[PATCH 2/2] pwm: pca9685: support staggered output ON times

Clemens Gruber
2020-11-12T16:45:51Z

The PCA9685 supports staggered LED output ON times to minimize current
surges and reduce EMI.
When this new option is enabled, the ON times of each channel are
delayed by channel number x counter range / 16, which avoids asserting
all enabled outputs at the same counter value while still maintaining
the configured duty cycle of each output.

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
---
drivers/pwm/pwm-pca9685.c | 31 +++++++++++++++++++++++++++++++
1 file changed, 31 insertions(+)

diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
index df214758256e..b993231ebb5e 100644
--- a/drivers/pwm/pwm-pca9685.c
+++ b/drivers/pwm/pwm-pca9685.c
@@ -75,6 +75,7 @@ struct pca9685 {
struct pwm_chip chip;
struct regmap *regmap;
int period_ns;
+ bool staggered_outputs;
#if IS_ENABLED(CONFIG_GPIOLIB)
struct mutex lock;
struct gpio_chip gpio;
@@ -251,6 +252,7 @@ static int pca9685_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
{
struct pca9685 *pca = to_pca(chip);
unsigned long long duty;
+ unsigned int on, off;
unsigned int reg;
int prescale;

@@ -286,6 +288,32 @@ static int pca9685_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
duty = (PCA9685_COUNTER_RANGE - 1) * (unsigned long long)duty_ns;
duty = DIV_ROUND_UP_ULL(duty, period_ns);

+ if (pca->staggered_outputs && pwm->hwpwm < PCA9685_MAXCHAN) {
+ if (duty_ns > 0) {
+ /*
+ * To reduce EMI, the ON times of each channel are
+ * spread out evenly within the counter range, while
+ * still maintaining the configured duty cycle
+ */
+ on = pwm->hwpwm * PCA9685_COUNTER_RANGE /
+ PCA9685_MAXCHAN;
+ off = (on + duty) % PCA9685_COUNTER_RANGE;
+ regmap_write(pca->regmap, LED_N_ON_L(pwm->hwpwm),
+ on & 0xff);
+ regmap_write(pca->regmap, LED_N_ON_H(pwm->hwpwm),
+ (on >> 8) & 0xf);
+ regmap_write(pca->regmap, LED_N_OFF_L(pwm->hwpwm),
+ off & 0xff);
+ regmap_write(pca->regmap, LED_N_OFF_H(pwm->hwpwm),
+ (off >> 8) & 0xf);
+ return 0;
+ }
+
+ /* Clear ON times */
+ regmap_write(pca->regmap, LED_N_ON_L(pwm->hwpwm), 0);
+ regmap_write(pca->regmap, LED_N_ON_H(pwm->hwpwm), 0);
+ }
+
if (pwm->hwpwm >= PCA9685_MAXCHAN)
reg = PCA9685_ALL_LED_OFF_L;
else
@@ -416,6 +444,9 @@ static int pca9685_pwm_probe(struct i2c_client *client,
reg &= ~(MODE1_ALLCALL | MODE1_SUB1 | MODE1_SUB2 | MODE1_SUB3);
regmap_write(pca->regmap, PCA9685_MODE1, reg);

+ pca->staggered_outputs = device_property_read_bool(
+ &client->dev, "staggered-outputs");
+
/* Clear all "full off" bits */
regmap_write(pca->regmap, PCA9685_ALL_LED_OFF_L, 0);
regmap_write(pca->regmap, PCA9685_ALL_LED_OFF_H, 0);
--
2.29.2