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1 Square Inch TTL CPU

A microprocessor built with TTL chips. In one square inch.

What would be cool for the square inch contest or Hackaday prize…. How about a homebrew CPU with TTL chips ?

This is the smallest TTL Homebrew CPU in the world !

Like the old CPU’s of the eighties, it has 40 connections, an 8 bit databus, and 16 bit addressbus. But it also has some very unusual properties….
Interesting is, that it occupies less area than an old 40-pin DIP processor…

The Square Inch processor is fully working ! A digital clock, driven by the square inch cpu, was designed to prove this.
This project is very much a race against the clock. The idea for this project occurred to me after the first week of september, leaving three weeks before the end of the Square Inch Contest. Since it involves processor architecture, hardware and pcb design, and software, this is hardly possible in three weeks when you also have a job.

The project was not finished in time for the square inch deadline, perhaps the Hackaday prize 2019 will bring more luck.

Well let’s start with the usual characteristics:

- 8 data lines
- 16 address lines
- Single 64kB memory space for program and data
- Memory Read and Memory Write lines
- Reset and clock inputs
- 5 volt power lines

- Zero page addressing
- Indirect Zero Page addressing
- Immediate addressing
- Load, store and compare instructions
- Stack
- Conditional branches
- Subroutine calls
- Memory-mapped I/O
- Microprogrammed

Two rows of 20 pins each, with 0.05 Inch (1.27 mm) pin distance
Microprogram is in FLASH memory and can be written with a Raspberry Pi as programmer
Programmer’s registers are in RAM
There is NO ALU
Only 8 IC’s, on both sides of a 1 square inch, 2-layer pcb.
The registers are in RAM. That has been done before (see TMS9900). This will not give you a speed devil, but it is needed to fit the design in one square inch. Another thing left out of the CPU for this reason is…… the ALU. ( I do not intend to connect the square-inch 4 bit TTL ALU to this cpu ).”

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