MCU dead due to bad code?

Could it be that bad code in the flash is crashing and locking up the MCU which also hangs the JTAG? Try forcing the chip select on the flash on power up so that the code in the flash doesn't get accessed. Or try to get the JTAG to connect as soon as you release the reset - you might get lucky.

Actually i didn't find anything about this MCU that could cause such a behaviour. The fast reset and jtag approach didn't work But now i'll disconnect the Flash and pray.

Even without the flash the MCU doesn't respond to JTAG. I'm affraid the chips are toasted, but how. It's very strange :((.

I am not familiar with the OKI processors, but is it possible that there is a clock configuration register in flash that is being mis-configured? I have seen clock and bus configuration registers on other processors that would cause this kind of problem. Another thought would be some kind of security bit.

Good Luck, Bob

You don't say if you tried going back to code that you know worked, or if you tried a power-cycle-wait-hard-reset ? It is believable that a CPU would have a JTAG disable that does not recover on RESET ( surprising how many chips have reset pins that are better called reset request ... )

Otherwise you may have stumbled onto a undocumented HCF opcode ?

-jg

It seems unlikely but it is possible. I had a touchscreen that turned its toes up at the same time as I made a tweak to it's driver software, purely by coincidence. Cue many hours of debugging a working driver.

I tried a power-cycle-wait-hard-reset. The wait part was several hours.

According to the documentation there is no possibility to disable JTAG (only by toasting the chip , as i did *grin). Of course there could be something with the ROM timig, but JTAG should always work, it even works without any extern ROM/RAM etc. Some processors have JTAG disabling flags, but this type don't. Even if i connect BSEL1 to GND (enabling the internal BootROM) i can't connect with JTAG. In that case the external ROM shouldn't cause any problem.

Hi!

The problem is solved! In my code the RMPCON register is set to 9 switching the DRAM bank to zero address. But at this point the SDRAM controller isn't initialized yet. This seams to disturb the MCU and even the JTAG interface gets locked. By activating the internal BootROM (BSEL1=0) the malicious code is disabled and it's possible to connect via JTAG. I tried that approach before, but somewho it didnt't worked. I can only guess that the connection of BSEL1 to GND was not right. OKI Germany was very helpfully and actually they found the solution. After checking the BSEL1 setting i was able to reactivate all my "toasted" boards :)).

Thanks to all who tried to help!