Elektronika MK-90 - hardware

LCD controller КА1835ВГ3

This chip transfers display data from the RAM to the LCD drivers КА1835ИД1. Additionally it serves as a memory controller.

$E800 - address register, word size, write-only: This register specifies the starting address of the display memory.
$E802 - control register (?), word size, write-only: This register of unknown function is initialised with a fixed value $88C6.

Keyboard controller КА1835ВГ1

This chip returns the keyboard scan code (row and column of a pressed key) through the Channel 2 of the serial bus controller КА1835ВГ4. Pressing a key also causes a VIRQ interrupt request to the vector at address $00C8. The scan frequency of the keyboard matrix is determined by the serial bus CLK signal.
Additionally the chip serves as an address decoder.

Serial bus controller КА1835ВГ4

The controller supports eight serial channels, out of which four are used in the MK-90. All devices hooked on the bus share common DATA and SELECT lines, but receive separate CLK signals.

Channel 0 and 1 are used by the SMP memory modules.
Channel 2 is used by the keyboard controller КА1835ВГ1.
The CLK output of the channel 3 drives the piezo buzzer.

$E810 - data register, byte size: Data written is serially transmitted through the serial bus.
Reading returns the contents of the shift register, then the next byte is shifted in from the serial bus.
$E812 - transfer rate register, word size: This register specifies the frequency of the CLK signal.
frequency = 800kHz / register_value
$E814 - control and status register, byte size: Function of individual bits:
bits 2-0 select the serial channel
bit 3 specifies the transfer direction: 0=input, 1=output
Setting the transfer direction to input causes a byte to be read from the serial bus to the shift register.
bit 4 enables the transfer when set, disables when cleared
bit 7 is set when the transfer is completed and the controller is ready for the next operation
$E816 - command register, byte size: Writing to this register activates the SELECT signal, then the written byte is serially transmitted through the serial bus or a byte is read from the serial bus to the shift register, depending on the bit 3 of the register $E814.
Reading returns the contents of the shift register and deactivates the SELECT signal.

Real time clock КА512ВИ1

It's a direct equivalent of the of the MC146818 real time clock. A 32768Hz crystal oscillator is used as the time base. The Square Wave Output SQW drives the EVNT interrupt input with a frequency of 32Hz.

The chip bus lines AD0-AD7 are connected to the system bus lines AD1-AD8. Therefore any data written to the chip have to be shifted left by one bit, and any data read from the chip have to be shifted right by one bit. Also the chip registers are selected by consecutive even addresses.

The chip occupies the address range $EA00-$EA7E.