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A project log for JACA 1 & 2 Homebrew Computer

JACA - Just Another CPU Again Homebrew CPU, starting by a simple POC 4-bit CPU on circuit simulator soft. (done), then 8-bit (in progress)

andre-baptistaAndre Baptista 04/02/2018 at 17:585 Comments

Two ideas I shamelessly stole from the fantastic Gigatron TTL:

- The MCP100/101 Power On Reset: a very common issue on most projects of this kind is that when you power up the circuit, it starts in a not-so-good state (it's not random, seems to be always the same, but it's not the initial state), so we need a small circuit called Power On Reset, to trigger a short RESET pulse when the system is powered on;

- Use the HCT family of TTL ICs instead of LS: it uses just 1/5th of the LS energy. I'm already ordering the HCT equivalents of my LS chips (when available, there is no HCT version for 74181 ALU, for example). Today the computer is using more than 200mA, not counting the ALU and registers modules. The Gigatron uses less than 90mA, but doesn't has the plethora of leds my computer has (useful for debug). Most of the leds are restricted by 4.7K resistors, making them drain just 0.7mA per led. This gives enought bright for leds in indoor use (not direct sunlight).

Thanks, and see you next time.

Discussions

Julian wrote 06/29/2018 at 21:17 point

"there is no HCT version for 74181 ALU, for example" ... given how hard to get 74181s are these days (and some other stuff, like multiport register file chips, which are pretty handy too!) , I've switched my design to using a CPLD instead.  Unfortunately it means I'll (1) have to do a proper PCB for it, as the only appropriate CPLDs I could find are surface mount and (2) I'll either have to dedicate a substantial amount of circuitry to voltage conversion (because the CPLD is 3.3v, not 5v) or switch entirely to 74HC so I can run the whole system at 3.3.

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K.C. Lee wrote 06/29/2018 at 21:50 point

You can still get Xilinx XC9500XL series - 3.3V with 5V tolerant inputs.

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K.C. Lee wrote 06/29/2018 at 21:54 point

Run the whole system at 3.3V.  There are no real reasons to stick with 5V.  The 74XX series is 50+ years old.

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Yann Guidon / YGDES wrote 04/19/2018 at 21:51 point

The power-on reset circuitry is quite classic, you can find a wide variety of such chips, from the simplest and cheapest to the more elaborate, from most IC manufacturers. MAX809/TPS809/ADP809 is a classic and I lost track of who cloned who...

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Marcel van Kervinck wrote 04/19/2018 at 18:12 point

Let us know how 74HC(T) works for you on the breadboard. I can imagine you really need bypass capacitors. My breadboard version uses 74LS because I saw Ben Eater's videos. I found it couldn't care less about having caps or not.. Look for the Vulcan 74 project on 6502.org forum. This is made from 74HC as well, so yours should definitely work. In fact, these CMOS chips accept a more comfortable voltage range.

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