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Playing around with SP0256

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Posted (edited)

Hello there,


I am currently working on a simple speech synthesizer circuit for my C64. It works with the SP0256 allophone chip. I didn't created anything, I just re-created the board presented in the "Commodore 64 interfacing blue book" by V. J. Georgiou. A full project with ready-made pcb is presented. Here are the schematics :




As you can see, nothing complicated. The code is also available in the book, is more or less POKE commands to give the circuit the allophone index.


Unfortunately, it doesn't works (weird sound). I manage to probe some points in the circuit with my scope:

-the reset goes from low to high at the C64 boot,

-some data is incoming on PBx data pins when the program is running,

-when powering the computer on, even if nothing is "asked", there is some garbage signal on the "Dout" pin of the IC. (Strange ?). When playing the allophones, I can hear (and see) some variations in the noise, but nothing clear.

-my crystal is a 3.27 one, didn't manage to find a 3.12. As explained in the project article, it should only make the voice "higher". I didn't have a nice TTL clock signal on pin 28 or 27, but I think it's normal. (I see high speed "vibrations")


My opinion is to think that the SP256 is damaged. I found it on ebay. Looks "new" but I'm not sure at all if it may be already used. Maybe I'll try to buy a new one from another seller...


Is someone here familiar with this chip ?


Thanks !

Edited by Yoruk

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No ideas ?


Meanwhile, here are some pictures of my prototype. I use a home made software that analyze and convert PCB pictures designs into a Eagle-CAD project. Holes and via are automatically detected, it also computes signals names continuity. 🙂


Original design from the book :




And my prototype.





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