[libopenstm32-devel] Systick clock source definition

[e39 <e3...@fr...>]

I'm experimenting on the STM32F103 MCU and I'am surprised about what 
this code does (in examples/other/systick/systick.c):

   /* 72MHz / 8 => 9000000 counts per second */
   systick_set_clocksource(STK_CTRL_CLKSOURCE_AHB_DIV8);

   /* 9000000/9000 = 1000 overflows per second - every 1ms one interrupt */
   systick_set_reload(9000);

I've found that the LED does not toggle every 1s...

According to the comment and the symbol STK_CTRL_CLKSOURCE_AHB_DIV8, 
systick'c clock source is core clock divided by 8.

Diving into the Core M3 datasheet, I've found that bit 2 of SysTick 
control and status register selects the clock source, not a divider:

   stk clk src = (STK_CTRL bit 2 == 1) ? core clock : ext ref clk

I do not know yet what is this external reference clock but I've tried:

   systick_set_clocksource(STK_CTRL_CLKSOURCE_AHB);
   systick_set_reload(72000);

and now the LED does toggle every 1s.

Maybe it would be wise to rename:

   STK_CTRL_CLKSOURCE_AHB_DIV8 as STK_CTRL_CLKSOURCE_EXT

and

   STK_CTRL_CLKSOURCE_AHB      as STK_CTRL_CLKSOURCE_CORE

or something like that.

Tell me if I'm completely wrong, I need to learn!

--
Christophe