4G NTRIP module — add USB, reset, LED + fix required power pins

Goal: firmware-update and reset the 4G modem in place on the EINARBoard (no more swapping it to the simpleRTK), and wire the module's mandatory pins that are currently floating.

Module pin map (XBee-form, per datasheet) → EINARBoard socket

J7 = module pins 1–10 (row 1), J8 = module pins 11–20 (row 2, numbered 20→11 top-to-bottom).

Module pin	Function	Socket pad	Current	Action
1 VCC	3.3–3.6 V, 2 A pk	J7.1	3V3_MODEM ✓	keep
2 TX	data out	J7.2	DOUT_4G ✓	keep
3 RX	data in	J7.3	DIN_4G ✓	keep
4 GND	ground	J7.4	NC	→ GND
5 RESET	low = reset	J7.5	NC	→ reset button → GND
6 V_USB	5 V enables USB	J7.6	NC	→ USB VBUS (5 V)
7 USB+	USB D+	J7.7	NC	→ USB D+
8 USB−	USB D−	J7.8	NC	→ USB D−
9 DTR	disabled	J7.9	NC	leave
10 GND	ground	J7.10	GND ✓	keep
11 GND	ground	J8.10	NC	→ GND
12 CTS	disabled	J8.9	NC	leave
13 LED	1.8 V LED drive	J8.8	NC	→ R + LED → GND
14 VREF	must = VCC	J8.7	NC	→ 3V3_MODEM
15 GND	ground	J8.6	NC	→ GND
16 RTS	disabled	J8.5	NC	leave
20 ON/OFF	GND = always ON	J8.1	NC	→ GND

⚠️ Must-fix even if we skipped USB: VREF (J8.7→VCC) and ON/OFF (J8.1→GND). Without ON/OFF tied low the modem may never power on. Confirm J8 pad numbers vs. the Ardusimple footprint before wiring.

Parts to add

Ref	Part	Notes
J9	USB-C receptacle, USB 2.0 (e.g. GCT USB4085 / TYPE-C-31-M-12)	D+/D−, VBUS, GND, 2×CC
R_cc1, R_cc2	5.1 kΩ 0402/0603	CC1, CC2 → GND (device-mode detect, required for Type-C)
R_dp, R_dm	0 Ω (optional series)	in D+/D− if you want the option; can be DNP
SW_4G	tactile 6 mm (or SMD)	RESET(J7.5) ↔ GND
D_4G	LED 0805	anode ← R_led ← LED(J8.8); cathode → GND
R_led	~1 kΩ	LED current limit (pin sources 1.8 V)

New nets

USB_VBUS (→ J7.6 V_USB + USB-C VBUS), USB_DP (J7.7 ↔ USB-C D+), USB_DM (J7.8 ↔ USB-C D−), MODEM_RST (J7.5 ↔ SW_4G ↔ GND), MODEM_LED (J8.8 ↔ R_led ↔ D_4G ↔ GND). Reuse existing 3V3_MODEM for VREF, GND for ON/OFF + spare grounds.

Placement

Open pocket right of / below the XBee sockets (board ≈ x200–212, y76–98). USB-C at the board edge (mechanical access), button + LED beside it.

Routing

USB D+/D−: keep the pair together and short; USB-2.0 FS/HS firmware speed is forgiving — no controlled-impedance needed, just route them side-by-side, similar length, GND nearby.
Everything else is low-speed/DC — trivial.
Do it by adding to the schematic first → re-import → place → FreeRoute the new nets (lock existing routing) → DRC → re-export fab.

STATUS 2026-07-05 — what's done, what's left

DONE (in the schematic hbox-watchdog-v3-PROD.kicad_sch, verified ERC 0 errors):

Modem must-fix pins wired: J8.1→GND (ON/OFF), J8.7→3V3_MODEM (VREF), J8.6/J8.10/J7.4→GND.
New parts added + net-labelled: J9 USB-C (USB_C_Receptacle_USB2.0_14P, fp GCT_USB4085), Rcc1/Rcc2 5.1k, SW4G reset (CK_PTS636 SMD), Rmled 1k, D4G LED (0805).
All connections: USB_VBUS→J7.6, USB_DP→J7.7, USB_DM→J7.8, MODEM_RST→J7.5+SW4G, MODEM_LED→J8.8 chain, CC resistors→GND, USB-C VBUS/GND/CC/D±. (Verified on a trial board sync — J7/J8 pads took the right nets.)
Fixed a reference collision: the schematic already had a Rled (10k, live circuit); the new LED resistor is Rmled so the existing part is untouched.

LEFT (do in the KiCad GUI — the MCP's board backend left spurious DRC shorts on sync, so the board was reverted to its clean pre-USB state; the schematic is the source of truth):

Tools → Update PCB from Schematic (F8) — pulls in J9, Rcc1, Rcc2, SW4G, Rmled, D4G and assigns the modem-pin nets. (GUI F8 is clean; don't use the MCP sync.)

Place the 6 parts in the pocket right of the XBee (board coords, mm):

Ref	x	y	note
J9 (USB-C)	199.5	79	fills pocket; pads clear J8 (left) & P3 (right)
Rcc1	198.5	75.3	CC1 5.1k, row above J9
Rcc2	201.2	75.3	CC2 5.1k
Rmled	203.9	75.3	LED 1k
D4G	206.4	75.3	LED
SW4G	202.5	90	reset, in the strip below the y≈87.5 trace

Courtyard "kisses" with J8/P3 are OK (pads clear) — same as the board's existing by-design kisses.

Route 9 nets. GND pads (J9 grounds+shields, Rcc·2, SW4G·2, D4G·1) drop to the In1 GND plane with a via each (THT grounds auto-connect). Signals: USB_VBUS/DP/DM run J7.6/7/8 → J9 (under the module is fine); MODEM_RST J7.5→SW4G; MODEM_LED J8.8→Rmled→D4G; CC1/CC2 J9→Rcc. Keep D+/D- together. FreeRouting the ratsnest is the fast path (as before).
Fill All Zones (B) → DRC → should be back to the pre-existing cosmetic set (0 shorts).
Re-export the JLCPCB fab package (fab-jlcpcb/) — the current zip predates all of this.

Backup of the clean board kept at kicad/hbox-watchdog-v3-PROD.kicad_pcb.bak-preusb.

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