DATAMATH CALCULATOR MUSEUM
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.
|TMC1991||TI-1000 (1977), Western Auto Citation||Basic||Die-up, Rigid-PCB|
|TMC1992||TI-1000 (1978)||Basic||Die-down, Flex-PCB|
|TMC1993||Little Professor (1978, early)||Educational||Die-down, Flex-PCB|
Little Professor (1978, later),
Little Professor (1980)
|Architecture||Single-chip Calculator||Basic, Educational|
|Category||Digit Processor||4-bit digits|
|ROM Size||8,192 Bits||1024 Words * 8 Bits|
|RAM Size||256 Bits||64 Digits|
|Outputs||8 Digits, 8 Segments, 6 Keyboard||Integrated Digit and Segment Drivers, State Time STA - STE, STG|
|Inputs||4 Keyboard||State Time (Segments) to Keyboard Scan-Matrix|
The TMC1990 was manufactured in a 6 um metal gate PMOS process (metal width = 0.25 mil / 6.0 um, metal spacing = 0.25 mil / 6.0 um, diffusion width = 0.20 mil / 5.0 um, diffusion spacing = 0.25 mil / 6.0 um).
The die size of the TMC1990 is approximately 160 mils * 125 mils / 4.0 mm * 3.1 mm.
The TMC1990 uses a 0.4” wide 28-pin SPDIP (Shrink Plastic Dual In-line Package with a 0.07” / 1.778 mm lead pitch) with standard (Die-up) or reverse (Die-down) pinout.
|1||O||Digit driver 4||28||O||Digit driver 3|
|2||O||Digit driver 5||27||O||Digit driver 2|
|3||O||Digit driver 6||26||O||Digit driver 1 (LSD)|
|4||O||Digit driver 7||25||O||Segment driver DP|
|5||O||Digit driver 8 (sign/MSD)||24||O||Segment driver C|
|6||O||State Time A||23||O||Segment driver D|
|7||O||State Time B||22||O||Segment driver E|
|8||I||Keymatrix input 1||21||O||Segment driver F|
|9||O||State Time C||20||O||Segment driver G|
|10||I||Keymatrix input 2||19||O||Segment driver A|
|11||O||State Time D||18||O||Segment driver B|
|12||I||Keymatrix input 3||17||V||Negative Voltage VDD|
|13||O||State Time E||16||V||Positive Voltage VSS|
|14||I||Keymatrix input 4||15||O||State Time G|
|1||O||Digit driver 3||28||O||Digit driver 4|
|2||O||Digit driver 2||27||O||Digit driver 5|
|3||O||Digit driver 1 (rightmost)||26||O||Digit driver 6|
|4||O||Segment driver DP||25||O||Digit driver 7|
|5||O||Segment driver C||24||O||Digit driver 8 (leftmost)|
|6||O||Segment driver D||23||O||State Time A|
|7||O||Segment driver E||22||O||State Time B|
|8||O||Segment driver F||21||I||Keymatrix input 1|
|9||O||Segment driver G||20||O||State Time C|
|10||O||Segment driver A||19||I||Keymatrix input 2|
|11||O||Segment driver B||18||O||State Time D|
|12||V||Negative Voltage VDD||17||I||Keymatrix input 3|
|13||V||Positive Voltage VSS||16||O||State Time E|
|14||O||State Time G||15||I||Keymatrix input 4|
|The Segment drivers A-G and DP (Decimal Point) are connected to the display in the pictured way.|
The keyboards of all calculators based on the TMC1990 family consist of a x/y-matrix connected to up to six State Time outputs STA, STB, STC, STD, STE, and STG (segments) and the keymatrix inputs K1, K2, K3, and K4.
Example for the TI-1000 with TMC1991, TMC1992:
Example for the Little Professor with TMC1993:
Calculators based on the based on the TMC1990 make use of 8-digit LED Displays with common cathode architecture.
If you have additions to the above datasheet please email: firstname.lastname@example.org.
© Sean Riddle and Joerg Woerner, April 20, 2021. No reprints
without written permission.