Flash writes take time to stabilize

Not a teensy bug per-se, but a good thing to be aware of with respect to the MK20DX256 controller.

Flash writes using the program_longword command to the first 16 or so bytes to a sector take a few seconds to stabilize. The data is 0xFF until an indeterminate time.

The CCIF command complete register cannot be trusted for these bytes.

I have no idea why this is, but it seems to be a bug in the FFTL controller.

My test was done at 0x2000 after a sector_erase command.
 
When using Teensy 3.6, there's also a larger 8K cache to keep in mind, and of course the same small cache memory inside the flash controller.
 
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