nrf52840 app_pwm.c m_use_ppi_delay_workaround

[AOF]

Hi there,

I have nrf52840 development board and I'm just looking through the pwm_library example code which uses the app_pwm.c.

There, I was wanting to use 16MHz clock for the timer but it keeps changing the prescaler to 1, which means it's usnig 8MHz.

As I further investigated, this was due to m_use_ppi_delay_workaround being set to true.

m_use_ppi_delay_workaround gets set to true because of the code in app_pwm_init:

#ifdef GPIOTE_SET_CLEAR_TASKS
if (((*(uint32_t *)0xF0000FE8) & 0x000000F0) == 0x30) {
    m_use_ppi_delay_workaround = false;
} else {
    m_use_ppi_delay_workaround = true;
}
#endif

Could someone explain what this is doing?

From researching around the internet, it's trying to workaround some sort of errata but I'd like to know what errata it's trying to work around.

Thanks,

AOF

[Jared]

Hi,

This is not a workaround for a specific errata but is there to ensure the maximum frequency of the pwm library of 8 MHz. It's limited to 8 MHz as the maximum frequency you can toggle a GPIO pin is 8 MHz. You should evaluate using the PWM peripheral if you need a higher frequency. 

regards

Jared

[AOF]

Hi Jared,

Thanks for your reply. 

That makes sense that it was trying to cap the frequency at GPIO pin speed.

I just want to point out that, however, that it doesn't always limit it to 8MHz.

I had two boards; nRF52840 DK board and Rigado BMD-340 evaluation board (nRF52840 on board).

Loaded exactly the same code and got different behaviours:

nRF52840 DK board was actually setting the clock to 16MHz without any problem. 

whereas Rigado BMD-340 was limiting the clock to 8MHz.

Thanks,

AOF

[Jared]

Hi,

I find it strange that you're using the exactly same code and got different behaviors with the same IC. Did you measure the output signal? What was the frequency of it?