8.5 mm depth extends inward. No ground pour or copper within 5 mm keep-out zone around the antenna radiating edge.
Matching network: Pi-network (2--3 pads, 0402) between modem RF pad and antenna feed. Tune with VNA after first prototype.
RF trace: 50 ohm controlled impedance trace from modem RF pad (pin 60) to matching network. Continuous ground plane beneath RF trace, no splits.
U.FL placement: Adjacent to matching network. 0R selector between chip antenna path and U.FL path (only one populated at a time).
Ground plane: Extension board GND is connected to the base board via B2B-L (14 GND pins, left connector near LTE antenna) and B2B-R (9 GND pins, right connector). The 23 total GND pins provide low-inductance symmetric ground coupling. Maximise ground plane area on the extension board, especially in the axis perpendicular to the chip antenna.
WiFi/LTE isolation: The diagonal placement + orthogonal polarization (LTE vertical, WiFi horizontal) provides ~20--30 dB isolation. Time-domain coordination is handled by the LTE_TX_BLANK coexistence signal.

6.4 Power Layout¶

3.8V LDO close to B2B-L power input pins (L1, L2) -- short, wide input traces from 5V rail
Wide power traces (≥3 mm) for VBAT from LDO output to modem power pins (2A burst current)
Bulk capacitors (≥100 µF total, recommended 300 µF) close to modem VBAT pins
Separate analog and digital ground regions where specified by SIMCom reference design
The modem VDD_1V8 output (pin 15) powers the level shifter low-side supply -- add 100nF decoupling cap close to pin 15

6.5 SIM Routing¶

SIM signals routed locally from modem SIM pins to SIM holder (short traces, same PCB)
No SIM signals cross the B2B connector
ESD protection (TVS diode array) close to SIM holder
Controlled impedance per ISO/IEC 7816 SIM specification

6.6 Level Shifter Placement¶

Place level shifter IC close to B2B-R connector (on the 3.3V signal path side)
Keep 1.8V traces between modem and level shifter short
VDD_1V8 reference from modem pin 15 to level shifter low-side supply -- add decoupling cap
PWRKEY and RESET use separate NPN/MOSFET circuits (not the TXB0106) due to VBAT voltage domain

6.7 Manufacturing¶

4-layer FR4 (same specification as 230220 -- see PCB Project Brief section 4)
Black solder mask
ENIG surface finish
SMT assembly compatible with same EMS as base PCB
Top-side-only component placement simplifies assembly
On-board chip antenna eliminates manual cable assembly step

7. Test Points¶

TP #	Signal	Expected Value	Notes
TP40	VIN_5V	5.0V	Power from base PCB via B2B
TP41	VBAT_3V8	3.8V	LDO output to modem
TP42	VDD_1V8	1.8V	Modem 1.8V reference (pin 15)
TP43	UART_TX (1.8V side)	Logic	AT command data (modem side, 1.8V)
TP44	UART_RX (1.8V side)	Logic	AT response data (modem side, 1.8V)
TP45	RF_OUT	--	RF signal to antenna / matching network
TP46	SIM_CLK	Logic	SIM clock (local, modem ↔ holder)
TP47	NETLIGHT	Blink pattern	Dev LED should blink when active (1.8V)
TP48	PWRKEY	VBAT domain	Modem power toggle (VBAT level)

LTE Extension Tests¶

8. LTE Modem Firmware Update¶

The SIM7672E modem runs its own firmware (baseband + protocol stack). SIMCom periodically releases firmware updates for bug fixes, band support improvements, and carrier certifications. This section documents how to update the modem firmware.

8.1 Checking Current Firmware Version¶

Query the modem's firmware version via AT commands through the ESP32 UART interface:

AT+SIMCOMATI        → Module identification (HW version, FW version)
AT+CGMR             → Firmware revision string
AT+CSUB             → Sub-version / build info

Record the output before and after any firmware update. Verify you have the correct firmware package for your exact hardware variant (SIM7672E, not SIM7672G or SIM7672S — wrong firmware can brick the module).

8.2 Update Methods¶

The SIM7672E supports two firmware update methods:

Method A: USB Download Mode (Recommended for Production / Factory)¶

This method uses the SIM7672E's USB 2.0 interface and SIMCom's dedicated flashing tool.

Item	Value
Tool	SIMCom SPT Tool (System Programming Tool) or Aboot Firmware Download Tool
Connection	USB 2.0 (module USB_DM / USB_DP pins)
Driver	SIMCom USB driver (Windows / Linux)
Mode entry	Pull USB_BOOT (pin 6) LOW during power-up
Speed	Full firmware flash in ~2--5 minutes

Procedure:

Install prerequisites: Install SIMCom USB drivers and the SPT Tool (or Aboot tool) on a PC. Download the correct firmware package from SIMCom (match HW version exactly using AT+SIMCOMATI output).
Connect USB: Connect the SIM7672E USB_DM / USB_DP pins to a USB cable. On this extension board, USB is not routed to an external connector by default — a temporary test jig or flying wires to TP43/TP44 area may be needed. For production, consider a pogo-pin test fixture.
Enter download mode: Power off the modem (de-assert PWRKEY). Pull USB_BOOT (pin 6) LOW externally (override the 10k pull-up). Power on the modem — it enters USB firmware download mode instead of normal boot.
Flash firmware: The SPT Tool detects the module in download mode. Select the firmware image and start programming. Wait for completion (do not interrupt power).
Exit download mode: Remove the USB_BOOT LOW override. Power cycle the modem. It boots normally with the new firmware.
Verify: Send AT+CGMR and AT+SIMCOMATI to confirm the new firmware version.

USB_BOOT Safety

In normal operation, USB_BOOT (pin 6) has a 10k internal pull-up and must not be pulled LOW. The firmware download mode is entered only when USB_BOOT is forced LOW during power-up. See section 3.9 for the circuit details.

USB Access on This Board

The SIM7672E USB pins are not routed to an external connector on the 232210 extension board (USB is not needed for normal AT command operation — the ESP32 communicates via UART). For firmware updates, access USB via test points or a dedicated jig. A future board revision may add a micro-USB or USB-C footprint (DNP by default) for convenient modem firmware updates.

Method B: FOTA (Firmware Over-The-Air)¶

The SIM7672E supports firmware updates over the cellular data connection using SIMCom's FOTA server or a custom HTTP/FTP server.

Item	Value
Protocol	HTTP or FTP download
AT Command	`AT+CFOTA` (SIMCom FOTA) or `AT+HTTPGET` (custom server)
Connection	Requires active data connection (PDP context)
Size	Full firmware image, typically 10--30 MB
Duration	Depends on network speed (typically 5--15 minutes on Cat-1)

Procedure:

Ensure connectivity: Verify PDP context is active (AT+CGACT?).
Initiate FOTA: Use AT+CFOTA=<url> with the firmware image URL. The module downloads, verifies, and installs the update automatically.
Module reboots automatically after successful installation.
Verify: Send AT+CGMR to confirm the new version.

FOTA via ESP32 Firmware

In production, the ESP32 gateway firmware orchestrates FOTA updates. The cloud backend pushes a firmware update URL to the ESP32 (via MQTT or HTTP). The ESP32 sends the AT+CFOTA command sequence to the modem. This enables remote modem firmware updates without physical access to the device.

8.3 Firmware Update Decision Matrix¶

Scenario	Recommended Method	Notes
Factory first-flash	USB download	Fastest, most reliable
Production line	USB download (jig)	Repeatable with pogo-pin fixture
Field-deployed device	FOTA	No physical access needed
Bricked module (no AT response)	USB download	Only option if modem is unresponsive
Development / debug	USB download	Direct control, can flash debug builds

9. Revision History¶

Revision	Date	Author	Changes
Rev A	TBD	TBD	Initial design (not in first PCB iteration)

232203 Base Bat+WiFi -- Base PCB this extension connects to
232211 Ext LTE US -- US variant of this extension
ADR #17: LTE Module and Antenna -- Decision rationale
Sales Articles -- Part of subassembly 232504 (EU)
datasheets/A7670_SIM7070_A7672X_Compatible_Design_V1.02.pdf -- SIMCom hardware design reference (pin-compatible family)
datasheets/SIM7672X_Sleep_Mode_Application_Note_V1.00.pdf -- Sleep mode configuration

232210 - EverTag Ext LTE EU¶

1. Overview¶

Key Features¶

2. Block Diagram¶

3. Schematics¶

3.1 LTE Modem -- SIMCom SIM7672E¶

SIM7672E Modem Pin Assignment (Used Pins)¶

Complete Signal Path: ESP32 → B2B → Level Shift → Modem¶

3.2 Board-to-Board Connectors (Dual Split -- 1.27 mm pitch)¶

B2B-L (Left Connector -- Power + GND, near LTE antenna)¶

B2B-R (Right Connector -- Signals + GND, opposite LTE antenna)¶

3.3 SIM Card Holder¶

3.4 On-Board Chip Antenna -- Placement and Coexistence¶

Antenna Placement: Left Edge (Opposite WiFi)¶

Ground Plane Budget¶

Design Notes¶

3.5 U.FL Test / Fallback Connector¶

3.6 Modem Power Supply (3.8V LDO)¶

3.7 Level Shifters (1.8V ↔ 3.3V)¶

3.8 NETLIGHT Development LED¶

3.9 USB_BOOT Strapping (Pin 6)¶

4. Pin-Out¶

4.1 B2B Connector Signals¶

4.2 Local Signals (Extension Board Only)¶

5. Component Selection¶

5.1 LTE Modem¶

5.2 Board-to-Board Connectors (Dual -- 1.27 mm pitch)¶

5.3 SIM Card Holder¶

5.4 On-Board Chip Antenna¶

5.5 U.FL Connector (Test / Fallback)¶

5.6 3.8V LDO (Modem VBAT Supply)¶

5.7 Level Shifter IC¶

5.8 NETLIGHT Development LED¶

5.9 Bulk Capacitors (Modem VBAT)¶

5.10 NPN Transistors (PWRKEY / RESET Drivers)¶

5.11 SIM ESD Protection (TVS Diode Array)¶

6. PCB Design Notes¶

6.1 Assembly Principle -- Top Side Components Only¶

6.2 Board Dimensions¶

6.3 RF Layout and Antenna Placement¶

6.4 Power Layout¶

6.5 SIM Routing¶

6.6 Level Shifter Placement¶

6.7 Manufacturing¶

7. Test Points¶

LTE Extension Tests¶

8. LTE Modem Firmware Update¶

8.1 Checking Current Firmware Version¶

8.2 Update Methods¶

Method A: USB Download Mode (Recommended for Production / Factory)¶

Method B: FOTA (Firmware Over-The-Air)¶

8.3 Firmware Update Decision Matrix¶

9. Revision History¶

Related Documents¶