Minimal STM

STM32F411 Minimum System Board

A custom-designed breakout/minimum-system board built around the STM32F411 microcontroller, designed from datasheet-driven first principles rather than copied reference schematics. This project exists to build genuine hardware design intuition — every component value and circuit decision below was derived by reading the relevant datasheet, not by mimicking an existing board.

This board also serves as a stepping stone toward a larger drone flight controller [blocked] project, sharing the same MCU family and several of the same design fundamentals (power regulation, motor driving, reset/boot circuitry).

Status

🚧 In progress — schematic capture / PCB layout stage. Not yet fabricated. See Success Criteria for what "done" means for this iteration.

Overview

Why This Board Exists

Prior PCB work (an ESP32 minimal board) was largely built by following a tutorial's schematic without fully understanding why each component and value was chosen. This project deliberately reverses that: every design decision below is backed by a specific datasheet reference, and wrong first attempts are documented rather than hidden, since the mistakes were as instructive as the final design.

Design Process

This board followed a structured, professional-style workflow rather than jumping straight into schematic capture:

1. Requirements Definition — functional requirements, power budget, and constraints written before opening any tool.
2. Block Diagram — system partitioned into functional subsystems (power, MCU, motor driver, communication, GPIO, boot/reset, clock) before component-level detail.
3. Component Selection — actual part numbers chosen and verified against datasheets.
4. Schematic Capture — drawn in KiCad using values derived from datasheets, not copied designs.
5. Simulation & ERC — power stage checked in SPICE where applicable; full schematic run through Electrical Rule Check.
6. Design Review — each block checked against the original requirements.
7. PCB Layout & DRC — physical placement and routing, checked against manufacturing rules.
8. (Fabrication and physical testing are out of scope for this iteration — see Success Criteria.)

This same process is intended to be reused for future hardware projects, including the drone flight controller.

Requirements

• Power delivery via a single-cell battery, stepped down to a regulated 3.3V rail
• Total current budget: ~150mA (STM32F411 draws up to ~100mA, plus margin) — separate and isolated from motor current, which is supplied directly from the battery through the motor driver IC, not through the logic supply
• Programming via SWD (SWDIO, SWCLK, Power, Ground exposed on a header)
• General-purpose I/O exposed: multiple I2C and UART pins, PWM-capable pins, analog/digital GPIO, ADC pins — for driving LEDs and low-current peripherals only
• Motor driver IC (DRV8833) with motor outputs exposed separately from the GPIO header — motors never connect directly to logic-level pins
• Reset and BOOT0 circuitry, each with a tactile switch
• ESP32-style board outline

Success criteria:

1. Full schematic passes Electrical Rule Check (ERC) — no unconnected pins, no duplicate reference designators, no power conflicts.
2. Power regulation subcircuit simulates correctly in SPICE — stable 3.3V output under a simulated 150mA load.
3. Every pin function (SWD, I2C, UART, PWM, ADC) is confirmed against a valid STM32F411 alternate-function mapping per the datasheet.
4. PCB layout passes Design Rule Check (DRC) — manufacturable trace widths, clearances, and via sizes.

(This board is not being fabricated in this iteration — "done" means a clean, DRC/ERC-passing, well-reasoned design, not a physically tested unit.)

Component Selection & Reasoning

A note on motor wiring (a mistake worth documenting)

An early draft of this design routed motors through the board's general-purpose 3.3V/5V header pins. This is wrong: GPIO pins max out around 20–25mA, while motors draw hundreds of mA to amps — wiring a motor directly to a logic pin would damage the MCU. The corrected design routes motors only through the DRV8833's dedicated output terminals, which draw current from the battery's VM line, completely separate from the MCU's logic-level supply and current budget.

Schematic Organization

This board uses a flat, single-sheet schematic rather than hierarchical sheets. Given the relatively small number of functional blocks (six), a flat sheet keeps every connection visible without needing to flip between pages — valuable while still building intuition for how these blocks interact. (Hierarchical sheets are used on the drone flight controller project instead, where repeated blocks — four identical ESC circuits — and a larger overall block count make that structure worthwhile.)

Text annotations are used liberally on the schematic (e.g., component value justifications referencing specific datasheet sections) rather than embedding component images — images belong in this documentation, not the schematic file itself.

Decisions Log

Tools

• KiCad — schematic capture and PCB layout
• draw.io — block diagram
• ngspice (via KiCad) — power stage simulation

Related Projects

• Drone flight controller (STM32-based, hierarchical schematic, four-ESC output) — builds directly on the fundamentals established here.

License

(Add a license — e.g., CERN-OHL or MIT for firmware, if/when this repository is made public.)