PowerPC MPC8245 JTAG

Question

I'm working on an MPC8245, a 603e family processor. I am using a BDI2000 JTAG device to debug the startup code.

There is an odd problem I run into.

Single stepping runs just fine until the configuration register, MCCR1 (0xF0) is written to. After the data value is written, all of ROM now reads back 0x00000000. Single stepping cannot continue.

The MEMGO bit is NOT set as part of the data written to MCCR1.

Is this a limitation? or am I doing something wrong?

Answer 1

It appears that it is a limitation of JTAG to read memory with MEMGO bit low.

It appears that the BDI2000 JTAG device requires that the MPC8245 memory controller be active in order to read from ROM or RAM, even though the documentation would lead one to think that MEMGO bit is for SDRAM only and not ROM.

If a configuration for the BDI2000 is created with no [INIT] section, the default value of the MEMGO bit is low on power up, the BDI2000 cannot read ROM. All zeros are read.

Thus any time the MEMGO bit is low, the JTAG device cannot be used to single step or read memory.