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ET-57M - Programmable mini-calculator

Clone of TI-57 calculator with ATmega8 processor.

User manual in PDF (so far in CZ, the EN version is being worked on)

The ET-57M mini-calculator is a variant of the ET-57 calculator (clone TI-57) with an effort for minimum dimensions, so that it can serve as a "wrist calculator", with the possibility of mounting on the hand instead of a watch. For this reason, it uses only 12 buttons and a 6-digit LED display. Nevertheless, it has complete equipment as its larger model.

The processor chosen was ATmega8 (in an MLF package), with 8 KB ROM, 1 KB RAM and operating at an internal oscillation frequency of 4 MHz. The calculator emulates the functions of the 1977 Texas Instruments TI-57 calculator and expands its functions. In the ET-57M calculator, programs for the TI-57 can be run mostly without the need for modifications, but it is necessary to take into account different numbering of instructions.

Note: Click on the images to enlarge them



The USB connector is used to power the +5V calculator from an external charger or from a USB port, as well as to charge a possible accumulator. Charging takes place simply via a resistor, without monitoring the degree of charge. When using a non-rechargeable battery, the battery must be removed when using external power. The processor uses an internal 4 MHz RC oscillator.


The ATmega8 in the MLF case, which is a bit more difficult to solder, was chosen as the processor. To minimize the dimensions, the programming connector is not connected, the connector pins must be soldered directly to the board and removed again after programming the processor.

Bottom side of the printed circuit board:

Upper side (wires are enough):


Printed circuit board printing:

Button labels:

The holes for the buttons are punched through the hole puncher of 4 mm on the skin. Printed paper can be coated with laminating foil or transparent double-sided insulating tape.

Construction - the battery holder and the temporary programming connector are soldered to the board from below:

Connection of programming connector:

Programming firmware

Setup fuses of ATmega8 processor: low E3, high D1.

Button meaning:


Source codes of the ET-57M firmware

Schematic diagram in Eagle Free

User manual (in CZ)

Graphic materials (diagram and printed circuit board)

Used components

Miroslav Nemecek

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