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https://github.com/lefticus/4bitalu
https://github.com/lefticus/4bitalu
Last synced: 2 days ago
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- Host: GitHub
- URL: https://github.com/lefticus/4bitalu
- Owner: lefticus
- Created: 2023-11-30T04:36:01.000Z (about 1 year ago)
- Default Branch: main
- Last Pushed: 2023-12-09T20:34:24.000Z (about 1 year ago)
- Last Synced: 2024-11-05T16:56:33.700Z (about 2 months ago)
- Size: 10.1 MB
- Stars: 5
- Watchers: 3
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
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README
# 4bitalu
[digikey BOM](https://www.digikey.com/en/mylists/list/D8YFZJU3KQ)
## Background
This project is derived from my ECPE 2504 class that I took at Virginia Tech. The project was submitted on March 18, 1998.
The goal of the class project was to create a "Nibble Slice ALU".
This was a project that had:
* A 4 bit register (SR Latch Flip-Flop)
* 4 bit opcode input
* 4 bit operand input
We were allowed to use 7400 series TTL logic chips
* 74153 4-bit multiplexers (x2)
* 74175 Flip-Flop
* 7483 4-bit adder
* 7400 Quad NAND (x3)
The projects were built on breadboards with jumpers, switches, chips provided by the university (no pictures of the original were taken)
Requirements:
* 'A' input is the 4bit flip-flop
* 'B' input is a 4bit dip switch
* OpCode is a 4bit dip switch
* 8 instructions were required
1. A <- A (Hold)
2. A <- shr A (Shift-Right A)
3. A <- A+B (Add B to A)
4. A <- A+B+1 (Add B to A + 1)
5. A <- A+1 (Increment A)
6. A <- A' (Complement A)
7. A <- 0 (Clear A)
8. A <- A' OR B (Imply)
8 instructions were required - but we have 16 possible opcodes. That means we ended up with an additional 8 "unintended" or "undocumented" OpCodes!
The original product requirements doc follows:
## Original Design
## 2023 Build
I decided 25 years later to try my hand at PCB design and to actually build this old project. This took 2 revisions:
1. [Rev 1](df868067a174cf4c2a293e076a1fb69d7d88c638) had several issues
* Power pins were not connected
* Clock signal was not connected
* All of the bits input to the adder were reversed because the new tool I was using (KiCad) had the pins in a different order compared to the schematics from 1998
* The register LEDs were actually connected to the output of the adder, not to the output of the Flip-Flop (so I was reading what would be, not what currently was!)
2. [Rev 2](fc047236eafd9e9970c500b91abf409b33e64aa5) fixed those problems, and did not introduce any new ones, it does work!
* Except that the pull-down resistors for the inputs are spec'd too high, so the carry-in bit to the adder was always around 1 volt.
* This should not have been a problem, those adders should not register high unless the value is at least 2v, but it was always Carrying-In an extra bit (effectively every result was the expected result + 1)
* Also many (optional) LEDs are added for observing the processes in play
3. Rev 3 is currently in process
* Adds power filtering capacitors to the VCC of the 74's chips
* Inverts the output of the clock switch (in rev2 down is high and up is low)
* Adds an LED to show the current state of the clock input
These project files are all in KiCad format, if anyone wishes to fork / build their own / test this stuff out too
## Images
## Rev 1
Rev 1 Empty
Rev 1 Front
Rev 1 Back with bodge wires
## Rev 2
Rev 2 Empty
Rev 2 Running!
