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 8 bit counter for breadboard computer 
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Joined: Fri Aug 01, 2014 3:00 pm
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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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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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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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Sun Apr 14, 2019 3:15 pm
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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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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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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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Joined: Thu Apr 04, 2019 6:20 pm
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Druzyek wrote:
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.


i've seen this posted a dozen times or more. it's a common issue. i've done some "discrete" builds before such as a logic recreation of the 74LS181 ( https://hackaday.io/project/25596-mega-one-8-one ), so i was considering doing a LVC version of the 74xx461 as a discrete build that could be dropped into a design. it would be a little bigger than a real 461 of course, but would be logic complete...

as a side note, one of the alternatives that a lot of people use is a GAL16V8 chip to replace the two 4-bit counters... here is one of those as an example - https://hackaday.io/project/164212-yatac78-the-www-ttl-computer/log/162273-cpu-state-machine


Tue Apr 23, 2019 1:11 am
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The Lattice GAL16V8 was apparently discontinued and it's marked as obsolete or unavailable by the usual electronic component providers. Fortunately, ATMEL produces the ATF16V8 which unless I'm mistaken is an updated alternative. I believe it's not 100% compatible with the GAL though. More informed guys may clarify that point.


Tue Apr 23, 2019 7:08 am
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joanlluch wrote:
The Lattice GAL16V8 was apparently discontinued and it's marked as obsolete or unavailable by the usual electronic component providers. Fortunately, ATMEL produces the ATF16V8 which unless I'm mistaken is an updated alternative. I believe it's not 100% compatible with the GAL though. More informed guys may clarify that point.


indeed, the ATF16V8 is closer to being a PAL, but technically it's a small CPLD. it is programmed using the same CUPL language as GAL/PALs. they are pretty easy to purchase and can be found surplus pretty easily. i purchase 10k of the ATF16V8 as part of an auction a few years ago, so i have a more "than a few" available for my personal use...


Tue Apr 23, 2019 4:08 pm
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prpplague wrote:
joanlluch wrote:
The Lattice GAL16V8 was apparently discontinued and it's marked as obsolete or unavailable by the usual electronic component providers. Fortunately, ATMEL produces the ATF16V8 which unless I'm mistaken is an updated alternative. I believe it's not 100% compatible with the GAL though. More informed guys may clarify that point.


indeed, the ATF16V8 is closer to being a PAL, but technically it's a small CPLD. it is programmed using the same CUPL language as GAL/PALs. they are pretty easy to purchase and can be found surplus pretty easily. i purchase 10k of the ATF16V8 as part of an auction a few years ago, so i have a more "than a few" available for my personal use...

Wow, 10k units seem a lot to me!. I found that the chip is available from mouser at around 1€  (or possibly 1$) even on single units base, so it looks certainly interesting. I may have some use of it in the future, so I would like to know more about it, but that's out of my current expertise.
Please could you provide me some starting points, such as the minimum set of tools required to program it (software?, hardware?). Please note that I'm proficient in software development but a complete novice on hardware. Thanks in advance.


Tue Apr 23, 2019 6:45 pm
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joanlluch wrote:
prpplague wrote:
joanlluch wrote:
The Lattice GAL16V8 was apparently discontinued and it's marked as obsolete or unavailable by the usual electronic component providers. Fortunately, ATMEL produces the ATF16V8 which unless I'm mistaken is an updated alternative. I believe it's not 100% compatible with the GAL though. More informed guys may clarify that point.


indeed, the ATF16V8 is closer to being a PAL, but technically it's a small CPLD. it is programmed using the same CUPL language as GAL/PALs. they are pretty easy to purchase and can be found surplus pretty easily. i purchase 10k of the ATF16V8 as part of an auction a few years ago, so i have a more "than a few" available for my personal use...

Wow, 10k units seem a lot to me!. I found that the chip is available from mouser at around 1€  (or possibly 1$) even on single units base, so it looks certainly interesting. I may have some use of it in the future, so I would like to know more about it, but that's out of my current expertise.
Please could you provide me some starting points, such as the minimum set of tools required to program it (software?, hardware?). Please note that I'm proficient in software development but a complete novice on hardware. Thanks in advance.


hehe yea i often by pallets of SMT components at auction. this one pallet happened to have around 10k of the ATF16V8 parts in it. i made a little hex-decoder project using them, but i'll never use them all... ( https://hackaday.io/project/163274-4-bit-full-hex-decoder )

the HDL code is written using a language called CUPL. the windows based tool is called WinCUPL and can be downloaded for free here: https://www.microchip.com/design-centers/programmable-logic/spld-cpld/tools/software/wincupl
once you have you code written, you can test it by flashing it to the ATF16V8 using the inexpensive (around $50USD) TL866 series eeprom programmer. i have a hackaday log entry about my first experiences with this programmer: https://hackaday.io/project/158510-ddl4-cpu/log/147152-adventures-in-eeprom-land

good luck!


Tue Apr 23, 2019 7:02 pm
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Do you have an idea of how much complexity you could squeeze into one? The product page says 250 gates, which seems like a lot. Can you implement flip-flops for example?


Tue Apr 23, 2019 7:07 pm
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Druzyek wrote:
Do you have an idea of how much complexity you could squeeze into one? The product page says 250 gates, which seems like a lot. Can you implement flip-flops for example?


yea, the unit does have a flipflop on the output of each pin, and it does have enough gates to create flips from the gates. i've not pushed the limits of what it can do, as the hex decoder is pretty simple. i have the code listed on the project page if you are interested in looking at it: https://cdn.hackaday.io/files/1632746985514816/gal-7seg.pld

side note, there are a lot of good CUPL tutorials out there to start from...

these units seem to be great for replacing blocks of "glue logic" as CPLDs are. you don't want to use them for more complicated stuff that a real FPGA would be used for...


Tue Apr 23, 2019 7:17 pm
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Hi prpplague,

Thank you very much for your info and pointers. That's exactly what I needed to know.


Tue Apr 23, 2019 7:36 pm
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joanlluch wrote:
Hi prpplague,

Thank you very much for your info and pointers. That's exactly what I needed to know.


glad to help when i can! let me know how it works out for you! try documenting your project at hackaday.io !

Dave


Wed Apr 24, 2019 4:50 am
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