PSTCLK pin of Freescale MCF5274CVM166

[aworthi]

We are in the middle of EMC testing and have noticed a signal radiating from PSTCLK pin of the
Freescale MCF5274CVM166 (BGA256) microprocessor.  My understanding is this
CLK should only be active during BDM device programming.  However, when the
DBM device is not connected, we are measuring a 150 Mz signal with amplitude
3-4V (p-p).  Is there some why to disable this constant oscillation from
the PSTCLK in the software?  This is resulting in radiated emissions
levels above the allowable limit.

[TomE]

Everybody runs into that one on a lot of these chips.

There are two ways to fix this. Search this forum for "PSTCLK" or read the manual again and again and again until you spot the trick.

You don't like the primary function of the PSTCLK pin, so check in "Table 2-1. MCF5274 and MCF5275 Signal Information and Muxing (Continued)" to see if it has a secondary function. It does. Enable that secondary function (JTAG TCLK which is now an Input).

Searching finds this previous answer that says to do this:

https://community.freescale.com/message/60946#60946

The simple fix is to put a pullup on JTAGEN. When you want to use the debugger, make sure plugging the debugger in pulls this pin to ground. The easiest way is to re purpose one of the ground pins on "Figure 31-41. Recommended BDM Connector" to ground JTAGEN, or use the "Developer Reserved" pin to do this and then ground that pin in your debug cable or adapter.

You should also write the PSTEN bit in "9.3.3.1 Chip Configuration Register (CCR)" to zero to stop the other debug pins from radiating as well.

Tom