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.

Family Members and Applications

Type Calculator/Product Application Comments
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
TMC1993 Little Professor (1978, later),
Little Professor (1980)
Educational Die-up, Rigid-PCB



  Description Comments
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

Technical Specifications

Item Min Typ Max Unit Comments
VSS   0   V   
VDD   -9.0   V   
CK  150 250 333 kHz  Internal oscillator


The TMC1990 is 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.

Pin Configuration

TMC1990 (Die-up)

Pin IO Function Pin IO Function
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 (rightmost)
4 O Digit driver 7 25 O Segment driver DP
5 O Digit driver 8 (leftmost) 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

TMC1990 (Die-down)

Pin IO Function Pin IO Function
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. 

Keyboard Scan-Matrix

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: 

  K1 K2 K3 K4
STG OFF +/- % ÷
STE 7 8 9 ×
STD 4 5 6
STC 1 2 3 +
O . =

Example for the Little Professor with TMC1993: 

  K1 K2 K3 K4
STE 7 8 9 ÷
STD 4 5 6 ×
STC 1 2 3


Calculators based on the based on the TMC1990 make use of an 8-digit LED-display with common cathode architecture. 

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If you have additions to the above datasheet please email:

Sean Riddle and Joerg Woerner, April 20, 2021. No reprints without written permission.