HAL and Micro Library

The micro/* packages are Flint’s cross-chip peripheral APIs. They provide a stable, typed interface to hardware without exposing registers, memory-mapped IO addresses, or vendor-specific SDK calls.

The Four Layers

Flint’s hardware support is organized into four layers:

┌─────────────────────────────────────────────┐
│              Application Code               │
├─────────────────────────────────────────────┤
│         micro/*   board/*                   │  ← What you use
├─────────────────────────────────────────────┤
│              hal/*                          │  ← Target-selected implementation
└─────────────────────────────────────────────┘

You import micro/* and board/*. You do not import hal/* directly.

Startup

When your main runs, the target hal/* package has already:

• Released resets
• Brought up the clock tree and PLLs
• Configured the watchdog
• Initialized the timer/timebase
• Configured XIP/flash for normal operation

You do not write startup code. You do not call SystemInit(). You write application code.

Builder-Style APIs

Peripherals with many options use builder-style initialization:

use micro/i2c

let mut bus = i2c.bus(0)
    .sda(gpio.pin(4))
    .scl(gpio.pin(5))
    .freq(400_000)
    .init()?

Builder methods shape configuration. The terminal init() (or open(), attach()) is the one fallible step. Once initialized, steady-state operations are infallible:

bus.write(0x68, data)?    // fallible: I2C can NACK
bus.flush()?              // fallible: flush can timeout

led.high()                // infallible: GPIO state change always works
led.low()                 // infallible

Capability Discovery

Optional features are exposed through Option<T>. Use standard control flow to adapt:

use board/pico
use micro/dma

if pico.capabilities().dma_channels > 0 {
    if let Some(mut ch) = dma.channel(0) {
        ch.start_transfer()?
    }
}

Because the selected target and board are known at compile time, constant-folding removes dead branches for unsupported capabilities.

What Is Available

Required:

• GPIO: digital I/O
• UART: serial communication
• ADC: analog-to-digital conversion
• PWM: pulse-width modulation
• I2C and SPI: bus protocols
• DMA: direct memory access
• PIO: RP2040 programmable IO state machines
• micro/cpu: CPU hints (WFI, breakpoint, spin-loop)
• micro/multicore: secondary core launch (RP2040)
• USB device (CDC, HID, MSC)
• XIP/SPI flash access
• SD card over SPI
• FAT filesystem
• Interrupt registration