stm32/ADC: Fix prescaler calculation to include max frequency. #4742
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Due to the integer rounding rules one has to subtract 1 from the numerator. For example: Let max clock be 55 and supplied clock be 110 110/55 = 2 which results in the divider being set to 4 and the clock after division ends up being 27 instead of 55 Subtracting 1 to the numerator get around the rounding issue 109/55 = 1 which results in the divider being set to 2 and the clock after division ends up being 55 which is exactly max clock
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After talking with some people they pointed out that this should be better documented. So I also added a Looking forward to your ideas and comments. :) |
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This also puts the explanation why the calculation has to be done that way into one place so it does not need to be copied all over the codebase.
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Due to the integer rounding rules one has to subtract 1 from the numerator.
For example:
Let max clock be 55 and supplied clock be 110
110/55 = 2 which results in the divider being set to 4 and the clock after division ends up being 27 instead of 55 Subtracting 1 to the numerator get around the rounding issue 109/55 = 1 which results in the divider being set to 2 and the clock after division ends up being 55 which is exactly max clock