Technology Is Not Dull: Raspberry Pi Pico Microcontroller

Sunday, 24 January 2021

Raspberry Pi Pico Microcontroller

Raspberry Pi have just released a new product into a new market for them a low cost microcontroller. And not just a microcontroller using an existing piece of silicon, no, this is a in-house custom designed processor.

Why a microcontroller?

General purpose computers like the Raspberry Pi are great at doing lots of things, but that comes at a price. They can do lots of things that appear to humans to be happening at the same time, but it is like juggling, if you are late getting to one of the items in the air, one or more of the items is going to come crashing down. By dedicating a microcontroller to the task, you can ensure the timely response.

Also, because microcontrollers do not have to contain all the clever hardware to handle lots of memory, task swapping and other things required for general purpose computers they are both economical in cost and energy requirements.

There are plenty of microcontrollers about such as the BBC Microbit and Arduino (and compatible), but most are built using an existing System On A Chip.

Raspberry Pi decided to build a dedicated microcontroller chip based on their experiences with the Sense Hat and the all in one Raspberry Pi 400.

What does Raspberry Pi Pico look like?

As you can see, it is much smaller than even the Raspberry Pi Zero (a WH example above).

Out of the box it has only limited built in semsors and outputs, this is not a Circuit Playground Express.

Basically it has a green LED on GPIO pin 25 and a chip temperature sensor.

What is it for?

The Raspberry Pi Pico can be used where other microcontrollers would be used. It lacks the existing Arduino Shield eco-system, so generally where it is the basic I/O that is required.

It is small, and frugal with the power so it can run independently on two or three AA batteries.

How do you get one?

They are available from the usual suspects, I have one on order from Pimoroni.

In addition, they are on the cover of issue 39 of the print version of the Hackspace magazine.

What can you program it in?

At the moment there is a full C/C++ SDK and ports of Micropython and CircuitPython.

There is currently no Arduino implementation, but there will be an official RP2040 based Arduino device so I suspect that will not be a long wait.

Raspberry Pi RP2040

There is a full description of the RP2040 here, but these are the highlights:

In-house design using dual core ARM Cortex M0+
264KB RAM
Upto 16MB of external Flash memory via a QSPI bus.
30 GPIO pins (four owhich can be configured as analogue input)
Two each of UART, SPI and I2C controllers
16 PWM channels
USB host and device support plus mass-storage boot mode for drag and drop programming
Eight Programmable I/O state machines

The eight PIO state machines are a particular innovation - they are programmable in a simple assembly language to perform tasks at set rates. Each instruction takes one cycle and is independent of the two main cores. This allows you to set up time sensitive operations at known speeds, irrespective of what the main processor is doing. Most microcontrollers would require bit-banging, using the processor to transmit or receive data by changing/reading the state of an input. The PIO allows this to be offloaded to a PIO with full control of the process. All the processor has to do is ensure that it is kept fed or emptied in time.

The chip is called an RP204 based on a naming system:

Cores: 2

CPU type: 0 ~ M0 - this is a loose description of the CPU type.

RAM: 4 = floor(log2(ram / 16k))

Flash: 0 = floor(log2(nonvolatile / 16k)) or zero in this case (the host board provides the Flash storage).