Raspberry Pi RP2040 Realtime OS

Learning project to create a pre-emptive, real-time OS support for the RP2040. Scheduler code is written in C to allow for easy experimentation. Current capabilities include:

Pre-emptive
Thread priority
Waits/delays
Cooperative yield
Mutexes
Stack checks (issues breakpoint)


The context switch times on the 125MHz Pico with four threads are:
No optimizations

	Context Time
No stats collection:	5.8 uS
With stats collecton:	9.3 uS
With stats/stack checks:	9.9 uS

Example code has a simple "top" like output every two seconds to the serial port. The Ctx= values are the percentage of time spent in the context switch. Util= is the percentage of time spent in a thread and Idle= percent spent in one of the cores idle threads.
The S or status indicator is simply: W-waiting, R-running, Z-zombied or thread returned.

Wall time 0:56:38
CPU0 Ctx=0.095%, Util=  1.229%  Idle= 98.771%
CPU1 Ctx=0.074%, Util=  0.081%  Idle= 99.919%
Thrd   Name     S PRI  CPU     LastCPU
   0 Red LED    W 100  0.001%  1
   1 Green LED  W 100  0.011%  0
   2 Report     R 255  1.115%  0
   3 Spinner    Z 150  0.000%  0

The current implementation consists of three files and a main example:

rp2040os.h    OS header
rp2040os.c    Implementation in C
func.s        Implementation function in assembly
main.c        Example

A simple use case to kick off two threads:

#include "rp2040.h"


static uint32_t blink1Stack[128];
void blink1() {
  while (true) {
    setGPIO(LED_PIN1);
    delayms(300);
    clrGPIO(LED_PIN1);
    delayms(300);
  }
}

static uint32_t blink2Stack[128];
void blink2() {
  while (true) {
    setGPIO(LED_PIN2);
    delayms(300);
    clrGPIO(LED_PIN2);
    delayms(300);
  }
}

int main() {
  stdio_init_all(); 
  gpio_init(LED_PIN1);
  gpio_set_dir(LED_PIN1, GPIO_OUT);
  gpio_init(LED_PIN2);
  gpio_set_dir(LED_PIN2, GPIO_OUT);
  
  addThread("Red LED", blink1, blink1Stack, sizeof(blink1Stack), 100);
  addThread("Green LED", blink2, blink2Stack, sizeof(blink2Stack), 100);
  setupSched(); // No return
}

### Stack sizes
Be aware that the more library functions you call, the more stack space you may require. For example, using the printf function will require more than 256 32bit values in a stack to handle the functions memory requirements. If the unit under test is attached to a debugger, the STACK_WATCH can be turned on and will trigger a BKPT (breakpoint) in the debugger if any of the stack guard values are modified, indicating a possible overflow condition.

### osConfig.h
This file lets you customize how the scheduler works. By default, it is set to the RP2040 specifications or two cores. Setting the USER_TASKS to the correct amount for your application will minimize the amount of memory used. Additional thread information such as naming, stats and stack monitoring also consume a little more memory, but can be very helpful with debugging.
The CONTEXTSW_PIN can be used with an oscilloscope to see the context switches in real-time.

Building

export PICO_SDK_PATH=path to your pico-sdk
Copy the $PICO_SDK_PATH/external/pico_sdk_import.cmake into main directory.

$> mkdir build
$> cd build
$> cmake .. -DCMAKE_BUILD_TYPE=Debug
$> make