Hi - I am hoping that someone could give some pointers as to how and attack a reset (double bus fault) problem with an embedded
68020 board that was originally designed by HP in a piece of test equipment that was mfg back in 1996. Since HP no longer supports this equipment, I'm on my own. I do have the schematics, however.Problem: There are times when the unit is powered up, that the 68020 will not reset properly and simply locks up. Upon investigation, I have determined that a double bus fault has occurred. There is a diagnostic led on the HALT line that shows this to be the case. This lockup occurs randomly on power up. I have two of these units, one unit exhibits this problem where the other does not. Both units are running the same ver of firmware that is mature. I'm convinced that the problem must be hardware related.
What I've done: The unit uses a TI supervisory reset part (TL7705) that has a reset delay of 120mSec (I'v verified this). I've used a digital storage scope to capture the rise time of the 5V power source, the reset line to the
68020, and the 68020 clock. All three signals behave as expected. The osc starts up within about 5 mSec, the 5V supply within about 1mSec and the reset pulse is approx 120mSec. However, about every dozen or so power cycles, the processor will double bus fault and lock up. The osc, by the way, is at 19.6MHz.Originally, I thought that the power supply was coming up in a strange manner or the TL7705 was malfunctioning, but I don't see it.
I need someone with 68000 HW design experience to give me some pointers as to how and solve this. Any help would be appreciated.
Thanks Jim