SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
The receive clock failure check circuit (see Figure 24-134) works off both the internal McASP interface clock and the high-frequency serial clock (AHCLKR) coming from the device clock generator. It continually counts the number of interface clocks for every 32 high rate serial clock (AHCLKR) periods, and stores the count in RCNT of the receive clock check control register (MCASP_RXCLKCHK) every 32 high rate serial clock cycles.
The logic compares the count against a user-defined minimum allowable boundary (RMIN) and automatically flags an event (RCKFAIL in MCASP_RXSTAT) when an out-of-range condition occurs. An out-of-range minimum condition occurs when the count is smaller than RMIN. The logic continually compares the current count (from the running interface clock counter) against the maximum allowable boundary (RMAX). This is in case the external clock completely stops, so that the counter value is not copied to RCNT. An out-of-range maximum condition occurs when the count is greater than RMAX. Note that the RMIN and RMAX fields are 8-bit unsigned values, and the comparison is performed using unsigned arithmetic.
An out-of-range count may indicate either that an unstable clock was detected or that the audio source has changed and a new sample rate is being used.
In order for the receive clock failure check circuit to operate correctly, the high-frequency serial clock divider must be taken out of reset.