Generating 50% duty cycle square waves greater than 1Mhz using PWM.

[himothy]

Hello,

I am attempting to use my nRF52810 (testing with the nRF52 DK) to generate square waves using PWM. Ideally, these square waves would operate at 8 or 16Mhz. So far I have used the base pwm_library and pwm_driver examples provided in the sdk to test this but have some uncertainty how to do it. I have been able to produce square waves with 50% duty cycles, but it seems that the frequency tops out at 1Mhz. 

When using the pwm_library, it seems as though the lowest period I can set is 1us (1Mhz). However I cannot provide a period less than 1us or the pwm output is blank. I have stepped through the code a bit to understand how it works, but unsure how to achieve a higher frequency output.

I have also done similar high level exploration into the pwm_driver library but have not gotten any further.

My questions are:

1) What is the max frequency output achievable using PWM (is it 16Mhz)?

2) How can I achieve this (or 8Mhz/16Mhz)? An example would be greatly appreciated.

[Sigurd]

1) What is the max frequency output achievable using PWM (is it 16Mhz)?

Using the PWM HW peripheral, then lowest allowed .top_value is 3, so the max frequency you can get with the PWM peripheral is 16(base or prescaler)/3(countertop) = 5.3333 MHz.

2) How can I achieve this (or 8Mhz/16Mhz)? An example would be greatly appreciated.

Using GPIOTE+TIMER+PPI, it possible to get max 8 MHz with duty ~50%. See e.g. this post:  nRF52 clock in and out 

[himothy]

Thanks for the help Sigurd. Very helpful and much appreciated.

Would just like to confirm that 8Mhz is the highest frequency achievable. There is no way to generate a higher frequency (16Mhz)?

I used the example you linked above and was able to generate a 8Mhz waveform.

Is there any way to adjust the waveform shape so that the output is more square? Additionally, can the amplitude of the waveform be adjusted?

[Sigurd]

himothy said:

Would just like to confirm that 8Mhz is the highest frequency achievable.

Yes, this is the highest you can get.

himothy said:

Is there any way to adjust the waveform shape so that the output is more square?

Maybe if you set the GPIO pin in high drive mode.

nrf_gpio_cfg(pin_number, NRF_GPIO_PIN_DIR_OUTPUT, NRF_GPIO_PIN_INPUT_DISCONNECT, NRF_GPIO_PIN_NOPULL, NRF_GPIO_PIN_H0H1, NRF_GPIO_PIN_NOSENSE);

himothy said:

Additionally, can the amplitude of the waveform be adjusted?

No, it should be around ~VDD

[himothy]

Thanks for the follow up Sigurd.

Setting the GPIO pin in high drive mode makes the waveform "more square." But a bit more of a clipped sine wave than a square wave. It also increases the amplitude too.