SAADC Sampling Frequency at 200kHz

[sakib]

Hi,

Aim :- To Configure SAADC Sampling Rate at 200Khz.

SAADC Configuration :-
1. The resolution, oversample, interrupt priority and low power mode are set as 12 bit, OFF, 6 and ON respectively.
2. We have enabled 5 channels in scan mode with acquisition time 3us and pin_p select to respective pins, rest all the channel config parameters are set to default.
3. PPI is used for sampling with the buffer value set to 5.
we are able to sample all the channels at a maximum frequency of 30khz

Problem :- when we go beyond 30khz the sampling doesn't happen.
Verification :- Toggled a LED in SAADC callback function and observed the frequencies on an oscilloscope.

Please let us know what would be the reason?

Thanks,

[Karl Ylvisaker]

Hello,

Problem :- when we go beyond 30khz the sampling doesn't happen.

We have enabled 5 channels in scan mode with acquisition time 3us

The total conversion time will increase as you increase the number of channels to sample according to the formula from the SAADC peripheral documentation: 

When multiple channels are enabled, they are sampled successively in a sequence starting with the lowest channel number. The time it takes to sample all enabled channels is given as follows:

Total time < Sum(CH[x].tACQ+tCONV), x is the number of enabled channels

This is because each channel is samples separately in a sequence during the scan of all enabled channels.

Verification :- Toggled a LED in SAADC callback function and observed the frequencies on an oscilloscope.

In general this method is ok for visualizing how often a callback is triggered, but I recommend that you set the LED/pin to toggle using PPI to connect it directly to the triggering event. This will avoid the issue of the CPU being busy with another higher priority task at the time of the event generation, skewing you measurements. I do not know if this is already how you have implemented this, but I mention it here just in case for future reference.

Best regards,
Karl

[sakib]

Thank you for timely response, we are able to sample at 200Khz after modifying these parameters.

sakib said:

We have enabled 5 channels in scan mode with acquisition time 3us and pin_p select to respective pins, rest all the channel config parameters are set to default.

Enabling only a single channel on which we wanted a sampling rate of 200kHz.

sakib said:

1. The resolution, oversample, interrupt priority and low power mode are set as 12 bit, OFF, 6 and ON respectively.

Disabling low power mode for that particular channel on which we wanted a sampling rate of 200KHz

sakib said:

PPI is used for sampling with the buffer value set to 5.

Modifying the buffer value from 5 to 200.

Thanks,
Sakib