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Project Drogheda |
Helgen CTO |
2026-06-20 |
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
(pin map must match) and the 03-Architecture-Hardware/BOM-master.
1. Power tree (corrected)
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)
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)
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.
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