Commits on Apr 12, 2021

1. Make ticker computation use shift-by-0 …

   Runtime code that analysed clock frequency to determine numerator and
   denominator for conversion to standard 1MHz failed to handle the case
   of either being 1 correctly.
   
   Although it would spot other values that could be performed as shifts,
   it failed to spot that 1 is "shift by 0", so would end up doing runtime
   multiply and/or divide by 1. The runtime divide by 1 could be slow on a
   Cortex-M0 device, increasing interrupt latency.
   
   UART character loss on STM32F0 devices has been traced to this incorrect
   code.
   
   Correct the `exact_log2` routine so that `exact_log2(1)` returns 0 to
   fix this.
   
   Original code had a single special no-multiply-or-divide case for
   hardware clock frequency being exactly 1MHz, as USTICKER is on STM32F0 -
   this code lacks that but has a more general special case that covers all
   shift-convertible frequencies like 500kHz or 8MHz, which should be
   similar speed as shifts are cheap.

   

   kjbracey committed Apr 12, 2021
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