---
title: EinarBoard 3.0 — On-Board MCU Integration — Design Brief & Execution Prompt
tags: [project-drogheda, hardware, einarboard, mcu-integration, stm32f411, dfu, design-brief, prompt]
project: Project Drogheda
author: Helgen CTO + Claude (decisions locked 2026-08-12)
date: 2026-08-12
status: Active — THE prompt for the 3.0 design sessions. Read this first, then execute the phases in §10.
authority: supplements [[watchdog-electrical-design-state]] (canonical) + the 1.1 brief (as executed in v4 = EinarBoard 2.0); where they conflict on 3.0 scope, THIS file wins.
---

# EinarBoard 3.0 — Design Brief ("the prompt")

> **How to use this doc.** Self-contained instruction set for turning `hbox-watchdog-v4`
> (EinarBoard 2.0, as sent to fab ~2026-08-01, git `b505cb3`) into **EinarBoard 3.0**
> (`kicad/hbox-watchdog-v5`). Standing rules unchanged from the 1.1 brief: **file-level
> KiCad edits only** (KiCad MCP backend clobbered disk state 2026-07-24 — read-only MCP
> queries are fine); git checkpoint after every phase; verify every structural edit with an
> independent parse; bench gates unchanged.

## 0. Decisions locked 2026-08-12 (Leo)

| Topic | Decision |
|---|---|
| MCU | **On-board STM32F411RET6 (LQFP64)** replaces the Blackpill module + J1/J2 sockets |
| Flash path | **USB DFU on a board-mounted USB-C** (BOOT0 + NRST buttons on-board, Blackpill parity) + SWD recovery pads |
| Base | `hbox-watchdog-v4` (EinarBoard 2.0: W5500 Ethernet, SC16IS752 serial, PCF8574 expander — all kept) |
| Modem / GNSS | **Stay modular** — XBee socket + simpleRTK2B Pro shield (stock already purchased) |
| Fixation | Shield **bolted through its UNO-pattern holes** into matching EinarBoard holes (M2.5); **2× M2.5 holes near the XBee** for a heatsink that also clamps the modem |
| Fan | **2-pin fan only** — 4-pin header + tach conditioning removed, PB10 freed |
| Naming | Product **EinarBoard 3.0** · KiCad project **`hbox-watchdog-v5`** |

## 1. Why the F411RET6 (and not something else)

Same silicon family as the Blackpill's F411CEU6 → **firmware v2.3/v3.x runs unchanged**.
Only the build target moves: Arduino board = *Generic F411RE* (or a copied Blackpill
variant) with `HSE_VALUE = 25 MHz` — a build-config change, not a port.

| | F411CEU6 (Blackpill chip) | **F411RET6 (chosen)** | G431CBT6 (rejected for now) |
|---|---|---|---|
| Package | UFQFPN48 (hot-air only) | **LQFP64 — hand-solder/rework OK** | LQFP48 |
| JLC price/stock (2026-08-12) | $3.31 · 1,333 (C2054095) | **$2.47 · 5,342 (C94355)** | $2.96 · 13,744 (C529355) |
| Extra pins | — | **PC0–PC12, PD2** (+ USART6 on PC6/PC7 = real 3rd UART; more ADC) | FDCAN (future CAN), crystal-less USB |
| Firmware | zero change | build-target change only | **full port — wrong timing mid-fleet** |

The 1.1 brief's "F411 has no third USART" was a **48-pin-package** limitation (USART6 lives
on PA11/PA12 there = USB). On LQFP64 it's on PC6/PC7 — reserve them. If vehicle CAN ever
becomes a requirement: MCP2518FD on SPI, or a G4 port as its own project.
Source **genuine ST via LCSC/JLC only** — the ROM bootloader behavior is the product feature.

## 2. ⚠ THE BOOT TRAP — read before touching pins

DFU entry on the F411 requires, at reset: **BOOT0 = 1 AND BOOT1 (PB2) = 0**
(BOOT0=1 + PB2=1 boots SRAM = dead end). v4 has **`ETH_INT` (W5500 INTn) on PB2**, per the
1.1 note "BOOT1: irrelevant with BOOT0=0; do not strap" — which was true only while the MCU
was a removable module. W5500 INTn **idles HIGH** → with the MCU soldered down, DFU would
be unreachable. Therefore in v5:

- **Move `ETH_INT` PB2 → PC0** (EXTI0 is free: PA0/PB0 don't use EXTI).
- **Strap PB2 → 10 k → GND.** Nothing else on PB2, ever.
- ROM bootloader also listens on **USART1 (PA9/PA10)** = the Modo-B pins. Flash with the
  RTK stack unpowered (current practice) so F9P TX2 traffic can't capture the bootloader;
  if field-reflash-with-stack-on is ever needed, add a series jumper on the PA10 tap then.

DFU bootloader clocking: the F411 has **no HSI48** — the ROM bootloader derives the 48 MHz
USB clock from the **external HSE crystal (4–26 MHz, auto-detected)**. Our 25 MHz HSE is
therefore **mandatory, not optional** (AN2606; same reason the Blackpill carries one).

## 3. NEW Block P — on-board MCU (Blackpill absorbed)

**U14 STM32F411RET6** (LQFP64, JLC C94355). Support circuitry, Blackpill-parity where noted:

- **HSE:** 25.000 MHz crystal (3225, CL 10–12 pF) + 2× load caps `C = 2(CL − C_stray)`
  (≈15 pF for CL=10 pF; finalize per chosen crystal). Short symmetric traces, GND guard.
- **LSE:** 32.768 kHz (3215/2012, CL 12.5 pF, e.g. Q13FC-class) + caps per CL. Fit it
  (Blackpill parity; RTC available). Firmware note: F411 LSE wants low drive level.
- **VCAP1:** 4.7 µF X5R ≥6.3 V at the pin (DS10314 — confirm value/pin in Phase A).
- **Decoupling:** 100 nF at **every** VDD pin + 4.7 µF bulk; **VDDA:** ferrite bead (or
  10 Ω) from 3V3 + 1 µF + 100 nF (BAT_SENSE/VEH5V/NTC ADC quality — free upgrade vs the
  module's direct tie). **VBAT → 3V3** + 100 nF.
- **BOOT0:** dedicated pin → **10 k pull-down** + tact switch to 3V3 (+ optional 2-pin jig
  pad). **PB2/BOOT1: 10 k pull-down (§2).**
- **NRST:** joins the existing `NRST` net (SW1 on-board + P10 external — already designed)
  + 100 nF at the pin.
- **LEDs:** `LED_C13` **active-LOW** (3V3 → 1 k → LED → PC13, Blackpill polarity so v2.3+
  blink code is unchanged) + PWR LED on 3V3.
- **USB-C (J11):** 16-pin USB 2.0 receptacle (TYPE-C-31-M-12 class) on a case-reachable
  edge. CC1/CC2 → 5.1 k to GND (UFP). D+ → PA12, D− → PA11 via **USBLC6-2SC6** ESD array.
  Shield → GND via RC (1 MΩ ∥ 4.7 nF) or direct — decide Phase A.
- **SWD:** 4 pads/test-points (3V3, PA13 SWDIO, PA14 SWCLK, GND) — DNP header. With a
  soldered MCU this is the brick-recovery + factory path; the 1.1 brief already wanted it.
- **PA0 note:** no KEY button (Blackpill's KEY is PA0 = our `BAT_SENSE`). Not a loss.

## 4. Power — who makes 3V3 now

The Blackpill's on-module LDO dies with the module, so:

- **U15 LDO 3V3:** AMS1117-3.3 (SOT-223, JLC Basic C6186) — or equal ≥500 mA. Budget:
  MCU ~50 mA + SHT40 + TMP102 + PCF8574 + SC16IS752 + MAX3232 + SP3485 TX bursts ≈
  150–250 mA worst → 1 A part with margin, ~0.3 W dissipation OK in SOT-223.
- **Input OR:** `LDO_IN = MAIN5V (via D_or1 SS34) ∥ USB VBUS (via D_or2 SS34)`.
  **Bench flashing on USB alone powers the 3V3 domain only** (MCU, sensors, expander,
  serial bridge): W5500 (3V3_ETH from SYS5V) stays dark, K1 stays NC (corrections path
  untouched), no backfeed into the LTC4040/MAIN5V. This replaces the module's
  VBUS-to-5V tie with something that can't fight the backed-up rail.
- **Phase-A sweep:** every net the module's 3V3 pin used to source (I²C pull-ups, sensor
  VDD, PCF8574, SC16IS752, IOREF row to MOD1, D5 clamp reference, NTC divider top…) moves
  to the new LDO output net — keep the net name `3V3`. Verify with a netlist diff.
- 3V3_MODEM and 3V3_ETH bucks: **unchanged**.

## 5. Fan — 2-pin, done

- Fan connector → **2-pin** (`5V_FAN`, `FAN_RET`). Remove the 1×04 header, `FAN_TACH` /
  `FAN_TACH_RAW` conditioning (pull-up/divider). Driver chain (PA6 `FAN_PWM` → Q → low-side)
  and the U2 fan buck stay exactly as-is.
- **PB10 freed** (tach DNP provision retired — fleet fans are 2-wire, decided 2026-06-24).
- Firmware v3.0 already plans tach-less fan-fault inference (temp climbs while PWM high).

## 6. NEW Block Q — fixation (mechanical)

- **Shield bolts:** 4× holes in EinarBoard at **exactly MOD1's UNO-pattern hole coords**
  (read the NPTH positions out of the `Arduino_UNO_R3` footprint in the v5 PCB — do not
  re-derive from a drawing). Drill **Ø2.75 mm (M2.5)**; bolts pass shield → standoffs →
  EinarBoard (→ optionally the mounting plate). **Bench check before freezing drill:**
  caliper the real simpleRTK2B Pro holes — UNO-pattern boards are usually Ø3.2 (M3); if so
  either keep M2.5 hardware (washers, slop is fine) or open ours to Ø3.2 and use M3. Keepout
  annulus top/bottom, NPTH, no copper — and respect the HV24/inner-layer clearance rule.
- **XBee heatsink:** **2× M2.5 holes near J7/J8** so a small heatsink/bracket bolts down and
  indirectly clamps the modem module. Position in Phase B with the case; keep clear of the
  RF side of the socket and the 24 V copper.
- Case/plate: bolt pass-through and heatsink height ride the ASA reprint — coordinate
  connector-edge plan (USB-C now needs an opening too) **before routing** (Phase B gate,
  same as 1.1).

## 7. Pin map delta (v4 → v5)

| Pin | v4 (EinarBoard 2.0) | v5 (EinarBoard 3.0) |
|---|---|---|
| PB2 | `ETH_INT` (W5500 INTn) | **BOOT1 strap: 10 k → GND. Nothing else.** |
| PC0 *(new pin)* | — (module had no PC0) | **`ETH_INT`** |
| PB10 | tach DNP provision | **free** (fan is 2-pin) |
| PA13/PA14 | not broken out (module USB only) | **SWD pads** (DNP header) |
| PC6/PC7 *(new)* | — | **reserved: USART6** (future 3rd UART) |
| PC1–PC5, PC8–PC12, PD2 *(new)* | — | free — do not spend without updating this table |
| PA11/PA12 | module USB | **J11 USB-C** D−/D+ |
| everything else | per [[watchdog-electrical-design-state]] §2 + 1.1 blocks M/N/O | **unchanged** |

## 8. Depopulation / cleanup (3.0)

REMOVE: J1/J2 2×20 sockets + Blackpill-L/R symbols and footprint (→ **errata E-J1 retired
by construction** — no module, no mirroring, no "never power a seated module" hazard);
1×04 fan header + tach parts; stale Blackpill silkscreen (module outline, header labels).
KEEP: SW1 + P10 reset, P11 (jumper plug), P12 NTC, J7/J8 XBee, MOD1 shield sockets, J7 USB
modem-config path. SILKSCREEN: USB-C "DFU: hold BOOT0" hint, SWD pad labels, hole callouts
(M2.5), fan "2-pin 5V".

## 9. Risks / open items

- **PB2 strap is load-bearing** (§2) — make it a Phase-D ERC-notes item like the SC16IS
  address strap was in 1.1.
- LQFP64 symbol/footprint: use KiCad lib `MCU_ST_STM32F4:STM32F411RETx` +
  `Package_QFP:LQFP-64_10x10mm_P0.5mm`; save copies into the project libs (same policy as
  the RevNum_EJ1 footprint).
- Blackpill variant nuances: confirm `Generic F411RE` Arduino variant boots USB CDC as
  today's build does (bench item F-1 on first article; SWD pads are the net if it doesn't).
- Shield hole diameter verify (§6). · LSE drive/caps verify on first article.
- DFU with stack powered = USART1 capture risk (§2) — procedure note, not hardware.
- v4 boards at fab: 3.0 starts now as design work; nothing in 3.0 blocks v4 bring-up, and
  v4 bench findings (modem rail, LTC4040 timer behavior) feed Phase D review.

## 10. Execution phases (each ends: git commit + parse-verify + snapshot)

- **A. Schematic** — copy v4 → `hbox-watchdog-v5` (done 2026-08-12 with this brief);
  Block P + LDO OR (§3/§4); ETH_INT move + PB2 strap (§2); 3V3 net sweep (§4); fan 2-pin
  (§5); depop §8; ERC = 0.
- **B. Outline & placement** — reclaim the Blackpill socket area; USB-C edge + case plan;
  Block-Q holes (shield coords from MOD1 footprint; XBee heatsink pair); **Gate: Leo
  approves connector-edge + hole plan before routing.**
- **C. Routing** — MCU fanout (crystal + USB first), re-route displaced nets, zones refill;
  HV24 rules stand.
- **D. Verify** — ERC/DRC clean; strap/boot checklist (BOOT0 pull-down, PB2 low, HSE
  present = DFU truth table); netlist diff v4→v5 reviewed; SVG/PDF review pack for Leo.
- **E. Outputs** — gerbers/BOM/CPL, LCSC pass (C94355 + new passives/USB-C/LDO), delta-doc
  §-update, PINOUT.md rewrite (RET6 + freed pins), errata page (E-J1 retired), firmware
  ticket: build target Generic F411RE + HSE flag + PC0 EXTI for ETH_INT.

*Fab gate unchanged: v5 doesn't go out before a v4-lineage bench validates the power path,
and first-article v5 must prove **USB DFU enumeration + a full flash cycle** before any
fleet quantity.*