Lab 07

Exercise Objectives

Introduce multithreading on Pico

Introduction

RP2040 microcontroler has two Cortex-M0 cores, Core0 and Core1. In the default mode of Raspberry Pi Pico program execution, Core0 executes all the tasks and Core1 remains idle or on standby mode. Using both the cores of RP2040 provides us with two threads of execution and hence a more powerful project with better processing speed.

Both Core0 and Core1 execute their assigned tasks independent of each other while sharing all the resources like memory space and program code with each other. Sharing the memory location between two cores can create race conditions and hence can cause trouble when mutual memory accessing is not assured. On the other hand, sharing program code with each other (Core0 and Core1) may sound troublesome but practically it is not. The reason is fetching code is a read instruction that does not create a race condition.

Library _thread

In order to run a parallel process on a microcontroller, it is necessary to use a suitable library that activates Core1. In MicroPython, the _thread library is available, which provides basic functionality that allows a user to work with multiple threads and also allow them to share global data space.

Examples

Simple theads

from machine import Pin
import utime
import _thread

We configure the pin of the internal led as an output and we assign to led.

led = Pin(25, Pin.OUT)

Function that will block the thread with a while loop which will simply display a message every second

def second_thread():
    while True:
        print("Hello, I'm here in the second thread writting every second")
        utime.sleep(1)

Function _thread.start_new_thread initializes execution in the second core. The second argument is a list or dictionary with the arguments that will be passed to the function.

_thread.start_new_thread(second_thread, ())

Second loop that will block the main thread, and what it will do that the internal led blinks every half second.

while True:
    led.toggle()
    utime.sleep(0.25)

Race condition

import _thread
from utime import sleep
 
def core0_thread():
  global lock
  while True:
    lock.acquire()
    print('o', end="") 
    sleep(0.25)
    print('n', end="")
    sleep(0.25)
    print('e', end="")
    sleep(0.25)
    print()
    lock.release()
 
def core1_thread():
  global lock
  while True:
    # lock.acquire()
    print('s', end="")
    sleep(0.25)
    print('i', end="")
    sleep(0.25)
    print('x', end="")
    sleep(0.25)
    print()
    # lock.release()
 
lock = _thread.allocate_lock()
second_thread = _thread.start_new_thread(core1_thread, ())
core0_thread()

Table of Contents