What's left for you to explore

We have barely scratched the surface! There's lots of stuff left for you to explore.

NOTE: If you're reading this, and you'd like to help add examples or exercises to the Discovery book for any of the items below, or any other relevant embedded topics, we'd love to have your help!

Please open an issue if you would like to help, but need assistance or mentoring for how to contribute this to the book, or open a Pull Request adding the information!

Topics about embedded software

These topics discuss strategies for writing embedded software. Although many problems can be solved in different ways, these sections talk about some strategies, and when they make sense (or don't make sense) to use.

Multitasking

Most of our programs executed a single task. How could we achieve multitasking in a system with no OS, and thus no threads? There are two main approaches to multitasking: preemptive multitasking and cooperative multitasking.

In preemptive multitasking a task that's currently being executed can, at any point in time, be preempted (interrupted) by another task. On preemption, the first task will be suspended and the processor will instead execute the second task. At some point the first task will be resumed. Microcontrollers provide hardware support for preemption in the form of interrupts. We were introduced to interrupts when we built our snake game in chapter 14.

In cooperative multitasking a task that's being executed will run until it reaches a suspension point. When the processor reaches that suspension point it will stop executing the current task and instead go and execute a different task. At some point the first task will be resumed. The main difference between these two approaches to multitasking is that in cooperative multitasking yields execution control at known suspension points instead of being forcefully preempted at any point of its execution.

Sleeping

All our programs have been continuously polling peripherals to see if there's anything that needs to be done. However, sometimes there's nothing to be done! At those times, the microcontroller should "sleep".

When the processor sleeps, it stops executing instructions and this saves power. It's almost always a good idea to save power so your microcontroller should be sleeping as much as possible. But, how does it know when it has to wake up to perform some action? Interrupts are one of the events that wake up the microcontroller but there are others. The ARM machine instructions wfi and wfe are the instructions that make the processor "sleep" waiting for an interrupt or event.

Microcontrollers (like our nRF52/nRF51) have many capabilities. However, many share similar capabilities that can be used to solve all sorts of different problems.

These topics discuss some of those capabilities, and how they can be used effectively in embedded development.

Direct Memory Access (DMA).

Some peripherals have DMA, a kind of asynchronous memcpy that allows the peripheral to move data into or out of memory without the CPU being involved.

If you are working with a micro:bit v2 you have actually already used DMA: the HAL does this for you with the UARTE and TWIM peripherals. A DMA peripheral can be used to perform bulk transfers of data: either from RAM to RAM, from a peripheral like a UARTE, to RAM, or from RAM to a peripheral. You can schedule a DMA transfer — for example "read 256 bytes from UARTE into this buffer" — and leave it running in the background. You can check some register later to see if the transfer has completed, or you can ask to receive an interrupt when the transfer completes. Thus, you can schedule the DMA transfer and do other work while the transfer is ongoing.

The details of low-level DMA can be a bit tricky. We hope to add a chapter covering this topic in the near future.

Interrupts

We saw button interrupts briefly in chapter 14. This introduced the key idea: in order to interact with the real world, it is often necessary for the microcontroller to respond immediately when some kind of event occurs.

Microcontrollers have the ability to be interrupted, meaning when a certain event occurs, it will stop whatever it is doing at the moment, to instead respond to that event. This can be very useful when we want to stop a motor when a button is pressed, or measure a sensor when a timer finishes counting down.

Although these interrupts can be very useful, they can also be a bit difficult to work with properly. We want to make sure that we respond to events quickly, but also allow other work to continue as well.

In Rust, we model interrupts similar to the concept of threading on desktop Rust programs. This means we also must think about the Rust concepts of Send and Sync when sharing data between our main application, and code that executes as part of handling an interrupt event.

Pulse Width Modulation (PWM)

In a nutshell, PWM is turning on something and then turning it off periodically while keeping some proportion ("duty cycle") between the "on time" and the "off time". When used on a LED with a sufficiently high frequency, this can be used to dim the LED. A low duty cycle, say 10% on time and 90% off time, will make the LED very dim wheres a high duty cycle, say 90% on time and 10% off time, will make the LED much brighter (almost as if it were fully powered).

In general, PWM can be used to control how much power is given to some electric device. With proper (power) electronics between a microcontroller and an electrical motor, PWM can be used to control how much power is given to the motor thus it can be used to control its torque and speed. Then you can add an angular position sensor and you got yourself a closed loop controller that can control the position of the motor at different loads.

There are some abstraction for working with PWM in the embedded-hal pwm module and you will find implementations of these traits in nrf52833-hal.

Digital inputs and outputs

We have used the microcontroller pins as digital outputs, to drive LEDs. When building our snake game, we also caught a glimpse of how these pins can be configured as digital inputs. As digital inputs, these pins can read the binary state of switches (on/off) or buttons (pressed/not pressed).

Digital inputs and outputs are abstracted within the embedded-hal digital module and [nrf52833-hal] does have an implementation for them.

(spoilers reading the binary state of switches / buttons is not as straightforward as it sounds ;-) )

Analog-to-Digital Converters (ADC)

There are a lot of digital sensors out there. You can use a protocol like I2C and SPI to read them. But analog sensors also exist! These sensors just output a reading to the CPU of the voltage they are sensing at an ADC input pin.

The ADC peripheral can thus be used to measure an "analog" voltage level — for example, 1.25 Volts — as a "digital" number — for example, 24824 — that the processor can use in its calculations.

There were generic ADC traits in embedded-hal, but they were removed for embedded-hal 1.0: see issue #377. The nrf52833-hal crate provides a nice interface to the specific ADC built into the nRF52833.

Digital-to-Analog Converters (DAC)

As you might expect a DAC is exactly the opposite of ADC. You can write some digital number into a register to produce a specific voltage on some analog output pin. When this analog output pin is connected to some appropriate electronics and the register is written to quickly with the right values you can do things like produce sounds or music.

Neither the nRF52833 nor the MB2 board has a dedicated DAC. One typically gets a kind of DAC effect by outputting PWM and using a bit of electronics on the output (RC filter) to "smooth" out the PWM waveform.

Real Time Clock

A Real-Time Clock peripheral keeps track of time under its own power, usually in "human format": seconds, minutes, hours, days, months and years. Some Real-Time Clocks even handle leap years and Daylight Saving Time automatically.

Neither the nRF52833 nor the MB2 board contains a Real-Time Clock. The nRF52833 does contain "Real-Time Counter" (RTC), a low-frequency ticking clock that is supported by nrf52833-hal. This counter can be dedicated to serve as a synthesized real-time clock. The key requirement, of course, is to keep the RTC peripheral powered even when the MB2 is not in use. While the MB2 does not have an on-board battery, the RTC should be able to run for a long time (possibly years) with a battery plugged into the battery port on the MB2 (for example, the battery pack provided with the micro::bit Go kit).

Other communication protocols

  • I2C: discussed in earlier chapters of this book
  • SPI: abstracted within the embedded-hal spi module and implemented by the [nrf52-hal]
  • I2S: currently not abstracted within the embedded-hal but implemented by the [nrf52-hal]
  • Ethernet: there does exist a small TCP/IP stack named smoltcp which is implemented for some chips. The MB2 does not have an Ethernet peripheral
  • USB: there is some experimental work on this, for example with the usb-device crate
  • Bluetooth: the nrf-softdevice wrapper provided by the Embassy MB2 runtime is probably the easiest entry into MB2 Bluetooth right now
  • CAN, SMBUS, IrDA, etc: All kinds of specialty interfaces exist in the world; Rust sometimes has support for them. Please investigate the current situation for the interface you need

Different applications use different communication protocols. User facing applications usually have a USB connector because USB is a ubiquitous protocol in PCs and smartphones. Whereas inside cars you'll find plenty of CAN buses. Some digital sensors use SPI, I2C or SMBUS.

If you happen to be interested in developing abstractions in the embedded-hal or implementations of peripherals in general, don't be shy to open an issue in the HAL repositories. Alternatively you could also join the Rust Embedded matrix channel and get into contact with most of the people who built the stuff from above.

General Embedded-Relevant Topics

These topics cover items that are not specific to our device, or the hardware on it. Instead, they discuss useful techniques that could be used on embedded systems. Most of what we will discuss here is not available on the MB2 — but most of it could easily be added by connecting a cheap piece of hardware to the MB2 edge-card connector, either driving it directly or using something like SPI or I2C to control it.

Gyroscopes

As part of our Punch-o-meter exercise, we used the Accelerometer to measure changes in acceleration in three dimensions. But there are other motion sensors such as gyroscopes, which allows us to measure changes in "spin" in three dimensions.

This can be very useful when trying to build certain systems, such as a robot that wants to avoid tipping over. Additionally, the data from a sensor like a gyroscope can also be combined with data from accelerometer using a technique called Sensor Fusion (see below for more information).

Servo and Stepper Motors

While some motors are used primarily just to spin in one direction or the other, for example driving a remote control car forwards or backwards, it is sometimes useful to measure more precisely how a motor rotates.

A microcontroller can be used to drive Servo or Stepper motors, which allow for more precise control of how many turns are being made by the motor, or can even position the motor in one specific place, for example if we wanted to move the arms of a clock to a particular direction.

Sensor fusion

The micro:bit contains two motion sensors: an accelerometer and a magnetometer. On their own these measure (proper) acceleration and (the Earth's) magnetic field. But these magnitudes can be "fused" into something more useful: a "robust" measurement of the orientation of the board, with less measurement error than that of any single sensor.

This idea of deriving more reliable data from different sources is known as sensor fusion.

So where to next?

First and foremost, join us on the Rust Embedded matrix channel. Lots of people who contribute or work on embedded software hang out there, including, for example, the people who wrote the microbit BSP, the nrf52-hal crate, the embedded-hal crates, etc. We are happy to help you get started or move on with embedded programming in Rust!

There are many other options:

  • You could check out the examples in the microbit-v2 board support crate. All those examples work for the micro:bit board you have.
  • If you are looking for a general overview of what is available in Rust Embedded right now check out the Awesome Rust Embedded list.
  • You could check out Embassy. This is a modern efficient preemptive multitasking framework that supports concurrent execution using Rust async/await.
  • You could check out Real-Time Interrupt-driven Concurrency RTIC. RTIC is a very efficient preemptive multitasking framework that supports task prioritization and deadlock-free execution.
  • You could check out more abstractions of the embedded-hal project and maybe even try and write your own platform agnostic driver based on it.
  • You could try running Rust on a different development board. The easiest way to get started is to use the cortex-m-quickstart Cargo project template.