Timer with a constant and stable period

Question

Hello! 
 
 I am trying to sample a digital signal with 12-bits (In a later stage I will need to sample two 12-bits signals). At the moment I am storing the recorded signal in an SD card, but the final goal is to send it via BLE. For this purpose, I was using an interrupt triggered by a timer, but the signal is not being correctly sampled: 
 
 Input:

Output:

I configured a GPIO pin to go high when entering the timer interrupt routine and low when leaving it. The measured timing profile is shown next: 
 
 As seen, the period is not stable. Searching here in the forum, I found that for a timer with a constant and stable period we should use PPI. But I don't see how to configure the PPI to read the GPIO_IN register. Could anyone guide me on how to do it? 
 
 Alternatively, is there any other way to perform a periodic task with a constant and stable period?

Karl Ylvisaker · Accepted Answer

Hello, You are correct that you will need to use the PPI peripheral to perform periodic sampling. The reason for this is that when you use the CPU to trigger the sampling you never really know when the task will be triggered - since the CPU might be busy with higher priority tasks. This is especially the case when you have enabled the SoftDevice, since the SoftDevice will take control of the CPU whenever it has a timing-critical deadline to meet, such as an upcoming BLE event. The PPI peripheral allows you to connect an event to a task trigger, effectively having the task be triggered without any CPU intervention. This is what allows you to do periodic sampling. You could see this demonstrated in the SAADC peripheral example , in which PPI is used to connect the TIMER's CC event to the SAADC SAMPLE task - triggering a sampling every time the timer event occurs, completely without CPU interaction. You could also take a look at this example by my colleague which demonstrates how to perform periodic sampling and send the measurements over a BLE connection afterwards . Best regards, Karl

Timer with a constant and stable period

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