Apple 1 Replica Video Section Not Working

3 replies [Last post]
Offline
Joined: Jan 29 2016
Posts: 4

Hi,

I'm trying to test the video section of the Apple 1 Replica but it doesn't seem to work. I was able to compile the source code after some minor modifications because the original code won't compile. I have the video section on a protoboard and I'm not seeing output (Q pin5) on the 74HC74 frequency divider. I noticed that it takes the clock signal directly from the XTAL of the Atmega8, and this signal is about 100mVp which is way too weak for the 74HC74 to work. Also this signal is close to a sine wave and I think the 74HC74 should have a square wave as a clock. Datasheet says VIH is about 2V, which means that it will never work. The circuit I'm testing is the one on page 235 of the "Apple 1 Replica Creation" book.
I'll appreciate any comments.

Thanks!

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.
Offline
Joined: Jan 29 2016
Posts: 4
Re: Apple 1 Replica Video Section Not Working

I fixed this problem. I had to enable CKOPT fuse to make it work. But now the line duration is way too off, I'm getting 280uS when it should be about 64uS for NTSC. Does anyone have the fixed version of the code?

Offline
Joined: Oct 9 2011
Posts: 1044
Re: Apple 1 Replica Video Section Not Working

I'm interested in what you are doing? Are you hand building a replica-1 according to the "book"?

It sounds like you are. Did you point to point solder? or did you wire-wrap. Love to see some pics....

Cheers,
Corey

Offline
Joined: Jan 29 2016
Posts: 4
Re: Apple 1 Replica Video Section Not Working

Hi, yes, I'm trying to build the Replica 1 according to the book. I'm using the Atmel uControllers for video and keyboard because the other uControllers are imposible to find here. I now have the video section working on a protoboard. I had to change the Atmega8535 that I was using to an Atmega8A because the 8535 has 512 bytes of SRAM while Atmega8 has 1Kb so that was causing the program to read out of boundaries and crash. Source code and schematics have issues that won't make it work if you build them exactly as they are.