RPi Pico: First impressions

2021-01-28 micropython / microcontrollers / electronics
My first impressions of the new Raspberry Pi Pico / RP2040, with MicroPython

First Impressions

I received a couple of Raspberry Pi Picos this morning from Little Bird Electronics and this is a quick write up of the out-of-box experience.

Booting and Flashing

As expected from RPi, the web documentation is fantastic. Right there on the site from day one is a Getting Started with MicroPython page, which is great to see. Downloading the UF2 file and copying it to the board (it appears as a mass storage device) takes only a moment, and as soon as it reboots a familar /dev/ttyACM0 is available.

MicroPython v1.13-290-g556ae7914 on 2021-01-21; Raspberry Pi Pico with RP2040
Type "help()" for more information.
>>> 

I haven’t tried this on Windows / Mac just yet but hopefully it will work in much the same way as other MicroPython boards

Unfortunately the mass storage interface is no longer available! This is a pity because this is a great feature especially for beginners. If that’s you, I’d recommend checking out Adafruit’s CircuitPython fork instead until MicroPython gets this going. More on that later.

Adafruit CircuitPython 6.2.0-beta.1 on 2021-01-27; Raspberry Pi Pico with rp2040
>>> 

The board itself it pretty minimal: there’s a single green LED on GPIO25, so at least we can run Embedded Hello World:

import machine
import time

led = machine.Pin(25, machine.Pin.OUT)
while True:
    led.toggle()
    time.sleep(1)

Pinouts

RPi Pico Pinout From raspberrypi.org

What might not be clear from this diagram is that there’s two each of the UART, SPI and I2C ports, and they can be moved around between different sets of pins, but not to totally arbitrary pins. So you can have a combination of those busses, but not all of them all at once. It seems like an odd way to do things but perhaps it will make board layouts simpler.

See Section 2.19.2 in the RP2040 Datasheet and once again curse the world of PDF documents and their inability to play nicely with hyperlinks.

The good news is that the USB connection is direct via it’s own PHY so unlike the a lot of microcontroller boards you don’t lose a serial port the console.

The Pico is surface mountable to your own boards too, or there’s a various smaller boards available and a Feather compatible board is on the way.

Where does this sit in the market?

With 2 x 144 MHz ARM cores and 264 KB of SRAM, it’s a lot more powerful than an AVR based Arduino, and the boards are really quite cheap.

It’s a pretty direct competitor to the ESP32 and similar chips, but without the WiFi interface. Will it be able to gain the kind of critical mass which has built up around those platforms? Only time will tell.

The big drawcard is the Programmable I/O which is a programmable, state-machine driven I/O peripheral. It’s really four tiny demi-CPUs which can handle simple I/O tasks, and potentially offers a way to offload timing-critical activities like driving NeoPixels.

This idea of having a tiny coprocessor to handle I/O is interesting, there’s a similar thing going on with the ESP32 ULP and it kind of makes sense.

Another option is something like the QuickLogic QORC chip which integrates an Cortex M4 and an FPGA, letting you move I/O wrangling off the CPU and onto gateware.

Only time will tell if these peripherals prove useful enough to win a place in the hearts and minds of embedded programmers!

UPDATE: If encoding two streams of DVI isn’t enough to endear this chip to you, nothing will.

Building MicroPython

MicroPython is right there on the RPi Pico page as a development platform, and it’s likely that a lot of new RPi Pico users will find it a great starting point for getting to know the platform.

Download and Build

The RPi Pico port isn’t merged into the main MicroPython repository yet, instead you can find it here in the ‘pico’ branch.

There’s instructions for building it here, but to summarize:

sudo apt install cmake gcc-arm-none-eabi libnewlib-arm-none-eabi build-essential
git clone https://github.com/raspberrypi/micropython/
cd micropython
git checkout pico
git submodule update --init -- lib/pico-sdk
cd lib/pico-sdk
git submodule update --init
cd ../..
make -C mpy-cross
cd ports/rp2
make

You should now have a build/firmware.uf2 file ready to copy onto your device.

Having another look at WebUSB

One thing which sets these modules apart from the ESP32 etc is the onboard USB. I’ve written a lot about this in the past, the use of serial ports is a serious limitation and a disadvantage to beginners. A technology like WebUSB could be a great help.

The RP2040 includes a USB PHY, or “Physical Layer”, but that’s really just some slightly different IO drivers on two pins, DP and DM. There’s also some integrated support for low level USB operations (see the RP2040 datasheet section 4.1) which should help out with performance compared to using V-USB or LUFA over GPIO pins.

Instead both the MicroPython and CircuitPython ports use TinyUSB. This is great because there’s even already an example in the TinyUSB repository.

Note that the code used in the RP2 build is the version under /lib/pico-sdk/lib/tinyusb/, which is a fork supporting RP2, not the general version under /lib/tinyusb/. This will probably get merged back eventually.

It isn’t working perfectly yet but there’s a demo of connecting to RPi Pico MicroPython via WebUSB here.