Q. Describe the operation of 8279. And also explain following terms:

(i) N key Roll over.

(ii) Key board debounce.

(iii) FIFO RAM.

Ans:

8279 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. Keyboard interface has built in first-in first-out (FIFO) buffer that allows it store up to 8 keystrokes before the microprocessor should retrieve a character. Display section controls up to 16 numeric displays from an internal 16 X 8 RAM which stores coded display information.

Keyboard section comprises eight lines that can be connected to eight columns of a keyboard, plus two extra lines as well as to shift and CNTL/STB keys. Key pressed are automatically debounced and keyboard can operate in two modes two -key lock out or n-key rollover. If two keys in two -key lock out mode are pressed concurrently, only first key is recognized. In N-key roll over mode, simultaneous key are recognized and their codes are stored in internal buffer.