Explain the operation of 8279.  Explain the following terms:

(i)  N key Roll over.

(ii)  Key board debounce.

(iii)  FIFO RAM.

Ans

The 8279 microprocessor is a programmable keyboard and display interfacing component which scans and encodes up to a 64-key keyboard and controls up to a 16-digit numerical display. This keyboard interface has built in first-in first-out buffer which allows this store up to eight keystrokes before the microprocessor should retrieve a character. There display section controls up to 16 numeric displays by an internal 16 X 8 RAM which stores the coded display information.

The keyboard sections contain eight lines which can be connected to eight columns of a keyboard, and also two additional lines with to shift and CNTL/STB keys. These key pressed are automatically debounced and the keyboard can function in two modes two key lock out or n-key rollover. When two keys in the two -key lock out mode are pressed concurrently, only first key is identified. In the N-key roll over mode, concurrent key are recognized and their codes are stored into the internal buffer.