Controlling a microservo from nRF51822 when BLE is activated

[tumaku]

I am trying to control a servo motor from nRF51822 when BLE is activated. I am using mbed development framework and libraries. My servo example works flowlessly if BLE/SoftDevice S110 is not used. However, when I use BLE for advertising or to establish a connection to a central device, my servo starts glitching/bumping. I have seen this example that shows how to use PWM together with SoftDevice

github.com/.../nrf51-pwm-library

However I have not been able to make it work with my servo. Is there any simple servo example?

Thanks!

[Asbjørn]

The PWM example is the best place to start. If your application stops when you activate BLE functionality I would recommend you to check if both your application and softdevice are trying to access and use some of the same resources on the chip. For example timers or PPI channels. Have a look in the S110 Softdevice specification.

[tumaku]

The basic Servo routine that I am using in mbed relies on the "core" mbed class PwmOut. I will check if they are using Timer0 there. Trying to reuse the Nordic PWM example, this is the init method that I have created to use PIN 15 as PWM control of the servo. Is this correct?

void pwm_init()
{
    nrf_pwm_config_t pwm_config = PWM_DEFAULT_CONFIG;
    
    pwm_config.mode             = PWM_MODE_MTR_100;
    pwm_config.num_channels     = 1;
    pwm_config.gpio_num[0]      = P0_15;
    
    // Initialize the PWM library
    nrf_pwm_init(&pwm_config);    
}

[Asbjørn]

This seems fine. Just make sure that PPI channels and timers doesn't overlap with what the softdevice requires.

[James Heires]

tumaku, Have you initialized the control input to your servo before applying power? If not, you should, because most DC servos such as the TowerPro SG90 units have an undefined state when power is applied, but no control signal has been set.