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fix(dshot): pick bit width per timer clock for accurate rate #27402

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fix(dshot): pick bit width per timer clock for accurate rate #27402

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26 changes: 25 additions & 1 deletion platforms/nuttx/src/px4/stm/stm32_common/include/px4_arch/dshot.h
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Original file line number Diff line number Diff line change
Expand Up @@ -36,8 +36,32 @@

#include <drivers/drv_pwm_output.h>
#include <stm32_dma.h>
#include <arch/board/board.h>

#define DSHOT_MOTOR_PWM_BIT_WIDTH 20u
// Pick WIDTH so floor(TIM_CLK / 600000) divides cleanly into it, keeping the
// prescaler integer. Guard skips F1 IO co-processors (no TIM8, no DSHOT).
#if defined(STM32_APB1_TIM5_CLKIN) && defined(STM32_APB2_TIM8_CLKIN)

#define _DSHOT600_TICKS_APB1 (STM32_APB1_TIM5_CLKIN / 600000U)
#define _DSHOT600_TICKS_APB2 (STM32_APB2_TIM8_CLKIN / 600000U)

#if (_DSHOT600_TICKS_APB1 % 20U == 0U)
# define DSHOT_MOTOR_PWM_BIT_WIDTH 20u
#elif (_DSHOT600_TICKS_APB1 % 19U == 0U)
# define DSHOT_MOTOR_PWM_BIT_WIDTH 19u
#elif (_DSHOT600_TICKS_APB1 % 18U == 0U)
# define DSHOT_MOTOR_PWM_BIT_WIDTH 18u
#else
# define DSHOT_MOTOR_PWM_BIT_WIDTH 20u
#endif

#if (_DSHOT600_TICKS_APB1 % DSHOT_MOTOR_PWM_BIT_WIDTH) != (_DSHOT600_TICKS_APB2 % DSHOT_MOTOR_PWM_BIT_WIDTH)
# pragma message "DSHOT bit width: APB1 and APB2 timer clocks emit DSHOT at different rates on this board"
#endif

#else
# define DSHOT_MOTOR_PWM_BIT_WIDTH 20u
#endif

/* Configuration for each timer to setup DShot. Some timers have only one while others have two choices for the stream.
*
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