8 x 8 Keyboard Interface(With Interrupt signal)

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Statement: Interface an 8 x 8 matrix keyboard to 8085 through 8279 in 2-key lockout mode and write an assembly language program to read keycode of the pressed key. The external clock frequency is 2MHz. Use I/O mapped I/O technique.

HARDWARE FOR 8 x 8 MATRIX KEYBOARD INTERFACE(With Interrupt)

Fig. shows the interfacing of 8 x 8 matrix keyboard in interrupt driven keyboard mode. In the interrupt driven mode interrupt line from 8279 is connected to the one of the interrupt input of 8085 except INTR. Here, INT line from 8279 is connected to the interrupt RST 7.5 of 8085. Other signal connections are same as in the non interrupt mode

Source program:
MVI A, 00H : Initialize keyboard/display in encoded
OUT 81H : scan keyboard 2 key lockout mode
MVI A, 34H
OUT 81H : Initialize prescaler count
MVI A, 0BH : Load mask pattern to enable RST 7.5
SIM : mask other interrupts
EI : Enable Interrupt
HERE: JMP HERE : Wait for the interrupt
Interrupt Subroutine:
MVI A, 40H : Initialize 8279 in read FIFO
OUT 81H : RAM mode
IN 80H : Read FIFO RAM (keycode)
EI : Enable Interrupt
RET : Return to main program

Note: In the interrupt driven keyboard, when key is pressed, key code is loaded into FIFO RAM and interrupt is generated. This interrupt signal is used to tell CPU that there is a keycode in the FIFO RAM. CPU then initiates read command with in the interrupt service routine to read key code from the FIFO RAM.