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Modification of the Elektronika MK-85

I got an MK-85 with a bad microprocessor. As the original microprocessor is rather unobtainable, I decided to replace it (along with the ROM and RAM chips) with an STM32F103C8T6 microcontroller. So the original BASIC interpreter (written in PDP-11 assembler) had to be ported as accurately as possible to the C language. Some known bugs have also been fixed.

Project files

Contents of the stmk85.zip archive:

• microcontroller software, to be compiled with the STM32CubeMX and STM32CubeIDE tools (64-bit)
• schematic and PCB layout created with the KiCad utility

Due to limited space in the calculator, the board thickness of about 1mm is recommended.

Simplified prototype using a Blue Pill module

Differences from the schematic:

• the LCD controller КА1013ВГ2 is connected directly to the microcontroller's SPI port, bypassing the 74HCT4053 chip used as a logic level translator
• the microcontroller is powered with 3.3V from an on-board RT9193 regulator instead of 2.5V from the TPS62745DSSR regulator
• the calculator cannot be turned off - the slide switch is not connected, the OV2 power rail is permanently shorted to OV1 (instead of through the 2N7002 transistor)

Final version

Apart from the original integrated circuits, the resistor R10=2MΩ and the capacitor C17=0.15μF must be desoldered from the calculator. The diode VD1=КД521А in the power path must be replaced with a jumper. The ON_SW signal is connected with a blue wire to the junction of the R10 resistor and the C17 capacitor.

The components of the original clock generator (resistors R1, R2 and capacitors C2, C3) are no longer needed. They can be left in place or removed. I soldered an 8MHz crystal resonator and two 15pF capacitors in their place. One day they may be wired to the PD0 and PD1 pins of the microprocessor, which would allow to use the USB port.

Signals from the board are available on the pads left after desoldering the ROM and RAM chips. The purple wires visible on the photo go to an ST-LINK programmer.

Measurement results (at the clock frequency of 2MHz)

Power consumption

• turned off: 7μA (originally < 1μA)
• waiting for a key press: 0.15mA (originally 0.35mA)
• program execution: 1.25 to 1.60mA depending on the intensity of sending data to the LCD controller (originally 1.45mA at normal clock frequency and 5.15mA at fast clock)

Speed

• The execution time for the following sample sine-drawing program is 1.7 seconds (originally 194 or 34 seconds):
  

  10 FOR X=0 TO 59
  20 DRAW X, 3.5+3.5*SIN(20*X)
  30 NEXT X

• Solving the eight queens puzzle takes 28 seconds.