I need some help debugging my Klyball 600D board which I recently finished assembling. I get video out (random characters) but it does not come up with the boot prompt on reset.
I tried to eliminate all the obvious issues: power supply, wrong chips inserted, solder open or shorts, etc. The CPU has been swapped with a
known good chip.
To debug it I programmed a monitor ROM to just have the reset and interrupt vectors jump to the start of the ROM, read a few memory locations in each ROM and RAM chip and the keyboard and ACIA, and then loop back and repeat. It doesn't require any RAM to run.
It runs instructions, but not consistently. I think it is not reliably reading the ROM. After reset I see the EPROM selected and the reset vector addresses FFFC and FFFD read.
As I understand it, when reading from the EPROM, the steps are:
- CPU puts the address on the bus (e.g F800).
- MCS/ goes low.
- When 02/ also goes low, the EPROM pins 18 and 20 go low and the EPROM is enabled.
- After the access time (e.g 450 nS), the EPROM puts the data on the bus.
- When 02 goes high, the CPU reads the data.
Below is a screen shot from a digital scope. The traces from top to bottom are RES/ (CPU pin 40), 02 (CPU pin 39), the EPROM CS2 (U13 pin 18), and data line D0 (CPU pin33).
- DS1Z_QuickPrint21.png (47.71 KiB) Viewed 11385 times
Is the speed of the EPROM critical? I've tried several 450 nS 2716 EPROMs. I've also tried some 2816 EEPROMs which are 250 nS.
Do these signals look correct? Anything I should look at and report here?