There are plenty of TTL CPU projects that have been created using discrete logic chips, including those that used the Harvard architecture, those based on the RISC instruction set, and even some offered in kit form for those who don’t want to go through the hassle of designing their own. Most of them have one thing in common — they are overly large, with some that require stacked PCBs in some cases. One exciting TTL project that reduces the size of the CPU layout was designed by hardware enthusiast “roelh,” with his 1 Square Inch TTL CPU, which he claims is the smallest in the world.
The TTL CPU offers eight data lines, 16 address lines, a single 64Kb memory space for programs and data, memory read/write lines, reset and clock inputs, and 5V power lines. The CPU also has some unusual features not found in most other TTL projects, including the ability to store a microprogram in Flash memory, which can be written using a Raspberry Pi as the programmer. The double-layer PCB houses only eight ICs (packed on both sides), and there is no ALU to carry out arithmetic and logic operations.
With the completion of the 1 Square Inch TTL CPU, roehl plans to use it to power his Kobold retro TTL 16-bit computer that’s currently in the design phase. roehl states the Kobold’s main features will consist of a simple ALU, some address registers, at least 256Kb of RAM, VGA video, PS/2 port, user-accessible I/O pins, and sound system, along with several others.
It will be fascinating to see the completed Kobold computer when it’s finished, those interested in following his progress can visit his project page.