This is a minor update where all tasks are made periodic. This is usefull to understand HYPERPERIOD.
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 |
Hyper period (H)=LCM(250,500,500,750,1000)
H = 3000 milliseconds
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.
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
For this, the function osSchedulerRoundRobinSinglePeriodicTask need to be uncommented in osKernel.c file and also in SysTick_Handler present in osKernel.s file.
Function osSchedulerRoundRobin is already uncommented and this is the default function enabled for execution
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
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
Using ADC, Value is displayed on LCD.
| ADC CHANNEL | BOARD PIN |
|---|---|
| ADC1_5 | PA5 |
| 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 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 |
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.
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)
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.
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 |