STM32F1 Embedded C Programming

Blue Pill: SysTick programming

A comprehensive guide to generating time-accurate delays with SysTick timer on Cortex M3 microcontrollers.

Do you know that “delay” means “do something, whatever. I don’t care, just let time pass” and that can be even

for(int i=0;i<10000;i++) (void) 0; ?

SysTick is a 24bit system timer, that counts down from reload value to zero, reloads, and counts from the subsequent clocks. It generates interrupts requests on a regular basis and can be used for timekeeping, time measurement, or as an interrupt source for tasks that need to be executed regularly.

In this article, you are going to learn about the SysTick timer, its configuration, and how to use its interrupt requests to generate accurate time delays.

SysTick timer — control flow

Configuration

We have to set the registers according to duration and interrupt requirements.

1. Programming reload value
   The reload value can be set using RELOAD register. This value is set to 1 less that the number of clock cycles needed for the interrupt as the timer counts both reload value as well as zero. e.g. If the SysTick interrupt is required every 100 clock pulses, set RELOAD to 99.
2. Clear current value
   This register can be accessed using the VAL variable. Bits 24:31 are reserved and 24-bit value can be read from bits 23:0. Writing any value this register sets it to zero along with setting COUNT_FLAG to zero.
3. Select clock source-
   The clock source can be set using CLKSOURCE bit (2) of CTRL register.
   0 - AHB/8
   1 - Processor Clock (AHB)
4. Enable Tick interrupt-
   To enable Tick interrupt set TICKINT bit (2) of CTRL register.
5. Start SysTick timer-
   ENABLE bit (0) of CTRL the register enables the counter. When ENABLE is set to 1, the counter loads the RELOAD value from the LOAD register and then counts down. On reaching 0, it sets the COUNTFLAG to 1 and optionally asserts the SysTick depending on the value of TICKINT. It then loads the RELOAD value again and begins counting.

Code

Configure pin C13 as output and add led blinking functionality

led_init()`: This function enables the clock for PORT C on the APB2 bus, configures pin c13 as general-purpose output with push-pull, and sets the speed to 2MHz.

led_on() : Turns the LED on by setting bit 13 of the BSRR register.

led_off() : Turns the LED off by setting bit 13 of the BRR register.

Pin C13 configuration and blinking functionality

Define the handler function for SysTick interrupt and define a delay function

msTicks : It stores the tick count i.e. number of times the SysTick interrupt has been triggered. We use this as a reference in the delay function.

SysTick_Handler() : The handler function for SysTick interrupt, updates/increments the global tick count.

delay() : This is the definition of the function to generate a time-accurate delay. It waits for the tick count to increase by the ms (milliseconds) specified from the initial tick count when the function was first invoked.

Main program body

In the main function, we are invoking the SysTick_Configure() API provided by CMSIS. It takes care of all configurations mentioned in the Configuration section. It takes the value to be loaded in the RELOAD register. For our specific application, the tick interrupt is required after every millisecond so we have passed SystemCoreClock/1000 (8MHz/1000) to it. It means that the interrupt will be triggered after the counter counts from 7999 to 0.
Explanation: For an 8MHz CPU clock the counter counts 800000 cycles in one second so it will count 8000 cycles in 1 millisecond. That’s why SysTick is configured to interrupt after counting 8000 cycles i.e. including 7999 and 0, thereby generating exactly a 1ms time base.

Source code and additional resources

Entire source code and information related to building and running code on the Blue Pill board can be found at SysTick Programming on STM32F1 Blue Pill by Rohit Nimkar.

Refer to the following article on How to get started with STM32 Blue Pill using Makefiles and VSCode.

Programming STM32 Blue Pill using makefile