COSMAC Elf Calculator Gets New Firmware

Everyone remembers their first. Their first CPU, that is. For many of us, it was the RCA 1802 thanks to the COSMAC Elf articles that ran in Popular Electronics. The later versions of the chip family were much better but were never as popular, but the 1805 did find its way into a printing calculator for dimensions from a company named Boyd. Some of these recently showed up on the surplus market and–of course–were subsequently hacked.

[Bill Rowe] is active in the groups that still work with the 1802. Because of some specialized uses you can still get the chips readily, some four decades after they were new. Other computers at the time were difficult to build and relatively expensive, while for $100 almost anyone could wire wrap a simple 1802-based computer together in a weekend or less.

While the 1802 was inexpensive and simple to use compared to other parts of its time, it didn’t have the most robust instruction set for modern languages like C. However, the assembly language is simple and regular, so it is easy to learn. [Josh Bensadon] disassembled the code and commented it, giving everyone a basis to start from.

[Bill] replaced the old 4K EPROM on the device with a 2K flash EEPROM. No UV erase cycle needed! The display used an IC that had an option to display hex, so that was easy. The reduced memory means [Bill] is linking without the math routines, which is ironic considering the device’s original function.

The new code implements a simple monitor and a small interpreted “language” to simplify writing new code. Oddly, the calculator has two other CPUs that drive the printer (but neither are 180x devices). No word if anyone’s going to break into those anytime soon.

If [Bill’s] name sound familiar, you might know him Olduino project. This is another 1802 board that can accept some Arduino shields and supports C programming–quite a feat for a machine with no built-in stack to speak of.

The Elf made the 1802 famous because it was the first computer you could reasonably build in a weekend with just a few parts you probably couldn’t scrounge. The design was simple and even if you bought everything as a kit, you’d be out about $100. That was more than it sounds like now, but way better than other small computers of the day. We’ve seen some great modern builds of the Elf. You can even run one in your browser if you don’t feel like melting solder.

Thanks [Eric] for the tip.

Filed under: classic hacks, Microcontrollers

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