1-Bit CPU DIY Kit Does One Operation Very Well, It Blinks an LED
Clever CPU design fully utilizes four 7400-logic chips.
One measure of a computer's bit width is the size of the CPU's registers. For example, several early 8-bit home computers used an "8-bit" 6502 processor. And while computers later came with 16-, 32-, 64-, and more bits, a Japanese electronics maker known as naoto64 went the opposite direction with the challenge of building a 1-bit CPU with logic chips. This computer might be the pinnacle of low-performance computers because it is only capable of two instructions, resulting in a blinking (or steady) LED.
The 1-bit CPU design uses four 7400-series logic integrated circuits (ICs) to implement all the basic CPU blocks on the simple printed circuit board (PCB). The PCB also contains four dip switches, USB-C (for basic power), four LEDs, and a push button.
At the core, a quad-NAND gate forms an XOR-based arithmetic unit. Of course, this design means the 1-bit CPU only has a single math operation: 1-bit ADD! However, it also has a JMP instruction that allows repeated use of the exclusive-or. Four dip switches make up the four-bit read-only memory (ROM.) Three possible programs control the LED programs on, off, or toggling.
Why is this a single-bit CPU? One element of the dual 7474 flip-flip forms a 1-bit data register, and the other forms a program counter by using the output and its complement. Three gates of a 7414 (Schmitt Trigger) inverter with passives create an approximate 1 Hz clock to drive the 1-bit CPU.
The spirit of this project design reminds us of the MyNOR single-board computer. Dennis Kuschel built a floating point calculator around a CPU made of only a single NOR gate in that project.
The design is clever even if the 1-bit CPU does not build a practical computer. Its schematic is worth looking over. One of the more impressive aspects of the design is that it fully utilizes all the elements in the four logic ICs!
The Japan-based online store Switch Science sells the 1-bit CPU kit for 2500 Yen. You can also download the KiCad design files and Gerbers from naoto64's GitHub repository. All the documentation is in Japanese, but machine translation services do a respectable job translating them.