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Using the CMU on the MPC5744 for IRCOSC Measurement

Question asked by Ahmed Soubky on Sep 20, 2016
Latest reply on Sep 21, 2016 by Petr Stancik



According to the MPC5744 reference manual ( Rev. 5, 04/2014 ):


I am using CMU_0 for the IRCOSC measurement and the reference clock is the XOSC.

When I write the value 0x64 in CMU_MDR[MD] register, I get the correct frequency which is, as per the above formula, 16 MHz given that the XOSC frequency is 40MHz.

When I write any other value in CMU_MDR[MD] register, I get incorrect values of the IRCOSC frequency. Do you have any explanation for this ?

Note: Also according to the reference manual,

MDR field is R/W and from my understanding we write into this register the number of clock cycles during which IROSC frequency measurement is done. Is that correct ?