---
tags: [project-drogheda, hardware, watchdog, schematic, pcb]
project: Project Drogheda
author: Helgen CTO
date: 2026-06-20
status: Active
---

# Watchdog — Wiring & Schematic Spec (STM32F411 Blackpill)

> The as-built watchdog circuit, formalized for the production PCB. Pairs with
> the firmware [[04-Source-Code/firmware/blackpillrtkcode_v2/blackpillrtkcode_v2|blackpillrtkcode_v2.ino]]
> (pin map must match) and the [[03-Architecture-Hardware/BOM-master|BOM]].

## 1. Power tree (corrected)

```text
Cabinet 24 V ─GX16-5─► HBOX
   └► Mean Well SD-15B-05 (24→5 V, 3 A) ─► 5 V rail ─► PowerBoost 1000C (USB in)
        PowerBoost (BAT=18650 1S4P) ─► PowerBoost 5V OUT ─► STM32 5V + IRF9540N source
        24 V ignition ─► DFR1015 (buck) ─► STM32 PA5 (ignition sense, ≤3.3 V)
```

## 2. STM32F411 pin map (matches firmware v2)

| Pin | Net | Function |
|---|---|---|
| PA0 | BAT_SENSE | 18650 voltage ÷ divider → ADC |
| PA4 | 5V_SENSE | PowerBoost USB (vehicle-power present) ÷ divider → ADC |
| PA5 | IGN_SENSE | ignition via **DFR1015** (digital, INPUT_PULLDOWN) |
| PA1 | PWR_CTRL | drives the RTK power switch (NPN→P-MOS) |
| PA2 | GNSS TX2 | UBX commands → simpleRTK2B RX1 |
| PA3 | GNSS RX | NMEA from simpleRTK2B (read-back) |
| PB0 | EXT_LED | external status LED (via N-MOSFET) |
| PC13 | onboard LED | active-LOW |

## 3. RTK power switch (high-side, P-channel)

```text
        5V ───┬──────────────[10kΩ pull-up]──┐
              │                              │
   IRF9540N Source                        Gate ── 2N3904 Collector
              │                              │
            Drain ──► simpleRTK2B 5V    2N3904 Emitter ── GND
                                             │
   STM32 PA1 ──[1kΩ]── 2N3904 Base ──────────┘
```

- PA1 **HIGH** → 2N3904 conducts → pulls IRF9540N gate **LOW** → P-MOS **ON** →
  5 V to the simpleRTK2B + 4G stack. PA1 LOW → gate held high by 10 kΩ → OFF.
- 10 kΩ gate pull-up = defined OFF at boot/reset (fail-safe: stack off until the
  MCU asserts).

> **PCB note:** replace the 2N3904 + IRF9540N + pull-up with an **integrated
> high-side load switch / eFuse** (e.g., TPS25200 / TPS2596) — current-limited,
> fewer parts, defined turn-on, no gate-pull-up gotchas. Keep the same fail-safe
> (load off until MCU enables).

## 4. Status LED (low-side N-MOSFET, logic-level)

```text
 3.3V ──[220–330Ω]──►|LED|──► 2N7000/BS170 Drain
                                     Source ── GND
        STM32 PB0 ── Gate ──┬── 10kΩ ── GND   (pull-down: LED off while booting)
```

- 2N7000/BS170 are logic-level (switch fully at 3.3 V). 10 kΩ pull-down keeps the
  LED off during MCU boot. PC13 onboard LED mirrors inverted (active-low).

## 5. Sense dividers

| Net | Source | Divider | At ADC (max) | Note |
|---|---|---|---|---|
| PA0 BAT | 18650 (3.0–4.2 V) | ÷2 (e.g. 10k/10k) | 1.5–2.1 V | firmware `DIVIDER_RATIO=2.0` |
| PA4 5V | PowerBoost USB ~5 V | ÷2 | ~2.5 V | "vehicle power present" |
| PA5 IGN | 24 V ignition | **DFR1015 buck** → ≤3.3 V | digital | isolated/scaled by DFR1015 |

> ⚠ The BOM lists a lone **1.5 kΩ "????"** resistor — confirm whether it's a
> divider leg or the LED resistor and pin it down on the schematic. Keep ADC
> inputs ≤3.3 V with margin.

## 6. Resistor / discrete BOM (watchdog)

| Ref | Value | Purpose |
|---|---|---|
| R1 | 1 kΩ | PA1 → 2N3904 base |
| R2 | 10 kΩ | IRF9540N gate pull-up (to 5 V) |
| R3 | 220–330 Ω | LED current limit |
| R4 | 10 kΩ | PB0 LED-MOSFET gate pull-down |
| R5/R6 | 10 kΩ/10 kΩ | PA0 battery divider (÷2) |
| R7/R8 | 10 kΩ/10 kΩ | PA4 5 V divider (÷2) |
| Q1 | 2N3904 (NPN) | gate driver |
| Q2 | IRF9540N (P-MOS) | RTK 5 V high-side switch |
| Q3 | 2N7000 / BS170 (N-MOS) | LED switch |

## 7. For the production PCB

- Integrated load switch (above) instead of the discrete trio.
- All ADC dividers with RC filtering; clamp/Zener protection on PA5 ignition.
- Bring out **SWD** + a **debug UART** (the v2 ANSI dashboard) on a keyed header.
- Add an external temp sensor near the 18650 / modem (v2 logs MCU temp; an
  external one near the battery is more representative).
- Keep switching-regulator noise away from the GNSS antenna feed (RF integrity).
- Single keyed **GX16-5** for the 24 V + ignition interface to the cabinet.

## Open Questions

- [ ] Pin down the 1.5 kΩ resistor's role; finalize all divider values.
- [ ] Confirm DFR1015 output level into PA5 (≤3.3 V) and whether it's analog or digital sense.
- [ ] Load-switch part choice + current limit (from the measured power budget).

## Related

- [[CLAUDE|Master Index]] · [[04-Source-Code/firmware/blackpillrtkcode_v2/blackpillrtkcode_v2|firmware v2]] ·
  [[03-Architecture-Hardware/hardware-respec-and-pcb-brief|Hardware re-spec + PCB brief]] ·
  [[03-Architecture-Hardware/BOM-master|BOM]]