machine: add BOOTSEL button support for RP2040 and RP2350

[rdon-key]

Add machine.BootselButton() to read the BOOTSEL button state on RP2040 and RP2350 boards.

Reading the BOOTSEL button requires temporarily overriding the QSPI flash chip-select pin, which makes flash inaccessible during the read. The implementation uses the existing RP2 flash-safe mechanism to prevent the other core from accessing flash during this operation.

The low-level reading function runs from RAM and follows the official Raspberry Pi Pico BOOTSEL example.

API note

This adds a new exported function, machine.BootselButton() bool, to the machine package for RP2040 and RP2350 targets.

Many RP2040 and RP2350 boards have a BOOTSEL button. On boards such as the Raspberry Pi Pico and Pico 2, it is the only user button. However, it cannot be read like a normal GPIO: reading it requires taking over the QSPI flash chip-select pin, running code from RAM, and keeping the other core away from flash. This is hard to do safely from application code, so I think it belongs in the machine package.

I'm happy to change the name or signature if there is a preferred convention.

Testing

Tested on physical Raspberry Pi Pico and Pico 2 boards.

Raspberry Pi Pico

• -scheduler=tasks: PASS
• -scheduler=cores: PASS
• BOOTSEL press and release detection: PASS
• Repeated button presses under concurrent goroutine workload: PASS

Raspberry Pi Pico 2

• -scheduler=tasks: PASS
• -scheduler=cores: PASS
• BOOTSEL press and release detection: PASS
• Repeated button presses under concurrent goroutine workload: PASS

Basic test

package main

import (
    "machine"
    "time"
)

func main() {
    for {
        println("BOOTSEL:", machine.BootselButton())
        time.Sleep(500 * time.Millisecond)
    }
}

Run on RP2040 with:

tinygo flash -target pico -scheduler=tasks -monitor main.go

Run on RP2350 with:

tinygo flash -target pico2 -scheduler=tasks -monitor main.go

Multicore workload test

package main

import (
    "machine"
    "sync/atomic"
    "time"
)

var heartbeat uint32

//go:noinline
func workload(n uint32) uint32 {
    n ^= n << 13
    n ^= n >> 17
    n ^= n << 5
    return n
}

func worker() {
    var n uint32 = 1
    for {
        for i := 0; i < 10000; i++ {
            n = workload(n)
        }
        atomic.AddUint32(&heartbeat, 1)
    }
}

func main() {
    go worker()

    var previous uint32
    for {
        pressed := machine.BootselButton()

        current := atomic.LoadUint32(&heartbeat)
        delta := current - previous
        previous = current

        println("BOOTSEL:", pressed, "heartbeat delta:", delta)
        time.Sleep(500 * time.Millisecond)
    }
}

Run on RP2040 with:

tinygo flash -target pico -scheduler=cores -monitor main_cores.go

Run on RP2350 with:

tinygo flash -target pico2 -scheduler=cores -monitor main_cores.go

Example output on Pico:

BOOTSEL: false heartbeat delta: 545356
BOOTSEL: false heartbeat delta: 545355
BOOTSEL: true heartbeat delta: 545353
BOOTSEL: true heartbeat delta: 545349
BOOTSEL: true heartbeat delta: 545340
BOOTSEL: false heartbeat delta: 545351
BOOTSEL: false heartbeat delta: 545358

Example output on Pico 2:

BOOTSEL: false heartbeat delta: 8644369
BOOTSEL: true heartbeat delta: 8644375
BOOTSEL: true heartbeat delta: 8644348
BOOTSEL: true heartbeat delta: 8644272
BOOTSEL: false heartbeat delta: 8644349

BOOTSEL press and release detection worked while the worker goroutine continued making progress. No hangs or failures were observed.

Note: this test does not verify that the worker goroutine actually runs on the second core.

[rdon-key]

I’m moving this back to draft for now.

After looking into RP2350 more closely, it seems the BOOTSEL handling is much closer to RP2040 than I initially thought, and the relevant errata does not affect the QSPI pads.

I’m going to add RP2350 support and test it on a Pico 2 before marking this ready for review again.

[rdon-key]

RP2350 support has been added and tested on a physical Pico 2 with both scheduler=tasks and scheduler=cores. CI is also passing, so I’ve marked this ready for review again.