Programmable Keyboard/Display Interface- 8279

Programmable Keyboard/Display Interface - 8279

A programmable keyboard and display interfacing chip. Scans and encodes up to a 64-key keyboard. Controls up to a 16-digit numerical display. Keyboard section has a built-in FIFO 8 character buffer. The display is controlled from an internal 16x8 RAM that stores the coded display information.

1 Pinout Definition 8279

ü A0: Selects data (0) or control/status (1) for reads and writes between micro and 8279.

ü BD: Output that blanks the displays.

ü CLK: Used internally for timing. Max is 3 MHz.

ü CN/ST: Control/strobe, connected to the control key on the keyboard

ü CS: Chip select that enables programming, reading the keyboard, etc.

ü DB7-DB0: Consists of bidirectional pins that connect to data bus on micro.

ü IRQ: Interrupt request, becomes 1 when a key is pressed, data is available.

ü OUT A3-A0/B3-B0: Outputs that sends data to the most significant/least significant nibble of display.

ü RD(WR): Connects to micro's IORC or RD signal, reads data/status registers.

ü RESET: Connects to system RESET.

ü RL7-RL0: Return lines are inputs used to sense key depression in the keyboard matrix.

ü Shift: Shift connects to Shift key on keyboard.

ü SL3-SL0: Scan line outputs scan both the keyboard and displays.

2 Block Diagram of 8279

Display section:

ü The display section has eight output lines divided into two groups A0-A3 and B0-B3.

ü The output lines can be used either as a single group of eight lines or as two groups of four lines, in conjunction with the scan lines for a multiplexed display.

ü The output lines are connected to the anodes through driver transistor in case of common cathode 7-segment LEDs.

ü The cathodes are connected to scan lines through driver transistors.

ü The display can be blanked by BD (low) line.

ü The display section consists of 16 x 8 display RAM. The CPU can read from or write into any location of the display RAM.

Scan section:

ü The scan section has a scan counter and four scan lines, SL0 to SL3.

ü In decoded scan mode, the output of scan lines will be similar to a 2-to-4 decoder.

ü In encoded scan mode, the output of scan lines will be binary count, and so an external decoder should be used to convert the binary count to decoded output.

ü The scan lines are common for keyboard and display.

ü The scan lines are used to form the rows of a matrix keyboard and also connected to digit drivers of a multiplexed display, to turn ON/OFF.

CPU interface section:

ü The CPU interface section takes care of data transfer between 8279 and the processor.

ü This section has eight bidirectional data lines DB0 to DB7 for data transfer between 8279 and CPU. It requires two internal address A =0 for selecting data buffer and A = 1 for selecting control register of8279.

ü The control signals WR (low), RD (low), CS (low) and A0 are used for read/write to 8279.

ü It has an interrupt request line IRQ, for interrupt driven data transfer with processor.

ü The 8279 require an internal clock frequency of 100 kHz. This can be obtained by dividing the input clock by an internal prescaler.

ü The RESET signal sets the 8279 in 16-character display with two -key lockout keyboard modes.

3 Keyboard Interface of 8279

The keyboard matrix can be any size from 2x2 to 8x8.Pins SL2-SL0 sequentially scan each column through a counting operation. The 74LS138 drives 0's on one line at a time. The 8279 scans RL pins synchronously with the scan. RL pins incorporate internal pull-ups, no need for external resistor pull-ups. The 8279 must be programmed first.

First three bits given below select one of 8 control registers (opcode).

000DDMMM

Mode set: Opcode 000.

DD sets displays mode.

MMM sets keyboard mode.

DD          field selects either:

ü 8- or 16-digit display

ü Whether new data are entered to the rightmost or leftmost display position.

MMM Field:

DD    Function

000    Encoded keyboard with 2 key lockout

001    Decoded keyboard with 2 key lockout

010    Encoded keyboard with N key rollover

011    Decoded keyboard with N key rollover

100    Encoded sensor matrix

101    Decoded sensor matrix

110    Strobed keyboard, encoded display scan

111    Strobed keyboard, decoded display scan

Encoded: SL outputs are active-high, follow binary bit pattern 0-7 or 0-15.

Decoded: SL outputs are active-low (only one low at any time).

Pattern output: 1110, 1101, 1011, 0111.

Strobed: An active high pulse on the CN/ST input pin strobes data from the RL pins into an internal FIFO for reading by micro later.

2-key lockout/N-key rollover: Prevents 2 keys from being recognized if pressed simultaneously/Accepts all keys pressed from 1st to last.

Fig 3.14 Display Interface of 8279