# What type of processor in usual 8-digits pocket calculator?

There are a lot of +,-,*,/,CE,MC,M+,M-,MR buttons 8-digits calculators on market with keyboard layout like:

+----+----+----+----+
| ON | MRC| M+ | M- |
+----+----+----+----+
|  7 |  8 |  9 |  ÷ |
+----+----+----+----+
|  4 |  5 |  6 |  × |
+----+----+----+----+
|  1 |  2 |  3 |  - |
+----+----+----+----+
|  0 |  . |  = |  + |
+----+----+----+----+


Some of them have % and √ (sqrt) functions.

I wander what type of processor is under black epoxy lump? What is clock speed, registers bit width?

There are a lot of information about ancient '60, '70, '80 calculator architecture, advanced TI calculators and DIY projects but I can't find any mentioning of modern pocket calculator.

• Probably some dedicated chip that sells for a few cents, made by a company you've never heard of, with the datasheets available only in Chinese. – The Photon Sep 21 '15 at 8:20
• LOL @ThePhoton exactly! – Junior Sep 21 '15 at 8:34
• Usually a 4 bit cpu programmed via mask rom. They generally perform calculations in base 10 via bcd so 4 bits is quite natural. – John Meacham Sep 21 '15 at 12:43
• Something that you can still read a part number on: youtu.be/_VvEO_m3Owk?t=627 Those are mid-1990s vintage. – Fizz Sep 22 '15 at 9:16
• Google seems to know absolutely nothing about HD61708B07 by the way, and that's part the used in that Casio/Tandy calculator, so not totally obscure brands. Does anyone recognize the logo on the chip (in the video)? – Fizz Sep 22 '15 at 9:24

There are surely a lot of different ICs around for that application.

One I found was EMPCD081A, which has all the features you mention, as you can see in the datasheet, it runs with a 200kHz clock during operation (while calculating I assume) and 18kHz during wait operation. It can't shut down completely as it has to refresh the LCD and register a key press on the matrix. Running this slowly allows it to operate from a solar cell under very dim lights. It just takes 20µA maximum during operation.

My quick search has not revealed anything more specific on the internals of the chip (bit width of the registers).

Doing some further digging, there are some sources which suggest that they internally work with a 4-bit logic:

• TC83220-0009: has a 4-bit oriented RAM (runs at 4MHz, but it can drive a printer, so it's a bit different)
• MM57109: This is a really old design, but in this file a block diagram is depicted showing a 4-bit bus.
• With your suggestion I found some other datasheets on category.alldatasheet.com/… thanks, +1! – gavenkoa Sep 21 '15 at 12:07
• @gavenkoa thanks, I updated my answer with a further research (still not extensive) which suggests that they use a 4-bit logic. As 4 bits = 1 digit that sort of makes sense. – Arsenal Sep 21 '15 at 13:22

With help from @Arsenal I start looking for chips on datasheet sites by 8-DIGIT CALCULATOR keywords.

Most refs have only clocking and interfacing info. Characteristics are very similar - Wait 18 kHz, Operate 200 kHz.

Among 40 reviewed datasheets I found: