MPC8280 datasheet(929/1386 Pages) FREESCALE

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Multi-Channel Controllers (MCCs)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

29-39

It is not possible to determine exactly which channel's FIFO experienced the underrun, therefore a GUN

is considered a global event affecting that entire MCC. Following the assertion of MCCE[GUN], the MCC

stops transmitting data on all channels and all ones are sent instead. The MCC must be reset either through

an MCC RESET command or a CPM RESET after this error and then the MCC may be reinitialized.

There are several possible causes for an MCC GUN error:

•

Glitching on the TDM clock. Refer to Section 29.8.1.2.1, “TDM Clock.”

•

Synchronization pulse (sync pulse). Refer to Section 29.8.1.2.2, “Synchronization Pulse.”

•

Misprogramming of SIRAM. Refer to Section 29.8.1.2.3, “SIRAM Programming,” and

Section 29.8.1.2.4, “MCC Initialization.”

•

CPM bandwidth. Refer to Section 29.8.1.2.5, “CPM Bandwidth.”

•

CPM priority. Refer to Section 29.8.1.2.6, “CPM Priority.”

29.8.1.2.1

TDM Clock

Glitches on the TDM clock may be interpreted as an overwhelming number of clocks that the SI may try

to process (amongst other anomalous behavior), thus draining the MCC FIFOs on that TDM much too

quickly. This results in a GUN.

29.8.1.2.2

Synchronization Pulse

A TDM frame length, as programmed in SIRAM, that is shorter than the actual gap between sync pulses

may cause an underrun condition (in other words, if SIRAM programming hits an entry with

SIxRam[LST] set and then encounters dead time before the next sync pulse on the line). This may result

in anomalous behavior in the SI and therefore an underrun condition.

To avoid these cases, pad out the SIRAM programming with “null entries”, entries with no CPM peripheral

specified (MCC=0 and CSEL = 0000 in SIRAM entry) at the end of the SIRAM programming. Make the

null SIRAM entries represent the appropriate amount of time so that the SI frame length matches the gap

between sync pulses.

29.8.1.2.3

SIRAM Programming

Failure to follow SIRAM programing guidelines can result in erratic behavior or possibly GUN errors. The

following is a list of common programming errors:

•

Failing to set “LAST” in the final entry of an even-numbered total of SIRAM entries (i.e. there

cannot be an odd total of SIRAM entries for a TDM)

•

Programming an SIRAM entry to use an un-initialized MCC channel (or if the MCC as whole has

not been initialized with the appropriate “INIT TX RX” commands).

•

Programming an SIRAM entry to use an MCC channel that is not assigned to that particular TDM.

•

Using a channel too often (which would overload the FIFO) without going to superchannels.

•

Turning on too many MCC channels at once in the SIRAM (we suggest not to enable more than 32

channels per TDM frame's worth of time when you are first programming the SI).

•

Other programming errors, such as incorrect values in the SIRAM entries, etc.

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