Saturday, October 31, 2009

Remembering the 8X300 Microcontroller

The Signetics 8X300 (originally made by Scientific Micro Systems) was fast when compared to the other microprocessors of the day.  It was implemented in bipolar Schottky and could execute instructions in 250ns.  The only other way to get those speeds was to roll your own semi-custom processor using bit slice components.  The down side was that it only had 8 instructions - two to move data, 3 to manipulate data, one to test data, and two to control execution.  But, you could do so much with so little.  Data could be rotated, masked, and manipulated with a single instruction.  This made the 8X300 a powerful controller for embedded applications.

In the day, we wrote code by hand, punched a card deck and compiled it on a large DEC machine.  After the compilation errors were resolved, the output was paper tape.  These were then entered into an in-circuit emulator.  Patches to the code were made by manually inserting a JUMP, adding the new code, and JUMPING back.  The small instruction set really helped with this manual process. 

I recently dug up the following old article from EDN Magazine. The author, the project lead on my 1st 8X300 project, describes how to add interrupts to an 8X300-based system.