Count
to 32
External
AHCLKX
pin input
Sync to
system
clock
8−bit
counter
Clear
Count
Prescale
/1 to
/256
McASP
clock
(A)
system
4
XCLKCHK[3−0]
XPS
Load
XCLKCHK[31−24]
XCNT
8
XMIN
XCLKCHK[15−8]
8
XCNT<XMIN?
8
XCLKCHK[23−16]
XMAX
8
Counter>XMAX?
8
OR
True
True
XCKFAIL
XSTAT.2
Set
Interrupt
mute
AND
XCKFAILSW
XCLKCHK.7
Switch to internal
AHCLKX1
reset McASP
transmitter,
enter underrun
(D15 mode only)
send BMC 0’s
when clock is bad
external
Functional Description
22.3.10.4.6.2
Transmit Clock Failure Check and Recovery
The transmit clock failure check circuit (
) works off both the internal McASP system clock and
the external high-frequency serial clock (AHCLKX). It continually counts the number of system clocks for
every 32 high rate serial clock (AHCLKX) periods, and stores the count in XCNT of the transmit clock
check control register (XCLKCHK) every 32 high rate serial clock cycles.
The logic compares the count against a user-defined minimum allowable boundary (XMIN), and
automatically flags an interrupt (XCKFAIL in XSTST) when an out-of-range condition occurs. An out-of-
range minimum condition occurs when the count is smaller than XMIN. The logic continually compares the
current count (from the running system clock counter) against the maximum allowable boundary (XMAX).
This is in case the external clock completely stops, so that the counter value is not copied to XCNT. An
out-of-range maximum condition occurs when the count is greater than XMAX. Note that the XMIN and
XMAX 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 transmit clock failure check circuit to operate correctly, the high-frequency serial clock
divider must be taken out of reset regardless if AHCLKX is internally generated or externally sourced.
Figure 22-31. Transmit Clock Failure Detection Circuit Block Diagram
A
Refer to device data manual for the McASP system clock source. This is not the same as AUXCLK.
3812
Multichannel Audio Serial Port (McASP)
SPRUH73H – October 2011 – Revised April 2013
Copyright © 2011–2013, Texas Instruments Incorporated