Texas Instruments TMC1990

Texas Instruments introduced in 1978 the 2nd Generation of its wildly successful Little Professor, an educational toy very similar to a basic calculator but having the user answer computer-generated math questions. Compared to the 1st Generation of the Little Professor introduced in 1976 and based on the TMS0970 single-chip calculator family, replaced its successor based on a TMC1993 chip the power switch with a set of [ON] and [OFF] keys known already from the TI-30 calculator centered around the more capable TMC0980. The TMC1990 design exhibits an unconventional approach of scanning the keyboard switch-matrix with 10 dedicated pins (6 row outputs, 4 column inputs) instead of using either the digit-driver outputs or segment-driver outputs for the keyboard rows. While spending 6 extra pins for this purpose sounds counterproductive with respect to cost savings, did it actually reduce the complexity of the printed circuit board (PCB) dramatically by reducing cross-points in the layout between keyboard, single-chip calculator circuit, display, and battery and allowing for single-sided PCBs without using jumper wires.

From a technical point of view the TMC1990 is closely related to the TMS0970 and maintains the TMS1000 architecture with 8,192 Bits Read-Only Memory (ROM, 1k*8 Bits) and 256 Bits Random-Access Memory (RAM, 4*16 Digits), a 4-bit Arithmetic unit, a programmable PLA for segment decoding and both integrated segment and digit drivers for an 8-digit LED Display. Main differences are:

• Integrated power latch and power transistor for [ON] and [OFF] keys
• Six of the eight State Time Signals used for segment scanning bonded on dedicated pins for keyboard scanning
• Package options with Die-up (standard pinout) or Die-down (reverse pinout) options

While the TMC1990 was introduced too late to be successful in electronic handheld calculators, proofed it to be very successful with the Little Professor manufactured between 1978 and 1982.

With permission of the Author