diff --git a/README.md b/README.md index 836e791..270a2a8 100644 --- a/README.md +++ b/README.md @@ -817,10 +817,10 @@ see that SysTick has four registers: - VAL - a current counter value, decremented on each clock cycle - CALIB - calibration register -Every time VAL drops to zero, a SysTick interrupt is generated. The SysTick -interrupt index in the vector table is 15, so we need to set it. Upon boot, -our board Nucleo-F429ZI runs at 16Mhz. We can configure the SysTick counter -to trigger interrupt each millisecond. +Every time VAL drops to zero, a SysTick interrupt is generated. +The SysTick interrupt index in the vector table is 15, so we need to set it. +Upon boot, our board Nucleo-F429ZI runs at 16Mhz. We can configure the SysTick +counter to trigger interrupt each millisecond. First, let's define a SysTick peripheral. We know 4 registers, and from the datasheet we can learn that the SysTick address is 0xe000e010. So: @@ -859,6 +859,15 @@ void SysTick_Handler(void) { } ``` +With 16MHz clock, we init SysTick counter to trigger an interrupt every +16000 cycles: the `SYSTICK->VAL` initial value is 15999, then it decrements +on each cycle by 1, and when it reaches 0, an interrupt is generated. The +firmware code execution gets interrupted: a `SysTick_Handler()` function is +called to increment `s_tick` variable. Here how it looks like on a time scale: + +![](images/systick.svg) + + The `volatile` specifier is required here becase `s_ticks` is modified by the interrupt handler. `volatile` prevents the compiler to optimise/cache `s_ticks` value in a CPU register: instead, generated code always accesses memory. That diff --git a/images/systick.svg b/images/systick.svg new file mode 100644 index 0000000..dc45614 --- /dev/null +++ b/images/systick.svg @@ -0,0 +1,162 @@ + + + + white + + + white + + + Main rect + + + systick rect + + + main rect + + + handler rect + + + main rect + + + interrupt + + + SysTick​interrupt + SysTick​interrupt + + + clock + clock + + current + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + dots + + + + + + + + + + + ... + ... + + + main + ... + ... + main() + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Time + + + + + + ... + time + ... + + + SYSTICK->VAL + SYSTICK->VAL + 15999 15998 ... 1 0 15999 15998 ... 1 0 15999 + + + s_tick + s_tick + 0 1 2 + ... + ... + + + SysTick_Handler + + + ... + stick++ + stick++ + + SysTick_Handler() + + + + + + + ... + +