A one-bit processor explained: the vintage MC14500B #VintageComputing @kenshirriff
The Motorola MC14500B is a 1-bit processor introduced in 1976. While a 1-bit processor might seem almost useless, it was marketed as an Industrial Control Unit for applications that made simple decisions based on Boolean logic, for example, air conditioning, motor control, or traffic lights.
The MC14500B has an unusual architecture, making it more of a building block than a complete microprocessor. In particular, the chip doesn’t include any support for memory or addresses; it didn’t even have a program counter. The program counter, instruction fetches, jumps, subroutine calls, and I/O needed to be implemented with external circuitry. This is a key reason that the chip was so simple. (The other reason, of course, was that it only supported one bit.)
Since the MC14500B was designed for industrial control applications, you’d expect it to be a microcontroller, but it’s the opposite of a microcontroller in many ways. A typical microcontroller is a computer-on-a-chip including RAM and ROM, with strong I/O support, providing a single-chip solution. The MC14500B, however, requires multiple external chips to make it usable.
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We used these as part of our electronics unit when I did my Phystcs degree back in the mists of time. They were a fun introduction to the nitty gritty of microcontrollers.
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