8279 Programmable Keyboard

8279 programmable keyboard/display controller is designed by Intel that interfaces a keyboard with the CPU. The keyboard first scans the keyboard and identifies if any key has been pressed. It then sends their relative response of the pressed key to the CPU and vice-a-versa.

How Many Ways the Keyboard is Interfaced with the CPU?

The Keyboard can be interfaced either in the interrupt or the polled mode. In the Interrupt mode, the processor is requested service only if any key is pressed, otherwise the CPU will continue with its main task.

In the Polled mode, the CPU periodically reads an internal flag of 8279 to check whether any key is pressed or not with key pressure.

How Does 8279 Keyboard Work?

The keyboard consists of maximum 64 keys, which are interfaced with the CPU by using the key-codes. These key-codes are de-bounced and stored in an 8-byte FIFORAM, which can be accessed by the CPU. If more than 8 characters are entered in the FIFO, then it means more than eight keys are pressed at a time. This is when the overrun status is set.

If a FIFO contains a valid key entry, then the CPU is interrupted in an interrupt mode else the CPU checks the status in polling to read the entry. Once the CPU reads a key entry, then FIFO is updated, and the key entry is pushed out of the FIFO to generate space for new entries.

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