Writing a Temperature-Sensor App on Tock

In this stage, we write a simple application that will ask the Tock kernel for our chip's current temperature and then print this value to the serial console. By the end of this submodule, you will know how to:

  1. Compile and flash the Tock kernel.
  2. Compile and flash a libtock-c application.
  3. Interact with the tock process console.
  4. Interact with Tock syscalls, callbacks, and Inter-Process Communication.

Compiling and Installing the Kernel

For this tutorial, we provide a Tock kernel configuration that exposes all required peripherals to userspace applications. It is based on the nrf52840dk base board defition and adds an additional driver instantiation for the Ssd1306 1.3" OLED screen we are using in this tutorial.

You can compile this configuration board by entering into its respective directory and typing make:

$ cd tock/boards/tutorials/nrf52840dk-thread-tutorial
$ make
   Compiling nrf52_components v0.1.0 (/home/leons/proj/tock/kernel/boards/nordic/nrf52_components)
   Compiling nrf52840dk v0.1.0 (/home/leons/proj/tock/kernel/boards/nordic/nrf52840dk)
    Finished `release` profile [optimized + debuginfo] target(s) in 11.09s
   text    data     bss     dec     hex filename
 233474      36   41448  274958   4320e tock/target/thumbv7em-none-eabi/release/nrf52840dk-thread-tutorial
cb0df7abb1...d47b383aaf  tock/target/thumbv7em-none-eabi/release/nrf52840dk-thread-tutorial.bin

To flash the kernel onto your nRF52840DK development board, make sure that you use the debug USB port (top-side, not "nRF USB"). Then type

$ make install
tockloader  flash --address 0x00000 --board nrf52dk --jlink tock/kernel/target/thumbv7em-none-eabi/release/nrf52840dk-thread-tutorial.bin
[INFO   ] Using settings from KNOWN_BOARDS["nrf52dk"]
[STATUS ] Flashing binary to board...
[INFO   ] Finished in 9.901 seconds

If these commands fail, ensure that you have all of rustup, tockloader, and the SEGGER J-Link software installed. You can test your connection with the integrated J-Link debug probe by running:

$ JLinkExe
SEGGER J-Link Commander V7.94a (Compiled Dec  6 2023 16:07:30)
DLL version V7.94a, compiled Dec  6 2023 16:07:07

Connecting to J-Link via USB...O.K.
Firmware: J-Link OB-SAM3U128-V2-NordicSemi compiled Oct 30 2023 12:12:17
Hardware version: V1.00
J-Link uptime (since boot): 0d 00h 39m 40s
S/N: 683487279
License(s): RDI, FlashBP, FlashDL, JFlash, GDB
USB speed mode: High speed (480 MBit/s)

Connecting to the Tock Kernel

You can connect to your board's serial console using tockloader or any other serial console application. If your development board presents two console devices, the lower-numbered one is usually correct. Select 115200 baud, 1 stop bit, no partity, no flow control. The following command should also do the trick:

$ tockloader listen

By default, a Tock board without any applications will respond with a message similar to:

Initialization complete. Entering main loop
NRF52 HW INFO: Variant: AAC0, Part: N52840, Package: QI, Ram: K256, Flash: K1024

If you don't see this prompt, try hitting ENTER or pressing the RESET button on your board (near the left-hand side USB port). In case you see the following selection dialog, the nRF52840DK exposes the chip's serial console on the first UART port (e.g., ttyACM0 instead of ttyACM1). If that does not work, simply try the available ports:

$ tockloader listen
[INFO   ] No device name specified. Using default name "tock".
[INFO   ] No serial port with device name "tock" found.
[INFO   ] Found 2 serial ports.
Multiple serial port options found. Which would you like to use?
[0]     /dev/ttyACM1 - J-Link - CDC
[1]     /dev/ttyACM0 - J-Link - CDC

Which option? [0] 1
[INFO   ] Using "/dev/ttyACM0 - J-Link - CDC".
Initialization complete. Entering main loop
NRF52 HW INFO: Variant: AAC0, Part: N52840, Package: QI, Ram: K256, Flash: K1024

The small shell above is called the "process console". It allows you to start and stop applications, and control other parts of the tock kernel. For instance, reset will completely reset your chip, re-printing the above greeting. Use help to obtain a list of commands.

CHECKPOINT: You can interact with the process console.

Compiling and Installing an Application

With the kernel running we can now load applications onto our board. Tock applications are compiled and loaded separately from the kernel. For this tutorial we will use the libtock-c userspace library, whose source is located outside of the kernel repository here.

We provide some scaffolding for this tutorial. Make sure to enter the following directory:

$ cd libtock-c/examples/tutorials/thread_network
$ ls

These applications represent checkpoints for different milestones of this tutorial. If you are ever stuck on something, you can try running or looking at the subsequent checkpoint. We'll start the tutorial off at checkpoint 00_sensor_hello. Whenever we reach a checkpoint, we indicate this through a message like the following:

CHECKPOINT: 00_sensor_hello

To compile and flash this application, we enter into its directory and run the following command:

$ cd 00_sensor_hello
$ make -j install
Application size report for arch family cortex-m:
Application size report for arch family rv32i:
   text    data     bss     dec     hex filename
   3708     204    2716    6628    19e4 build/cortex-m0/cortex-m0.elf
  13944     816   10864   25624    6418 (TOTALS)
   text    data     bss     dec     hex filename
   4248     100    2716    7064    1b98 build/rv32imac/rv32imac.0x20040080.0x80002800.elf
  51432    1000   27160   79592   136e8 (TOTALS)
[INFO   ] Using openocd channel to communicate with the board.
[INFO   ] Using settings from KNOWN_BOARDS["nrf52dk"]
[STATUS ] Installing app on the board...
[INFO   ] Flashing app org.tockos.thread-tutorial.sensor binary to board.
[INFO   ] Finished in 1.737 seconds

Once the binary is flashed to the board, you can connect to its serial port using tockloader listen. Upon reset, the board should now greet you:

$ tockloader listen
Initialization complete. Entering main loop
NRF52 HW INFO: Variant: AAC0, Part: N52840, Package: QI, Ram: K256, Flash: K1024
Hello World!

Congratulations, you have successfully installed and run your first Tock application! You can manage apps installed on a board with Tockloader. For instance, use the following commands to install, list, and erase applications:

$ tockloader install            # Installs an app
$ tockloader list               # Lists installed apps
$ tockloader erase-apps         # Erases all apps

Making Your First System Call

The goal of the sensor application is to sample this chip's internal temperature sensor, and to provide this value to other applications using Tock's Inter-Process Communcation facility.

However, an application in Tock runs as an unprivileged process, and as such it does not have direct access to any chip peripherals. Instead, the application needs to ask the Tock kernel to perform this operation. For this, libtock-c provides some system call wrappers that our application can use. These are defined in the libtock folder of the libtock-c repository. For this particular application, we are mainly interested in talking to Tock's sensor driver subsystem. For this, libtock-c/libtock/temperature.h provides convenient userspace wrapper functions, such as libtocksync_temperature_read:

#include <libtock-sync/sensors/temperature.h>

// Read the temperature sensor synchronously.
// ## Arguments
// - `temperature`: Set to the temperature value in hundredths of degrees
//   centigrade.
// ## Return Value
// A returncode indicating whether the temperature read was completed
// successfully.
returncode_t libtocksync_temperature_read(int* temperature);

For now, let's focus on using the API to make a system call to read the temperature. For this, we can extend the provided 00_sensor_hello application's main.c file with a call to that function. Your code should invoke this function and pass it a reference into which the temperature value will be written. You can then extend the printf call to print this number.

With these changes, compile and re-install your application by running make install again. Once that is done, you should see output similar to the following:

$ tockloader listen
Initialization complete. Entering main loop
NRF52 HW INFO: Variant: AAC0, Part: N52840, Package: QI, Ram: K256, Flash: K1024
Hello World, the temperature is: 2600

CHECKPOINT: 01_sensor_ipc

Implementing an IPC Service

In our next step, we want to extend this application into an IPC service, such that we can provide the most recent temperature reading to other applications as well.

Because we do not want to make a system call every time we get such an IPC request, we instead change the main function to run a loop and query the temperature periodically, such as once every 250 milliseconds. For this, we can use the libtocksync_alarm_delay_ms function:

#include <libtock-sync/services/alarm.h>

int main(void) {
  // Perform initialization, declare variables

  for (;;) {
    // Read temperature into global variable

    // Wait for 250ms

  return 0;

It is worth noting at this point that the libtocksync_alarm_delay_ms function does not perform busy-waiting. It instead blocks this application from executing for some time, and unblocks it by notifying it after 250 ms. This notification comes in the form of a callback. A callback is a kernel-scheduled task in the userspace application that can run at specific, pre-determined points in the application: so-called yield-points. In contrast to, e.g., signal handlers on Linux, an application will not receive a callback between any arbitrary instructions. libtocksync_alarm_delay_ms is such a yield-point and allows any number of callbacks to be invoked until the 250ms wait-time has expired. When an application has no work to be done, the kernel is free to schedule other applications or place the chip into a low-power state.

In the above example, libtocksync_alarm_delay_ms internally configures an appropriate handler for the callback that is invoked when its wait-time has expired. However, other types of events require a developer to write and register a callback manually -- for instance, for IPC service requests. We do so by invoking the ipc_register_service_callback, defined in ipc.h:

#include <libtock/kernel/ipc.h>

// Registers a service callback for this process.
// Service callbacks are called in response to `notify`s from clients and take
// the following arguments in order:
//   pkg_name  - the package name of this service
//   callback  - the address callback function to execute when clients notify
//   void* ud  - `userdata`. data passed to callback function
int ipc_register_service_callback(const char *pkg_name,
                                  subscribe_upcall callback, void *ud);

In the above, ipc_register_service_callback takes a "package name" under which the IPC service will be reachable by clients. By convention this should be the same name that the application uses -- in our example, it should be org.tockos.thread-tutorial.sensor as defined in the Makefile. When a client sends an IPC request to a service, the provided callback will be invoked in the service application. This callback is invoked with some parameters provided by the IPC client, and is passed the ud pointer provided in the call to ipc_register_service_callback. This callback has a function signature as follows:

static void sensor_ipc_callback(int pid, int len, int buf, void *ud) {
  // Callback handler code

Here, pid is an identifier that can be used to send a notification back to the requesting client, using the following call:


IPC clients and services communicate through memory sharing. In particular, an IPC client can share a region of its own memory with the IPC service, provided some constraints on buffer size and alignment. This shared buffer is then provided to the IPC service callback through the len and buf parameters, where buf should be cast to the appropriate pointer type.

EXERCISE: Implement an IPC service callback for your sensor application that writes the current temperature value into the provided buffer.

You should write the temperature value into a global variable in the main loop, and read this variable in the callback handler. You may use something along the lines of:

memcpy((uint8_t*) buf, (uint8_t*) &current_temperature, sizeof(current_temperature))

After copying the value, notify the calling client using the ipc_notify_client call.

Install the application.

CHECKPOINT: 02_sensor_final

Testing your IPC Service

To test whether this IPC service works we also need an appropriate IPC client. For this, we provide a client application that also forms the basis of our control application.

CHECKPOINT: 03_controller_screen

EXERCISE: Install the provided 03_controller_screen application next to the sensor IPC service. A tockloader list command should show both applications as being installed:

$ tockloader list
[INFO   ] Using jlink channel to communicate with the board.
[INFO   ] Using settings from KNOWN_BOARDS["nrf52dk"]
│ App 0                                            |
  Name:                  org.tockos.thread-tutorial.controller
  Version:               0
  Enabled:               True
  Sticky:                False
  Total Size in Flash:   16384 bytes

│ App 1                                            |
  Name:                  org.tockos.thread-tutorial.sensor
  Version:               0
  Enabled:               True
  Sticky:                False
  Total Size in Flash:   8192 bytes

[INFO   ] Finished in 4.381 seconds

When both applications are flashed onto a Tock board, the provided 03_controller_screen application should indicate that it is making repeated IPC calls to the sensor and retrieving a temperature value, which can look like the following. You can also trigger these prints by pressing button 1 or 2 on the board.

$ tockloader listen
NRF52 HW INFO: Variant: AAC0, Part: N52840, Package: QI, Ram: K256, Flash: K1024
[controller] Discovered sensor service: 1
[controller] TODO: update screen! Measured temperature: 2500
tock$ [controller] TODO: update screen! Measured temperature: 2600
[controller] TODO: update screen! Measured temperature: 2700
[controller] TODO: update screen! Measured temperature: 2600
[controller] TODO: update screen! Measured temperature: 2500
[controller] TODO: update screen! Measured temperature: 2500

Take a moment to look at the 03_controller_screen/main.c implementation. It implements the IPC client logic by defining a sensor_callback, quite similar to the service callback we defined above. This callback is fired whenever the service notifies the client. This app also defines some logic to handle button presses and change a "set-point temperature", which it displays on the console. This part will be relevant in the next stage of the tutorial.

This concludes the first stage of this tutorial. In the next step, we will extend the controller application to utilize a more involved peripheral: an attached OLED screen. This screen, alongside the four buttons present on the nRF52840DK development board will serve as the user interface for our HVAC control system.

Continue here.