4bitalu

4bitalu

digikey BOM

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 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 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!