Maximum sampling rate on nRF51822 digital I/O?

[sd]

My application involves driving a digital-output (PDM) MEMS microphone at 1 -- 3.25 MHz with a clock generated on a GPIO.

devzone.nordicsemi.com/.../

The output of the microphone is a digital signal with data at the rate of the supplied clock. I'd like to sample this I/O at the highest rate possible; is this a scenario I can achieve with the nRF51822? (Assume a 16 MHz input clock to the nRF51822.)

[JohnBrown]

I'd say you might need to drop down into assembly language for this - I assume you're not trying to do this while a soft device is running? If so, then I think you're doomed.

[sd]

Yes, the SoftDevice will be part of the application. By 'running' do you mean transmitting? If so, then no, I know that that isn't possible. Otherwise, can you elaborate why it's a 'doom' scenario? (I'm willing to sacrifice quality to make it work.)

[JohnBrown]

Even if you're not transmitting, the SD will inevitably be using some CPU time, which is likely to interfere with what, essentially, is a synchronous clocked data stream. On the other hand, I don't know quite what it is you are trying to do. It could be that, if you are only interested in detecting a loud noise, for example, you might be able to subvert the SPI block into reading the 8 MSBs of the value.

[Stefan Birnir Sverrisson]

Hi

I am not very sure on what you intend to do but if you want to sample analog input signal then the internal nRF51 ADC has the potential to do that. However, the highest sampling frequency of the ADC is 50kHz with the lowest 8-bit resolution. When you use the softdevice, the sampling frequency is further limited, as JohnBrown has already pointed out. You could read more about the sampling limitations here.

[Charles Manning]

The soft device needs the CPU for its own function. There are times when the soft device can hog the processor for, if I recall correctly, over 1 millisecond. During this time you have no CPU - not even interrupts.