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Texas Instruments TMS0950

• Features

Texas Instruments announced on September 17, 1971 with the TMS1802NC the first available standard calculator building block on a chip, it was later renamed into TMS0102. The chip integrates 3,520 Bits Read-Only program Memory (ROM, 320 Words x 11 Bits), a 182-bit Serial-Access Memory (SAM, 3 Registers * 13 Digits, 2 * 13 Bit-Flags) and a decimal arithmetic logic unit as well as control, timing, and output decoders but no drivers for the display. These function blocks of the chip add up to an overall complexity of roughly 5,000 transistors.

While the TMS0100 product family was an impressive achievement combining the electronics of two Integrated Circuits (ICs) housed in large 40-pin packages into a single IC with a much smaller 28-pin package, could Texas Instruments’ advertisement "calculator-on-a-chip" be misleading. The Bill of Materials (BOM) of a typical calculator based on the TMS0100 design like the TI-2500 Datamath included not only the calculator chip, housing, keyboard assembly, display assembly, rechargeable batteries and a power [ON-OFF] switch, we did count on the printed circuit board (PCB) of the already cost-optimized TI-2500B introduced in January 1974 a myriad of components:

• TMS0119 Calculator Chip
• 2 SN75494 Digit Driver Chips
• 2 SN27915 Segment Driver Chips
• 5 Transistors
• 12 Diodes
• 1 Inductor
• 20 Resistors
• 7 Capacitors

Consequently the suggested retail price of the TI-2500B was marked with an astonishing $69.95, about $349 in 2000’s money and more than $500 in 2020’s money!

Texas Instruments was able to significantly drop the manufacturing costs of the Datamath with the introduction of the TI-2500 II in November 1974 based on the TMS0800 Chip with the integration of both the segment drivers and clock oscillator into a small 28-pin package

Credits for the first "real" single-chip calculator circuit go to the TMS0950 Chip introduced early in 1975 based on the TMS1000, the World’s first Microcomputer. While switching from the chip-size efficient Register Processor architecture of the TMS0100 design to a more flexible Digit Processor architecture of the TMS1000, more than doubling the program memory from 3,520 Bits to 8,192 Bits, increasing the data memory from 182 Bits to 256 Bits resulting in an overall complexity of about 8,000 transistors, could the BOM of a typical electronic handheld calculator be reduced dramatically due to a series of measures:

• Integration of both segment and digit drivers for a 9-digit LED display
• Lowering the two supply voltages from -10.0V and -15.8V to a single supply voltage of -9V provided by a disposable alkaline battery
• Integration of clock circuitry and power-on reset circuitry

Due to a smart pin-out of the 28-pin package matching both the pin-out of the display assembly and keyboard assembly and the use of only 6 discrete components (4 resistors, 1 capacitor and [ON-OFF] switch) avoiding any cross-points in the layout of the circuit, the whole electronics of a calculator could be placed easily on a simple, single-sided PCB.

The TI-1200 was introduced in March 1975 with a MSRP of $24.95, ditching the rather expensive Klixon keyboard and paving the way to the sub-$10 calculator leading to casualties like Bowmar Industries in 1976 during the Calculator War.

Texas Instruments even further optimized the design of the TMS0950 with the introduction of the pin-compatible TMS0970 in March 1976 and avoiding any passive components other than the [ON-OFF] switch on the PCB.

Due to its short product cycle of just one year only three calculators are based on the original TMS0950 design, while the TMS0970 – later renamed to TMC0970 and TMC0900 - could be considered one of the most successful calculator chip designs next to the TMC0980.

• Family Members and Applications

Type Calculators Digits
TMS0952 TI-1200, TI-1250 sign + 8
TMS0954 TI-1260 sign + 8

 

• Architecture

  Description Comments
Architecture Single-chip Calculator Basic
Category Digit Processor 4-bit digits
Related TMS1000
TMS0970
Generic design
Improved design
ROM Size 8,192 Bits 1024 Words * 8 Bits
RAM Size 256 Bits 64 Digits
Outputs 9 Digits, 8 Segments Integrated Digit and Segment Drivers
Inputs 4 Keyboard
1 Oscillator Mode
1 Reset
Segments to Keyboard Scan-Matrix

• Technical Specifications

Item Min Typ Max Unit Comments
VSS   0   V   
VDD   -9.0   V   
CK   180   kHz Rext= 68 kOhm, Cext= 68 pF

• Technology

The TMS0950 was manufactured in a 8.0 um metal gate PMOS process (metal width = 0.30 mil / 8.0 um, metal spacing = 0.35 mil / 9.0 um, diffusion width = 0.25 mil / 6.0 um, diffusion spacing = 0.30 mil / 8.0 um).

The die size of the TMS0950 is approximately 200 mils * 210 mils / 5.2 mm * 5.3 mm.

• Packaging

The TMS0950 uses a standard 0.6” wide 28-pin DIP (Dual In-line Package with a 0.1” / 2.54 mm lead pitch).

• Pin Configuration

Pin IO Function Pin IO Function
1 O Digit driver 7 28 O Digit driver 6
2 O Digit driver 8 27 O Digit driver 5
3 O Digit driver 9 (leftmost) 26 O Digit driver 4
4 V Negative Voltage VDD 25 O Digit driver 3
5 I Keymatrix input 1 24 O Digit driver 2
6 I Keymatrix input 2 23 O Digit driver 1 (rightmost)
7 I Keymatrix input 3 22 R 6k8 to VDD
8 I Keymatrix input 4 21 V Reference voltage
9 I Key or power on reset 20 V Common Voltage
10 O Segment driver DP 19 I OSC2 (Ext. CLK = VSS)
11 O Segment driver G 18 I OSC1 (Cext, Rext) or Ext
12 O Segment driver F 17 O Segment driver A
13 O Segment driver E 16 O Segment driver B
14 O Segment driver D 15 O Segment driver C
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 TMS0950 family consist of an x/y-matrix connected to six segment-driver outputs and the keymatrix inputs K1 to K4. The Power on reset is usually not connected to a key.

Example for the TI-1250 with TMS0952: 

  K1 K2 K3 K4
Seg. DP MC MR M− M+
Seg. F C +/− % ÷
Seg. E 7 8 9 ×
Seg. D 4 5 6
Seg. C 1 2 3 +
Seg. B CE 0 . =

• Display

Calculators based on the TMS0950 make use of a 9-digit LED-display with common cathode architecture. The TI-1200/TI-1250 uses the LED-stick 233.

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If you have additions to the above datasheet please email: joerg@datamath.org.

© Joerg Woerner, February 02, 2001. No reprints without written permission.