Question on timings

What have you built or do you want to design to go with your replica 1

Question on timings

Postby Kallikak » Jun Tue 23, 2009 7:24 pm

I've just tried a little circuit on my Replica 1 using a 74LS377 and some LEDs to test something else I want to build. I tied the 377 enable to /A000 from the Replica 1 expansion - this signal goes low when I write to any Axxx address. Then I simply connect the data bus from the 6502 to the inputs of the 377, and the outputs to some LEDs. My question is what is the easiest way to do the rising edge trigger to latch the data to the LEDs? After studying some timing diagrams I thought using phi1 would be ok - when writing to A000 it seemed to go high only when the /A000 signal was low and the data safely on the databus, but although the data settles properly I am noticing an initial flash. For the LEDs this is not a big deal, but for my real circuit this will be a problem. What I would like to avoid is having to gate phi2 or phi1 with r/w for example, since that would mean adding an additional chip, but of course if I have to then I have to.

Ken
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Postby vbriel » Jun Wed 24, 2009 7:23 am

Without timing it with r/w I would just run the clock pin 11 to ground and enable pin 1 to A000 decoding. If you try and time it with phase 1, you will run into a problem if you don't time it correctly.

This setup will only lightly flash the outputs of the 377 because it isn't a latch, and if you just write to that address, it is only sending that data for a few ms.

If you want the data to stay on the output until you clear it, then use a LS245 or LS244. LS245 is bidirectional or unidirectional if you just want it as output.

Vince
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Postby Kallikak » Jun Thu 25, 2009 12:00 am

Thanks for the suggestion Vince. I'll try with a 244. When enable is high, is the output stable for address decoding say? (I haven't played with tri-state outputs before.)
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