# pico-blinky

Rust firmware for the Raspberry Pi Pico. Flashes the on-board LED with the
digits of pi: three flashes, a pause, one flash, a pause, four, one, five,
then a longer pause before it repeats.

## Board

This targets the **RP2040**, the original Pico. It will not run on a Pico 2.
Check which one you have by holding BOOTSEL while plugging it in and looking
at the volume that mounts:

| Volume label | Chip    | This firmware |
|--------------|---------|---------------|
| `RPI-RP2`    | RP2040  | yes           |
| `RP2350`     | RP2350  | no            |

## Build

```
rustup target add thumbv6m-none-eabi
cargo install elf2uf2-rs --locked
cargo build --release
elf2uf2-rs target/thumbv6m-none-eabi/release/blinky pi-blink.uf2
```

## Flash

Hold the BOOTSEL button down **before** the cable goes in, keep it held for a
second after, then release. The button is only read at power-on, so pressing
it while the board is already running does nothing.

The board mounts as `RPI-RP2`. Copy the `.uf2` onto it. The volume ejects
itself once the write completes, which is the bootloader confirming it took
the image and jumped to it.

Prebuilt images are in `uf2/` if you just want to drag one across.

## Getting back into the bootloader

The firmware exposes a USB serial port, so after the first flash you do not
need the button again. Either:

- send `b` to the serial port, or
- open the port at 1200 baud and drop DTR, the convention `picotool` and the
  Arduino IDE use

Both call `reset_to_usb_boot`, and the board comes back up as `RPI-RP2`.

The LED pattern is driven from the hardware timer rather than blocking sleeps,
so USB stays responsive while the sequence runs.

## Layout

```
src/main.rs           pi digits + USB reboot-to-bootloader
alt/simple-blink.rs   plain 250 ms on/off blink, the first version
memory.x              flash/RAM layout, reserves 0x100 for the boot2 stage
build.rs              puts memory.x on the linker search path
uf2/                  prebuilt images
```

## Timing

Edit the constants at the top of `src/main.rs`:

```rust
const FLASH_ON_US:  u64 =   150_000;
const FLASH_OFF_US: u64 =   200_000;
const DIGIT_GAP_US: u64 =   800_000;
const CYCLE_GAP_US: u64 = 3_000_000;
```

`DIGIT_GAP_US` is the one worth raising if the sequence is hard to count by
eye, since 1 and 4 sit next to each other and can run together.