BLE + PWM motor driver

[ypearson]

The project I'm using the peripheral BLE UART that has been modified to use freeRTOS and PWM.

I'm using two channel PWM to drive a resonance motor at 25.6 kHz. If I disable the BLE thread the standard deviation of the PWM signal is ~10 Hz. However if the BLE thread is enabled, the PWM standard deviation jumps to ~ 150Hz. And unfortunately that is enough of a frequency drift to bring the motor out of resonance. Is there anything I can do to minimize this effect, and why does BLE increase the drift of the PWM signal ?

The PWM base is 16 MHz

thanks,

[ovrebekk]

Hi

Could you try to request high frequency clock when using BLE?

If you don't request HF clock the BLE stack will enable and disable the HF clock manually every time the radio is needed, and this switch could introduce some jitter to the 16MHz clock rate (the RC osc could be off by 5% or more). If you request the HF clock then it will always run from the crystal, and you will have a much more accurate clock.

[ypearson]

@Torbjørn Øvrebekk That worked, here is the lines of code that I used to fix it

// Start the external high frequency crystal
NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
NRF_CLOCK->TASKS_HFCLKSTART = 1;
// // Wait for the external oscillator to start up
while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0) {}

[ovrebekk]

That's the way to do it. I am glad to hear this solved your issue :)

[Namklak]

Yes, no more jitter.  But when I'm done with motion, I'd like to put the hfclk back in control of the ble softdevice, for minimum power use, how do I do that?

[ovrebekk]

Hi 

When using the SoftDevice you should use the sd_clock_hfclk_request() function to enable the external HF crystal, and the sd_clock_hfclk_release() function to release it.

Best regards
Torbjørn