This time round, I’ll wrap up my coverage of the key ARMv6-M Thumb instructions and mnemonics that you can use to command the Raspberry Pi RP2040. There are not many instructions left that were not covered in parts one and two, and I won’t be including all the remaining mnemonics, only those you’re likely to use frequently.
I was travelling when the Raspberry Pi Foundation launched the Pico W, so I had to wait to get back before I could get my hands on one. I have one now, and to try it out, I decided to port my network-oriented PicoWeather app, this time creating a MicroPython version — it was released for CircuitPython.
A number of the Cortex-M0+ Thumb ops I covered last time update the core’s Program Status Register (PSR) based on the outcome of the operation. The ops that do so have an S appended to their mnemonics and they only work with the core’s ‘low’ registers, R0-7.
Last time, I covered the basics of doing ARM assembly programming on the Raspberry Pi Pico’s RP2040 microcontroller. Now it’s time to get to grips with the dozens of instructions to which the RP2040’s Cortex-M0+ cores respond.
When I got my first microcomputer, I already knew Basic programming. My machine had a different Basic dialect from the one I’d learned at school, and there was a stack of graphics and sound functionality to get to grips with too, but it wasn’t long before I felt I’d mastered the high-level stuff and that it was time to move on to machine code. That’s how I’ve come to feel about the Raspberry Pi Pico’s RP2040 chip. The time’s right to learn ARM assembly programming on the Pico.
I made use of FreeRTOS’ timer functionality in the most recent post in this series, but I didn’t go into detail because the post was focused on other features. It’s time to address that deficiency. Today I’m talking about timers.
Last year, at the recommendation of a work colleague, I grabbed one of my spare Raspberry Pi 4s and installed the DNS proxy and content blocker Pi-Hole. It’s now handling all the DNS queries on my home network. Recently, I upgraded my Pi-Hole server to make its DNS requests over HTTPS.
One of the reasons why an embedded application developer might choose to build their code on top of a real-time operating system like FreeRTOS is to emphasise the event-driven nature of the application. For “events” read data coming in on a serial link or from an I²C peripheral, or a signal to a GPIO from a sensor that a certain threshold has been exceeded. These events are typically announced by interrupting whatever job the host microcontroller is engaged upon, so interrupts are what I’ve chosen to examine next in my exploration of FreeRTOS on the Raspberry Pi RP2040 chip.
FreeRTOS scheduling is hard in as much at can be difficult to decide how to configure it. I wanted to try and figure out the options.
The popular real-time operating system provides the configUSE_TIME_SLICING and configUSE_PREEMPTION as settings values. You can add them to your FreeRTOSConfig.h file Tasks themselves can be assigned priority values, and there are API calls to allows tasks to sleep, to yield up the CPU, and be suspended and subsequently resumed.
While documenting Twilio’s in-development Microvisor IoT platform, I’ve been working with FreeRTOS, the Amazon-owned open source real-time operating system for embedded systems. Does FreeRTOS work with the Raspberry Pi Pico’s RP2040 chip? I wondered. It turns out that it can, and this is how you set up a very basic FreeRTOS project which also serves as a demo.
smittytone messes with micros 