Saturday, October 24, 2009

Clock Signal Fixed...

Note the square wave on the scope... I finally have a clock input signal on pin 37 of the 6502 processor. I found that replacing the Philips 74HCT04N with a National Semi 7404N (D12 in the schematic) gave me a square wave. I found this problem by first checking pin 37 on the 6502, then checking the schematic, and seeing that the output from the crystal clock circuit is sent to pin 9 on the 74175 (C13 in the schematic). I then checked pin 9 with my scope and what I saw was certainly not a square wave, so I immediately thought this chip must be bad and replaced it (the 7404). After powering down, then back up again, I was pumped to see my expansion card LEDs flashing away as they should be! Finally. In the image above, you can see the square wave generated while I have my left hand holding the oscilloscope probe on pin 37, and the camera in my right.
The video is still not working, but it is late (5:30 AM) so I will take a closer look at the video issue "in the morning".

-- 9-dec-2009 UPDATE: The video section needs a 7404. 74HCT04 and 74LS04 will not work. I replaced the 7404 with a new one and now have video.

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