Texas Instruments TMC0920

• Features

With the Calculator War starting in 1975, manufacturing costs of basic 4-function electronic calculators were under extreme pressure and the calculator chip was one of the main cost drivers. The manufacturing costs of an Integrated Circuit (IC) are calculated with:

• IC cost = (Die cost + Testing cost + Packaging cost) / Final test yield

With the die cost roughly proportional to the die area, testing and packaging costs roughly proportional to the pin count, and the final test yield mostly inverse proportional to the die area, goals are well defined: Keep the die size as small as possible for a set of requirements agreed on. With both ROM (Read-Only Memory) and RWM (Read-Write Memory) sizes the main contributors to the die area and shift-register based data memory (SAM, Serial-Access Memory) of Register Processor denser than RAM (Random-Access Memory) of Digit Processors, Texas Instruments introduced with the TMC0920 in June 1977 a very cost effective single-chip calculator circuit for the TI-1025 and TI-1050.

The TMC0920 chip includes a minimalistic 4,608 Bits ROM (512*9 Bits – only 511 words accessible due to the design of the program counter) and 200 Bits SAM (5 Registers * 10 digits), a 4-bit Arithmetic unit, a programmable PLA for segment decoding and both segment and digit multiplexing for a 9-digit VF-Display. To further reduce the Bill of Material of an electronic calculator powered by a single 9V battery features the TMC0920 both an integrated charge pump driver to generate the high voltage (around -22V) for the Anodes and Grids of the VF-Display and integrated drivers for the Filament (heater) of it. Additional cost savings could be achieved by replacing a conventional [ON-OFF] switch with a simple push-button [ON] key.

While the TI-1025 was a very successful calculator marketed aggressively against the much more practical basic calculators using the then new LC-Display in a much slimmer housing, did it mark the last generation of TI’s VFD calculators and consequently not too many products were designed with the TMC0920 chip. Texas Instruments recycled the design of the TMC0920 for use with LC-Displays and introduced in June 1978 with the TI-1030 their first US-built product based on the TP0310 single-chip calculator circuit manufactured in power-saving CMOS technology and sporting an identical architecture.

• Family Members and Applications

Type Calculator Digits Functions
TMC0921 TI-1050, Koh-I-Noor Hardtmuth 1050, Western Auto Citation sign + 8 Memory, sqr(x), %
TMC0923 TI-1025, HEMA TI-1025, Western Auto Citation sign + 8 Memory, %


• Architecture

  Description Comments
Architecture Single-chip Calculator Basic, Memory
Category Register Processor 40-bit registers (10 digits * 4 Bits
ROM Size 4,608 Bits 512 Words * 9 Bits
SAM Size 200 Bits 5 Registers * 10 digits (SAM)
Outputs 8 Digits, 8 Segments Integrated Digit and Segment Drivers, Filament and Charge pump Drivers
Inputs 4 Keyboard Digit to Keyboard Scan-Matrix

• Technical Specifications

Item Min Typ Max Unit Comments
VSS   0   V   
VDD   -9.0   V   
VBIAS   -22.0   V   
CK  150 250 333 kHz  Internal oscillator

• Technology

The TMC0920 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 TMC0920 is approximately 160 mils * 125 mils / 4.0 mm * 3.2 mm.

• Packaging

The TMC0920 uses a 0.4” wide 28-pin SPDIP (Shrink Plastic Dual In-line Package with a 0.07” / 1.778 mm lead pitch).

• Pin Configuration

Pin IO Function Pin IO Function
1 V Positive Voltage VSS 28 V Negative Voltage VDD
2 I Keymatrix input 4 27 V Display Voltage VBIAS
3 I Keymatrix input 3 26 O Voltage Tripler Output
4 I Keymatrix input 2 25 O Filament driver 2
5 I Keymatrix input 1 24 O Filament driver 1
6 O Digit driver 1 (rightmost) 23   n.c.
7 O Digit driver 2 22 O Segment driver A
8 O Digit driver 3 21 O Segment driver B
9 O Digit driver 4 20 O Segment driver C
10   n.c. 19 O Segment driver D
11 O Digit driver 5 18 O Segment driver F
12 O Digit driver 6 17 O Segment driver G
13 O Digit driver 7 16 O Segment driver E
14 O Digit driver 8 (leftmost) 15 O Segment driver DP
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 TMC0920 family consist of a x/y-matrix connected to six digit-driver outputs and the keymatrix inputs K1, K2, K3, and K4. 

Example for the TI-1050 with TMC0921: 

  K1 K2 K3 K4
D5 OFF +/- % ÷
D4 7 8 9 ×
D3 4 5 6
D2 1 2 3 +
O . =

Example for the TI-1025 with TMC0923: 

  K1 K2 K3 K4
D6 MC MR M- M+
D5 OFF +/- % ÷
D4 7 8 9 ×
D3 4 5 6
D2 1 2 3 +
O . =

• Display

Calculators based on the based on the TMC0920 make use of a 9-digit VF-Display. 

horizontal rule

If you have additions to the above datasheet please email:

© Sean Riddle and Joerg Woerner, April 8, 2021. No reprints without written permission.