ET-57M - Programmable mini-calculator

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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

Properties

ATmega8 processor (4MHz, 8KB ROM, 1KB RAM, 512B EEPROM)
Powered by CR2032 battery, LiR2032 battery or USB
External power supply and battery charging from the USB connector
Dimensions only 43 x 43 mm
Calculations in BCD code
Accuracy of 11-digit calculations
Data display on max. 6 valid digits
Exponent 2 digits, range + - 99
10 program spaces
Each space 50 program steps (total 500 steps)
User program stored in EEPROM (without battery)
10 basic registers (accessible by direct addressing)
90 extended registers (accessible by index addressing)
6-digit LED display
12 buttons
Calculator code completely written in AVR assembler
Exponential and logarithmic functions
Trigonometric functions
Factorial
Random number generator
Index access to variables
Scientific display mode with exponent
Statistical function
10 labels in each program space
Function calls and jumps between program spaces
Fully open source hardware and software

Diagram

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.

Construction

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):

Mounting:

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.