Application Note 002: Scanning Motorola MPC8xx processors

[The following was written for an MPC860, but applies to all MPC8xx processors, though some of the bit positions may be off or the manual references may not be the same]

Universal Scan works great on these processors, but there is a trick you have to be aware of.

The Power PC family of processors doesn?t naturally come up in a JTAG compatible mode.  You have to force them into JTAG mode.

JTAG shares pins with the BDM port on the processor ? you get either BDM or JTAG ? not both.

During reset, the 860 reads a vector off of the data bus to determine what its startup configuration should be.

Look in the data book under Hard Reset Configuration Word (it was in Chapter 12.3 last time we looked in the MPC860 manual).  You'll find that bits 11,12 control the JTAG mode.  The processor defaults to 00, which is BDM mode.

You want that to be a "0,1" so you need to make sure data bit 12 is high during reset.  Most 860 designers know about this and provide that control as an option via a header/jumper in the schematic.

If you do that, you'll find Universal Scan works great with the 860 - we have a lot of customers that use it exclusively for that processor!

NOTE: One other caveat: Firmware folks have a tendency to clear all configuration registers to a known state on boot.  Make sure the firmware folks don't reset or clear bits 11,12 in the SIUMCR register. That will force the processor BACK INTO BDM MODE!

Motorola has a great AppNote discussing this: AN2387/D - last time we checked it was here.

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