Blue Pill - SysTick programming

Configure the systick timer to generate periodic interrupts. Use this timer for generating accurate delay function.

SysTick Timer

The processor has a 24 bit system timer, SysTick, that counts down from the reload value to zero, reloads and counts down on the subsequent clocks.
System Tick Time (SysTick) generates interrupt requests on regular basis. This allows an os to perform context switching to support multitasking. For applications that do not require an OS, the SysTick can be used for time keeping, time measurement or as an interrupt source for tasks that need to be executed regularly.
SysTick register can only be accessed using word access.

Control flow

1. Program the reload value:
   The reload value can be loaded by setting LOAD 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 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. Configure SysTick and start:
   1. Select clock source-
      Clock source can be set using CLKSOURCE bit (2) of CTRL register.
      0 - AHB/8
      1 - Processor Clock (AHB)
   2. Enable Tick interrupt-
      To enable Tick interrupt set TICKINT bit (2) of CTRL register.
   3. Start SysTick timer-
      ENABLE bit (0) of CTRL 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.

Project Working

This project configures SysTick timer and uses it to generate time accurate delay for blinking an LED. The onboard LED connected to pin C13 blinks every second.

Dependencies

• make
  Make utility is required for configuring and building this project. You can install make on linux by running command:

  sudo apt install build-essential

• gcc-arm-none-eabi toolchain
  ARM cross-platform toolchain is required to build applications for arm mcus. Toolchain can be installed by running following command:

  sudo apt install gcc-arm-none-eabi

• openocd
  It is an Open On Circuit Debugging tool used to flash and debug arm micro controllers. You can install openocd on linux by running command:

  sudo apt install openocd -y

• Cortex Debug extension
  This extension for VSCode is helpful for debugging the application on Blue Pill. The contents of registers as well as memory are visible in the context menu. Launch VS Code Quick Open (Ctrl+P), paste the following command, and press enter.

  ext install marus25.cortex-debug

Project Structure

• src directory contains all source files for the project
• include directory contains all header files for the project

Source file description

• STM32F103C8TX_FLASH.ld - linker script
• src\main.c - application code
• src\startup_stm32f103c8tx.s - assembly startup script for blue pill board
• system_stm32f1xx.c - clock configuration and system initialization functions
• STM32F103.svd - contains the description of the system contained in Arm Cortex-M processor-based microcontrollers, in particular, the memory mapped registers of peripherals.

Run Locally

Running the project is super easy. Just clone, build, and flash.

Clone the project

1. Using https

   git clone https://github.com/csrohit/bluepill-systick.git
   cd bluepill-systick

2. Using ssh

   git clone git@github.com:csrohit/bluepill-systick.git
   cd bluepill-systick

Configuration

All the configuration required for building this project is given below.

1. Build output directory In Makefile, output directory can be configured using variable BUILD_DIR.

2. Build type In Makefile, build type can be configured using variableDEBUG. Possible values are Debug and Release.

3. Binary name In CMakeLists.txt, output binary name can be configured using project(<binary-name>) macro. ** update above info in .vscode/launch.json as well for debugging to work.

Build

Run following command in terminal to generate flashable binaries for blue pill board. Build files will be written to Build Output Directory as configured.

make all

Flash

1. Connect STlink to PC and blue pill board using swd headers.
2. Put blue pill board in programming mode.
3. Run following to flash board with binary.

make flash

Output

Onboard led connected to Pin C13 can be observed to be blinking every second.

Debug

Click in Run and Debug option in VsCode sidebar. Then launch Cortex Debug target.

Happy debugging....