initial pulse while starting a PWM application

Question

Hi, 
 A short summary, while using the app_PWM on our application I noticed an initial pulse on the PWM pin, its a 3V 1ms pulse that happens about 1 sec after powering on the nRF52832... 
 The PWM configuration and initializations code is quite standard: 
 int main(void)

{
 initialize();
 // Initializing the PWM instance in main() function
 ret_code_t err_code;
 app_pwm_config_t pwm1_cfg = APP_PWM_DEFAULT_CONFIG_1CH(1000L, ON_OFF_LED);
 pwm1_cfg.pin_polarity[1] = APP_PWM_POLARITY_ACTIVE_LOW;
 err_code = app_pwm_init(&PWM1,&pwm1_cfg,pwm_ready_callback);
 app_pwm_enable(&PWM1);
 while ( app_pwm_channel_duty_set(&PWM1, 0, MIN_LIGHT_INTENSITY) == NRF_ERROR_BUSY);
 APP_ERROR_CHECK(err_code);

execution_start(start); 
 Is there something I need to configure / disable / initialize on the PWM or the Startup sequence so I don't get this initial pulse?

Einar Thorsrud · Accepted Answer

Hi, 
 There is a problem with the code you described in your last post, which is that you should call app_pwm_enable() before you call app_pwm_channel_duty_set(). Calling it the other way around causes the library to misbehave. This is unfortunately not documented as far as I can see and not obvious. There is also a problem that you set the duty cycle to 1000, which is not in the valid range (0 - 100). 
 You could update the code to be like this: 
 app_pwm_config_t pwm1_cfg = APP_PWM_DEFAULT_CONFIG_1CH(1000L, ON_OFF_LED);
pwm1_cfg.pin_polarity[0] = APP_PWM_POLARITY_ACTIVE_LOW;
err_code = app_pwm_init(&PWM1,&pwm1_cfg,pwm_ready_callback);
app_pwm_enable(&PWM1);
while ( app_pwm_channel_duty_set(&PWM1, 0, 100) == NRF_ERROR_BUSY); 
 In this case the pin state will be like the following: 
 
 After reset before the pin is configured it will be the default state (disconnected input) = high impedance. 
 After executing line 3 the pin will be configured as an output pin and set to the non-active state - high in this case. 
 After executing line with the duty cycle is set to 100%, the pin should stay in the active state - low in this case. 
 
 So in this case you get the 1 ms pulse you have described and that is expected. 
 If you do the trick I mentioned you would have something like this instead. In that case you will not see any pulse: 
 app_pwm_config_t pwm1_cfg = APP_PWM_DEFAULT_CONFIG_1CH(1000L, ON_OFF_LED);
pwm1_cfg.pin_polarity[0] = APP_PWM_POLARITY_ACTIVE_HIGH;
err_code = app_pwm_init(&PWM1,&pwm1_cfg,pwm_ready_callback);
app_pwm_enable(&PWM1);
while ( app_pwm_channel_duty_set(&PWM1, 0, 100-100) == NRF_ERROR_BUSY);

initial pulse while starting a PWM application

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