KEYBOARD AND DISPLAY INTERFACE USING INTEL 8279 MICROPROCESSOR

In a microprocessor b   system,   when   keyboard   and   7-segment   LED   display   is interfaced using ports or latches then the processor has to carry the following task.

• Keyboard scanning
• Key debouncing
• Key code generation
• Sending display code to LED
• Display refreshing

Interfacing 8279 with 8085 processor:

 •    A  typical  Hexa  keyboard  and  7-segment  LED  display  interfacing  circuit  using  8279  is shown.

•    The  circuit  can  be  used  in  8085  microprocessor  system  and  consist  of  16  numbers  of hexa-keys and 6 numbers of 7-segment LEDs.

•    The 7-segment LEDs can be used to display six digit alphanumeric character.

•    The 8279 can be either memory mapped or I/O mapped in the system. In the circuit shown
is the 8279 is I/O mapped.

•    The address line A0 of the system is used as A0 of 8279.

•    The clock signal for  8279 is obtained by dividing the output clock signal of  8085 by  a clock divider circuit.

•    The chip select signal is obtained from the I/O address decoder of the 8085 system. The chip select signals for I/O mapped devices are generated by using a 3-to-8 decoder.

•    The address lines A4, A5 and A6 are used as input to decoder.

•    The address line A7 and the control signal IO/M (low) are used as enable for decoder.

•    The chip select signal IOCS-3 is used to select 8279.

•    The I/O address of the internal devices of 8279 are shown in table.

•    The circuit has 6 numbers of 7-segment LEDs and so the 8279 has to be programmed in encoded  scan.  (Because  in  decoded  scan,  only  4  numbers  of  7-segment  LEDs  can  be interfaced):

•    In encoded scan the output of scan lines will be binary count. Therefore an external, 3-to-
8 decoder is used to decode the scan lines SL0, SL1  and SL2  of 8279 to produce eight scan lines S0 to S7.

•    The decoded scan lines S0 and S1  are common for keyboard and display.

•    The decoded scan lines S2  to S5  are used only for display and the decoded scan lines S6
and S7 are not used in the system.

•    Anode and Cathode drivers are provided to take care of the current requirement of LEDs.

•    The pnp transistors, BC 158 are used as driver transistors.

•    The anode drivers are called segment drivers and cathode drivers are called digit drivers.

•    The 8279 output the display code for one digit through its output lines (OUT A0  to OUT
A3 and OUT B0 to OUT B3) and send a scan code through, SL0- SL3.

•    The display code is inverted by segment drivers and sent to segment bus.

•    The  scan  code  is  decoded  by  the  decoder  and  turns  ON  the  corresponding digit  driver. Now  one  digit  of  the  display  character  is  displayed.  After  a  small  interval  (10  milli- second, typical), the display is turned OFF (i.e., display is blanked) and the above process
is repeated for next digit. Thus multiplexed display is performed by 8279.

•    The keyboard matrix is- formed using the return lines, RL0 to RL3 of 8279 as columns and decoded scan lines S0 and S1 as rows.

•    A hexa key is placed at the crossing point of each row and column. A key press will short the row and column. Normally the column and row line will be high.

•    During scanning the 8279 will output binary  count on SL0  to SL3, which is decoded  by decoder to make a row as zero. When a row is zero the 8279 reads the columns. If there is
a key press then the corresponding column will be zero.

•    If 8279 detects a key press then it wait for debounce time and again read the columns to generate key code.

•    In encoded scan keyboard mode, the 8279 stores an 8-bit code for each valid key press. The keycode consist of the binary value of the column and row in which the key is found and the status of shift and control key.

•    After a scan time, the next row is made zero and the above process is repeated and so on. Thus 8279 continuously scan the keyboard.

Interfacing 8279 with 8086 processor:

• A  typical  Hexa  keyboard  and  7-segment  LED  display  interfacing  circuit  using  8279  f
8086 based system is shown.

•        The system consists of 16 numbers of hexa-keys and numbers of 7-segment LEDs. The 7- segment LEDs can be used to display eight-digit alphanumeric character.

•    The 8279 can be either memory mapped or I/O mapped in the system. In the circuit shown
is I/O mapped.

•    The address line A1 of the system is used as A0 of 8279.

•    The clock signal for 8279 is obtained by dividing the PCLK (peripheral clock) of 8284 by
a clock divider circuit.

•    The chip select signals, for I/O mapped devices are generated by using a 3-to-8 decoder.

•    The address lines A5, A6 and A7 are used as input to decoder.

•    The address line A0  and the control signal M  /IO (low) are used as enable for decoder. The chip select signal IOCS-3 is used to select 8279.

•    The I/O address of the internal devices of 8279 is shown in table.

•    The circuit has 6 numbers of 7-segment LEDs and so the 8279 has to be programmed in encoded  scan.  (Because  in  decoded  scan,  only  4  numbers  of  7-segment  LEDs  can  be interfaced):

•    In encoded scan the output of scan lines will be binary count. Therefore an external, 3-to-
8 decoder is used to decode the scan lines SL0, SL1  and SL2  of 8279 to produce eight scan lines S0 to S7.

•    The decoded scan lines S0 and S1  are common for keyboard and display.

•    The decoded scan lines S2  to S5  are used only for display and the decoded scan lines S6
and S7 are not used in the system.

•    Anode and Cathode drivers are provided to take care of the current requirement of LEDs.

•    The pnp transistors, BC 158 are used as driver transistors.

•    The anode drivers are called segment drivers and cathode drivers are called digit drivers.

•    The 8279 output the display code for one digit through its output lines (OUT A0  to OUT
A3 and OUT B0 to OUT B3) and send a scan code through, SL0- SL3.

•    The display code is inverted by segment drivers and sent to segment bus.

•    The  scan  code  is  decoded  by  the  decoder  and  turns  ON  the  corresponding digit  driver. Now  one  digit  of  the  display  character  is  displayed.  After  a  small  interval  (10  milli-
second, typical), the display is turned OFF (i.e., display is blanked) and the above process
is repeated for next digit. Thus multiplexed display is performed by 8279.

•    The keyboard matrix is- formed using the return lines, RL0 to RL3 of 8279 as columns and decoded scan lines S0 and S1 as rows.

•    A hexa key is placed at the crossing point of each row and column. A key press will short the row and column. Normally the column and row line will be high.

•    During scanning the 8279 will output binary  count on SL0  to SL3, which is decoded  by decoder to make a row as zero. When a row is zero the 8279 reads the columns. If there is
a key press then the corresponding column will be zero.

•    If 8279 detects a key press then it wait for debounce time and again read the columns to generate key code.

•    In encoded scan keyboard mode, the 8279 stores an 8-bit code for each valid key press. The keycode consist of the binary value of the column and row in which the key is found
and the status of shift and control key.

•    After a scan time, the next row is made zero and the above process is repeated and so on. Thus 8279 continuously scan the keyboard.