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About LPSPI CLK frequency on RT1051

Question asked by Massimiliano Cialdi on Apr 10, 2019
Latest reply on Apr 10, 2019 by Massimiliano Cialdi

I have problems setting the SCK frequency for LPSPI on RT1051.

here is my configuration:

I am quite sure about PLL3_PFD0 because I can indirectly measure it on CLKO2 pin:


Using SDK 2.4 APIs I set the LPSPI for a SCK running @25MHz, I debug it step-by-step to verify. The registers are:


And this should be correct, the nearest frequency is 24MHz


but on my oscilloscope I see (green is SCK and cyan is CLKO2):

As expected CLKO2 is 30MHz (so FLEXSPI_ROOT_CLK sould be 120MHz, so PLL3_PFD0 should be 720MHz)


But SCK (the green one) is 12MHz... a half of expected 24MHz


but I try to change only the mux:


Now the actual SCK should be 21.12MHz (the registers are the same values)

And on the oscilloscope I see


that is correct.


So where am I wrong? Why with PLL3_PFD0 I have SCK frequency half of what expected?


best regards