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 8 bit counter for breadboard computer 
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Joined: Fri Aug 01, 2014 3:00 pm
Posts: 16
Hi everyone, I started working on a little TTL computer project that I am trying to keep relatively small. So far, I'm using 74HC193s for the program counter. Is there an 8 bit counter in HC that would save some space over two 4 bit counters? There are lots of great ones in LS but 74HC593 is all I can find in HC and it doesn't seem to be made any more. Even better if it has an /OE pin.


Sat Apr 13, 2019 4:17 pm
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Joined: Fri May 05, 2017 7:39 pm
Posts: 18
The only small 8 bit counter I know is the 74HC(T)393 :( . But that one is not suitable for a PC - unless you never need to branch or jump :geek: .


Sun Apr 14, 2019 9:21 am
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Joined: Sat Feb 02, 2013 9:40 am
Posts: 795
Location: Canada
You might want to consider a counter with synchronous inputs for count and load signals like the '161 or '163. I believe the '193 uses asynchronous inputs. However the counters can only count up. The program counter shouldn't need to count down.

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Robert Finch http://www.finitron.ca


Sun Apr 14, 2019 3:15 pm
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Joined: Fri Aug 01, 2014 3:00 pm
Posts: 16
I found some 161s from a few years ago after I had ordered the 193s. What is the advantage of synchronous inputs for those pins? As I have it now, the lower 4 bits of my microcode ROM are driven by a 193 that increments every cycle. The rising edge of the clock increments the 193 and the falling edge is inverted so that the microcode ROM is latched out on the falling edge by a 74HC574. My thought was that this would give the ROM half a cycle to settle. With a 161, I wouldn't need to latch out the signal that drives the PC counters since what comes out of the ROM doesn't matter to the 161 until the clock rises, right?


Mon Apr 15, 2019 5:52 pm
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Joined: Fri Mar 22, 2019 8:03 am
Posts: 42
Location: Girona-Catalonia
I have been looking at the 74xx867 and 74xx869. They are synchronous, presettable, 8-bit up/down counters that seem to have everything that is needed.

http://www.ti.com/lit/ds/symlink/sn74as867.pdf

I have them on my list for a possible implementation of the Stack Pointer register for my 74xx based CPU. Unfortunately, they only seem to be manufactured for the AS and ALS hardware families, so I'm not sure if they are still usable on our respective projects.


Tue Apr 16, 2019 7:24 am
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Joined: Fri Aug 01, 2014 3:00 pm
Posts: 16
Ya there is a long list of chips that look like they would work but none of them are HC it seems.


Tue Apr 16, 2019 12:53 pm
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