djsundog
/
circuitpython
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

rp2/machine_pwm: Fix overflows with freq > 268 MHz. 

There were several places where 32-bit integer could overflow with
frequencies of 2^28 Hz or above (~268 MHz).  This fixes those overflows and
also introduces rounding for more accurate duty_ns computations.

Signed-off-by: Paul Grayson <pdg@alum.mit.edu>
This commit is contained in:
Paul Grayson 2022-12-14 19:19:21 -08:00 committed by Damien George
parent b5823604a0
commit f0f5c6568d
1 changed files with 37 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

49
ports/rp2/machine_pwm.c
View File

@ -100,11 +100,33 @@ STATIC void mp_machine_pwm_deinit(machine_pwm_obj_t *self) {
pwm_set_enabled(self->slice, false);
}
STATIC mp_obj_t mp_machine_pwm_freq_get(machine_pwm_obj_t *self) {
// Returns: floor((16*F + offset) / div16)
// Avoids overflow in the numerator that would occur if
// 16*F + offset > 2**32
// F + offset/16 > 2**28 = 268435456 (approximately, due to flooring)
uint32_t get_slice_hz(uint32_t offset, uint32_t div16) {
uint32_t source_hz = clock_get_hz(clk_sys);
if (source_hz + offset / 16 > 268000000) {
return (16 * (uint64_t)source_hz + offset) / div16;
} else {
return (16 * source_hz + offset) / div16;
}
}
// Returns 16*F / denom, rounded.
uint32_t get_slice_hz_round(uint32_t div16) {
return get_slice_hz(div16 / 2, div16);
}
// Returns ceil(16*F / denom).
uint32_t get_slice_hz_ceil(uint32_t div16) {
return get_slice_hz(div16 - 1, div16);
}
STATIC mp_obj_t mp_machine_pwm_freq_get(machine_pwm_obj_t *self) {
uint32_t div16 = pwm_hw->slice[self->slice].div;
uint32_t top = pwm_hw->slice[self->slice].top;
uint32_t pwm_freq = 16 * source_hz / div16 / (top + 1);
uint32_t pwm_freq = get_slice_hz_round(div16 * (top + 1));
return MP_OBJ_NEW_SMALL_INT(pwm_freq);
}
@ -114,22 +136,27 @@ STATIC void mp_machine_pwm_freq_set(machine_pwm_obj_t *self, mp_int_t freq) {
#define TOP_MAX 65534
uint32_t source_hz = clock_get_hz(clk_sys);
uint32_t div16;
uint32_t top;
if ((source_hz + freq / 2) / freq < TOP_MAX) {
// If possible (based on the formula for TOP below), use a DIV of 1.
// This also prevents overflow in the DIV calculation.
div16 = 16;
// Same as get_slice_hz_round() below but canceling the 16s
// to avoid overflow for high freq.
top = (source_hz + freq / 2) / freq - 1;
} else {
// Otherwise, choose the smallest possible DIV for maximum
// duty cycle resolution.
// Constraint: 16*F/(div16*freq) < TOP_MAX
// So: div16 = ceil(16*F/(TOP_MAX*freq))
// So:
div16 = get_slice_hz_ceil(TOP_MAX * freq);
div16 = (16 * source_hz + TOP_MAX * freq - 1) / (TOP_MAX * freq);
// Set TOP as accurately as possible using rounding.
top = get_slice_hz_round(div16 * freq) - 1;
}
// Set TOP as accurately as possible using rounding.
uint32_t top = (16 * source_hz + div16 * freq / 2) / (div16 * freq) - 1;
if (div16 < 16) {
mp_raise_ValueError(MP_ERROR_TEXT("freq too large"));
@ -167,17 +194,15 @@ STATIC void mp_machine_pwm_duty_set_u16(machine_pwm_obj_t *self, mp_int_t duty_u
}
STATIC mp_obj_t mp_machine_pwm_duty_get_ns(machine_pwm_obj_t *self) {
uint32_t source_hz = clock_get_hz(clk_sys);
uint32_t slice_hz = 16 * source_hz / pwm_hw->slice[self->slice].div;
uint32_t slice_hz = get_slice_hz_round(pwm_hw->slice[self->slice].div);
uint32_t cc = pwm_hw->slice[self->slice].cc;
cc = (cc >> (self->channel ? PWM_CH0_CC_B_LSB : PWM_CH0_CC_A_LSB)) & 0xffff;
return MP_OBJ_NEW_SMALL_INT((uint64_t)cc * 1000000000ULL / slice_hz);
return MP_OBJ_NEW_SMALL_INT(((uint64_t)cc * 1000000000ULL + slice_hz / 2) / slice_hz);
}
STATIC void mp_machine_pwm_duty_set_ns(machine_pwm_obj_t *self, mp_int_t duty_ns) {
uint32_t source_hz = clock_get_hz(clk_sys);
uint32_t slice_hz = 16 * source_hz / pwm_hw->slice[self->slice].div;
uint32_t cc = (uint64_t)duty_ns * slice_hz / 1000000000ULL;
uint32_t slice_hz = get_slice_hz_round(pwm_hw->slice[self->slice].div);
uint32_t cc = ((uint64_t)duty_ns * slice_hz + 500000000ULL) / 1000000000ULL;
if (cc > 65535) {
mp_raise_ValueError(MP_ERROR_TEXT("duty larger than period"));
}