Record data to SD card at a high sampling rate
This sketch acquires data and stores it on an SD card. In my tests using a Teensy 3.6 and a SanDisk Ultra 16GB SDHC micro SD card, I was able to record an analog pin at 25 kHz.
It relies on the beta version of the SdFat library written by Bill Greiman, which is available at https:/greiman/SdFat-beta. I have tested this sketch with revision ffbccb0 of SdFat-beta. This code was inpired by SdFat's LowLatencyLogger and TeensySdioDemo examples. It uses the same binary file format as LowLatencyLogger, but with a bigger block size.
Here is how the code works. We have four buffers that are used to store the data. The main loop is very simple: it checks to see if there are any full buffers, and if there are, it writes them to the SD card. When is the data acquired? Data is acquired in the function yield(), which is called whenever the Teensy is not busy with something else. yield() is called by the main loop whenever there is nothing to write to the SD card. yield() is also called by the SdFat library whenever it is waiting for the SD card.