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stm32/stm32l4 PWM: While attempting to output a 70 MHz square wave from the timer output of a STM32 clocked at 140 MHz (which works fine in baremetal C), I stumbled on what I believe to be an error in arch/arm/src/stm32/stm32_pwm.c. Line 1304 we are told that

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reload = timclk / info->frequency;

which I belive to be incorrect, it should be

reload = timclk / info->frequency - 1;

since starting to count from 0, if I want to output half of the TIM clock, I must count to 1 and not to 2.

Surely enough, the original code did output 140/3=47 MHz, while this correction does allow the output up to 70 MHz.

I am not sure this affects most users generating slow PWM (e.g. PX4) but for frequencies
close to the PCLK, indeed the difference becomes significant.
This commit is contained in:
JM 2017-06-13 06:01:13 -06:00 committed by Gregory Nutt
parent af27e8e0a9
commit 7903a8a46c
2 changed files with 2 additions and 2 deletions
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2
arch/arm/src/stm32/stm32_pwm.c
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@ -1300,7 +1300,7 @@ static int pwm_timer(FAR struct stm32_pwmtimer_s *priv,
timclk = priv->pclk / prescaler;
reload = timclk / info->frequency;
reload = timclk / info->frequency - 1;
if (reload < 1)
{
reload = 1;

2
arch/arm/src/stm32l4/stm32l4_pwm.c
View File

@ -841,7 +841,7 @@ static int stm32l4pwm_timer(FAR struct stm32l4_pwmtimer_s *priv,
timclk = priv->pclk / prescaler;
reload = timclk / info->frequency;
reload = timclk / info->frequency - 1;
if (reload < 1)
{
reload = 1;