RIOT/boards/rpi-pico/doc.txt

/**
@defgroup    boards_rpi_pico Raspberry Pi Pico
@ingroup     boards
@brief       Support for the RP2040 based Raspberry Pi Pico board

## Overview

The Raspberry Pi Pico is sold by the Raspberry Pi foundation for about 4 USD. It features the
RP2040 MCU, a custom dual core ARM Cortex-M0+ MCU with relatively high CPU clock, plenty of RAM and
some unique peripheral (the Programmable IO).

## Hardware

![Raspberry Pi Pico](https://www.raspberrypi.org/homepage-9df4b/static/rp2040@2x-d1b9dae9345ad2bd15a23c6a567edb5c.jpg)

### MCU

The Programmable IO (PIO) peripheral and the SSI/QSPI peripheral that supports execution from
flash (XIP) are the most distinguishing features of the MCU. The latter is especially important,
since the RP2040 contains no internal flash.

| MCU        | RP2040                                                       |
|:-----------|:------------------------------------------------------------ |
| Family     | (2x) ARM Cortex-M0+                                          |
| Vendor     | Raspberry Pi                                                 |
| RAM        | 264 KiB                                                      |
| Flash      | 2 MiB (up to 16 MiB)                                         |
| Frequency  | up to 133 MHz                                                |
| FPU        | no                                                           |
| PIOs       | 8                                                            |
| Timers     | 1 x 64-bit                                                   |
| ADCs       | 1x 12-bit (4 channels + temperature sensor)                  |
| UARTs      | 2                                                            |
| SPIs       | 2                                                            |
| I2Cs       | 2                                                            |
| RTCs       | 1                                                            |
| USBs       | 1 (USB 2.0)                                                  |
| Watchdog   | 1                                                            |
| SSI/QSPI   | 1 (connected to flash, with XIP support)                     |
| Vcc        | 1.62V - 3.63V                                                |
| Datasheet  | [Datasheet](https://www.raspberrypi.org/homepage-9df4b/static/rp2040@2x-d1b9dae9345ad2bd15a23c6a567edb5c.jpg) |

### User Interface

1 button (also used for boot selection) and 1 LED:

| Device | PIN              |
|:------ |:---------------- |
| LED0   | 25               |
| SW0    | QSPI_SS_N (*)    |

(*) Since the switch is connected to the chip-select pin of the QSPI interface the flash chip RIOT
is running from via XIP, the switch is difficult to read out from software. This is currently not
supported.

### Pinout

![Pinout Diagram of RPi Pico](https://projects-static.raspberrypi.org/projects/getting-started-with-the-pico/f009ad94826c2f0cd7573a295897e76955301096/en/images/Pico-R3-Pinout.png)

## Flashing the Board

### Flashing the Board Using OpenOCD

Currently (June 2021), only two methods for debugging via OpenOCD are supported:

1. Using a bit-banging low-level adapter, e.g. via the GPIOs of a Raspberry Pi 4B
2. Using a virtual CMSIS-DAP adapter provided by the second CPU core via
   https://github.com/majbthrd/pico-debug

Since option 2 requires no additional hardware, this is currently the default. However, you need to
first "flash" the gimme-cache variant of [pico-debug](https://github.com/majbthrd/pico-debug)
into RAM using the UF2 bootloader. For this, plug in the USB cable while holding down the BOOTSEL
button of the Pico and copy the `pico-debug-gimmecache.uf2` from the
[latest pico-debug release](https://github.com/majbthrd/pico-debug/releases) into the virtual FAT
formatted drive the bootloader provides. Once this drive is unmounted again, this will result in
the Raspberry Pi Pico showing up as CMSIS-DAP debugger. Afterwards run:

```
make BOARD=rpi-pico flash
```

@warning    The `rpi-pico` virtual debugger is not persistent and needs to be "flashed" into RAM
            again after each cold boot.

@note       As of July 2021, the latest stable release of OpenOCD does not yet support the RP2040
            MCU. Instead, compile the current `master` branch from the upstream OpenOCD source.
            The OpenOCD fork of the Raspberry Pi foundation is incompatible with OpenOCD
            configuration provided, so please stick with upstream OpenOCD.

### Flashing the Board Using the Bootloader

Connect the device to your Micro-USB cable while the button (labeled `BOOTSEL` on the silkscreen
of the PCB) is pressed to enter the bootloader. Afterwards run:

```
make BOARD=rpi-pico PROGRAMMER=uf2conv flash
```

### Flashing the Board Using J-Link

Connect the Board to an Segger J-Link debugger, e.g. the EDU mini debugger is relatively affordable,
but limited to educational purposes. Afterwards run:

```
make BOARD=rpi-pico PROGRAMMER=jlink flash
```

## On-Chip Debugging

There are currently (June 2021) few hardware options for debugging the Raspberry Pi Pico:

1. Via J-Link using one of Seggers debuggers
2. Via OpenOCD using a low-level bit-banging debugger (e.g. a Raspberry Pi 4B with the GPIOs
   connected to the Raspberry Pi Pico via jump wires)
3. Via a recently updated [Black Magic Probe](https://github.com/blacksphere/blackmagic)

In addition, a software-only option is possible using
[pico-debug](https://github.com/majbthrd/pico-debug). The default linker script reserved 16 KiB of
RAM for this debugger, hence just "flash" the "gimme-cache" flavor into RAM using the UF2
bootloader. Once this is done, debugging is as simple as running:

```
make BOARD=rpi-pico debug
```

***Beware:*** The `rpi-pico` virtual debugger is not persistent and needs to be "flashed" into RAM
again after each cold boot. The initialization code of RIOT now seems to play well with the
debugger, so it remains persistent on soft reboots. If you face issues with losing connection to
the debugger on reboot, try `monitor reset init` in GDB to soft-reboot instead.

## Known Issues / Problems

### Early state Implementation

Currently no support for the following peripherals is implemented:

- Timers
- ADC
- SPI
- I2C
- USB
- PIO
- RTC
- Watchdog
- SMP support (multi CPU support is not implemented in RIOT)

 */