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Track unadjusted PWM duty cycle to avoid accumulating conversion errors

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Fixes #2086

When the frequency of a `PWMOut` is change it re-sets the PWM's duty cycle as
well, since the registers have to be re-calculated based on the new frequency.
Unfortunately, `common_hal_pulseio_pwmout_get_duty_cycle`
will return a value very close to, but not exactly, the value passed to `common_hal_pulseio_pwmout_set_duty_cycle`. If the frequency is modified
without the calling code also re-setting the duty cycle then the duty cycle
will decay over time. This fixes that problem by tracking the unadjusted duty
cycle and re-setting the duty cycle to that value when the frequency is changed.
This commit is contained in:
Thea Flowers 2019-10-22 10:37:36 -07:00 committed by Thea Flowers
parent af1fab1915
commit 4fdf518251
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GPG Key ID: 3CA6321E8CACD921
2 changed files with 10 additions and 2 deletions
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11
ports/atmel-samd/common-hal/pulseio/PWMOut.c
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@ -135,6 +135,7 @@ pwmout_result_t common_hal_pulseio_pwmout_construct(pulseio_pwmout_obj_t* self,
bool variable_frequency) {
self->pin = pin;
self->variable_frequency = variable_frequency;
self->duty_cycle = duty;
if (pin->timer[0].index >= TC_INST_NUM &&
pin->timer[1].index >= TCC_INST_NUM
@ -322,6 +323,13 @@ void common_hal_pulseio_pwmout_deinit(pulseio_pwmout_obj_t* self) {
}
extern void common_hal_pulseio_pwmout_set_duty_cycle(pulseio_pwmout_obj_t* self, uint16_t duty) {
// Store the unadjusted duty cycle. It turns out the the process of adjusting and calucating
// the duty cycle here and reading it back is lossy - the value will decay over time.
// Track it here so that if frequency is changed we can use this value to recalcuate the
// proper duty cycle.
// See https://github.com/adafruit/circuitpython/issues/2086 for more details
self->duty_cycle = duty;
const pin_timer_t* t = self->timer;
if (t->is_tc) {
uint16_t adjusted_duty = tc_periods[t->index] * duty / 0xffff;
@ -415,7 +423,6 @@ void common_hal_pulseio_pwmout_set_frequency(pulseio_pwmout_obj_t* self,
break;
}
}
uint16_t old_duty = common_hal_pulseio_pwmout_get_duty_cycle(self);
if (t->is_tc) {
Tc* tc = tc_insts[t->index];
uint8_t old_divisor = tc->COUNT16.CTRLA.bit.PRESCALER;
@ -450,7 +457,7 @@ void common_hal_pulseio_pwmout_set_frequency(pulseio_pwmout_obj_t* self,
#endif
}
common_hal_pulseio_pwmout_set_duty_cycle(self, old_duty);
common_hal_pulseio_pwmout_set_duty_cycle(self, self->duty_cycle);
}
uint32_t common_hal_pulseio_pwmout_get_frequency(pulseio_pwmout_obj_t* self) {

1
ports/atmel-samd/common-hal/pulseio/PWMOut.h
View File

@ -36,6 +36,7 @@ typedef struct {
const mcu_pin_obj_t *pin;
const pin_timer_t* timer;
bool variable_frequency;
uint16_t duty_cycle;
} pulseio_pwmout_obj_t;
void pwmout_reset(void);