The status of two additional input keys i.e., shift and control are also stored within FIFO RAM. It has 8*8 FIFO RAM that works on the First– In– First– Out approach and can store 8 keycodes (code of pressed key) at a time in a sequential manner. While N-key rollover mode, whenever it is found that two keys are pressed at the same time then key codes of both of them get stored with the FIFO and are further serviced. In the 2-key lockout mode of operation, if two keys are pressed at the same time then the first recognized key will be considered by the system. The two operating modes are 2-key lockout and N-key rollover. Shift and Control/Strobe are the two additional inputs provided to this unit. It holds the 8 return lines denoted by RL 0 to RL 7 that forms the column of the keyboard matrix. Keyboard Section: This section is composed of Return Buffer and Keyboard Debounce and Control. Let us understand the operation of this architecture by considering its 4 separate sections.ġ. The functional block diagram of the Intel 8279 keyboard/display controller is shown below: There is a 40 pin IC of 8279 which exists in the Dual In-line Package (DIP). So, in this case whenever the CPU gets a signal that shows the pressed key then the requested service is served. Polled Mode: In the polled mode of operation the CPU itself checks for any pressed key in a periodic manner by reading the flags of 8279.Interrupt Mode: In this mode of operation, the CPU continues to perform its original task until and unless any request is generated to that a key is pressed and is required to be serviced.

Mainly there are two ways by which the keyboard can be interfaced with the CPU, these are as follows: These two operations take place within the system in a synchronized manner without disturbing the other ongoing operations of the CPU. Also, after the operation, the data which the CPU forwards it, is transmitted to the display device. If a pressed key is detected, then the keycode of the pressed key is sent to the CPU. It works in a way that the keyboard display interface keeps the keyboard under scanning in order to check if any key is pressed. The RAM is loaded by the CPU however, once data loading is done, the 8279 performs data displaying and refreshing functions. It has a display RAM of 16*8 matrix, however the arrangement can be organized in dual 16*4 RAM. Also, through 8279 multiplexed interfacing can be obtained for 7-segment LEDs as well as other popular display devices. It provides interfacing for 64-contact keys that are arranged in 8*8 matrix format. Intel’s 8279 is a dedicated controller designed by Intel that offers simultaneous keyboard and display operations. Content: Keyboard/Display Controller – 8279

Transistor keyboard or controller