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v3.2.1

This is a minor update where all tasks are made periodic. This is usefull to understand HYPERPERIOD.

Theory:

The list of tasks in our board are and their periods (periods are taken for )

Task Name Period in milliseconds LED
Task0 250 P0_DELL_SMALLGREEN
Task1 500 P1_DELL_YELLOW
Task2 500 P2_DELL_HDD
Periodic_Task3 750 P3_MY_LED
Periodic_Task4 1000 P4_LED

Calculations

Hyper period (H)=LCM(250,500,500,750,1000)

            H = 3000 milliseconds

Implementation

We will set QUANTA to 1 millisecond, so SysTick_Handler will be executed every millisecond. Now during this time, our funtion osSchedulerRoundRobin will decide which task to be executed.

LED animations from the tasks Task0,Task1,Task2 are deleted. I still dont know if the task addition function is relevant or not. I did not modify it yet.

v3.2

Here in second method of thread scheduling, we are going to modify the SysTick_Handler. Till Now we moved to another thread using this SysTick_Handler, but now we shall do it using C function which will give us more control on the context switching portion.

Capability to switch to next thread is implemented in C function instead of Assembly. Implementation is again considered for following conditions

Only 1 periodic thread to be executed

For this, the function osSchedulerRoundRobinSinglePeriodicTask need to be uncommented in osKernel.c file and also in SysTick_Handler present in osKernel.s file.

Multiple periodic threads to be executed

Function osSchedulerRoundRobin is already uncommented and this is the default function enabled for execution

v3.1

Here in this version and lesson, we are going to talk abount periodic schedulers. Now there might be many ways to achieve this.

First type of periodic scheduler is described in this version. Recall from version v2 we have a basic round robin scheduler which consists of Task0, Task1 and Task2 that are called every time by the SysTick_Handler.

Now let us say we need another task which is periodic (say should execute every 100milli seconds). Method explained in this version is by creating a Timer (TIM4 is used in the example) of period 100 milliseconds and executing our task in this interrupt.

Again, HAL Library is used as we are not learning the bare metal programming. Only exception to the application of HAL library is the Clock configuration. Its observed that by using HAL library to configure clock, Systick timer was getting started by HAL, but in our application osKernelLaunch function.

We will not use this method of periodic scheduling. So lets see for version v3.2

v3:

This is a pre-release of the actual RTOS

Now that we have understood context switching (using SysTick Timer), we shall now try to move towards more practical example. So consider a scenario where a reading from Potentiometer is displayed on the LCD screen. and our existing 3 LEDs shall also blink as per the QUANTA time. The 3 LEDs are now called as Probe LEDs.

Since our main intention is to study the RTOS and integration of peripherals around it, we are not going to do bare-metal programming here. So I have used HAL library provided by ST microelectronics in KEIL IDE.

Below are the peripheral details and their pin assignment

Board : STM32F411CEUTx

ADC :

Using ADC, Value is displayed on LCD.

ADC CHANNEL BOARD PIN
ADC1_5 PA5

LCD 20X4 DISPLAY

LCD PIN BOARD PIN
DB0 PB0
DB1 PB1
DB2 PB2
DB3 PB3
DB4 PB4
DB5 PB5
DB6 PB6
DB7 PB7
RS PA8
RW PA9
EN PA10

PROBE LEDs

Probe LEDs are used for animation as well as Oscilloscope verification of our task switching.

Probe Name LED PIN DEFINITION PIN
Probe 0 P0_DELL_SMALLGREEN_Pin PA0
Probe 1 P1_DELL_YELLOW_Pin PC13
Probe 2 P2_DELL_HDD_Pin PC14
Probe 3 P3_MY_LED_Pin PC15

v2:

Till now we gave 1000 quanta size to each thread, but it might be possible that some threads might be completed in less time. So we can give the remaining time to trigger other threads. This can be done only by context switching.

To do this context switch before quanta 1000 is to raise the systick interrupt again. One method is to set the PEND bit in ICSR register of SysTick this will set the PENDING bit to ON for SysTick and trigger SysTick_Handler interrrupt and hence context switch will happen.

Now in the context switch, next thread will be fetched immediately. In this code example, Task0 is yielding, means, it will execute only once in the while loop and raise a PEND bit of SYSTICK timer That causes the context switch to Task1.

Notice that LED ON for Task0 WILL NOT BE visible as its almost done im microseconds, Oscilloscope only shall do that.

v1:

In this version of ChiliRTOS, we are creating a simple Round Robin Scheduler.

Features of this scheduler is:

  • Each task is assigned with a time slice of the duration "QUANTA"
  • Task will run till QUANTA and then the next task will be taken up for the same QUANTA duration and so on
  • Even if the tasks are finished early, next task is not taken up (means, thread is not yielded)

v0:

This is a basic blinky that runs using simple delays of for loops

  • Clock configuration is done using CMSIS library
  • GPIO initialization is done using CMSIS library
  • Delays are generated using Systick timer configured at 1 millisecond tick.

Setup information

WE WILL BLINK TWO LEDs as part of tasks

List of LEDs configured in the code are

LED DESCRIPTION CHIP PIN
ONBOARD LED (SINK) PC13
DELL_HDD_LED PA2
DELL_YELLOW_LED PA1
DELL_SMALLGREEN_LED PA0