Keyboards are organized in a matrix of rows and columns. When a key is pressed, a row and a column make a contact; otherwise, there is no connection between rows and columns.
The CPU accesses both rows and columns through ports; therefore, with two 8-bit ports, an 8 x 8 matrix of keys can be connected to a microprocessor. The rows are connected to an output port and the columns are connected to an input port.
To detect a pressed key, the microcontroller grounds all rows by providing 0 to the output latch, then it reads the columns. If the data read from the columns is D3 – DO = 1111, no key has been pressed and the process continues until a key press is detected. However, if one of the column bits has a zero, this means that a key press has occurred. For example, if D3 – DO = 1101, this means that a key in the Dl column has been pressed. After a key press is detected, the microcontroller will go through the process of identifying the key. Starting with the top row, the microcontroller grounds it by providing a low to row DO only; then it reads the columns. If the data read is all Is, no key in that row is activated and the process is moved to the next row. It grounds the next row, reads the columns, and checks for any zero. This process continues until the row is identified. After identification of the row in which the key has been pressed, the next task is to find out which column the pressed key belongs to. This should be easy since the microcontroller knows at any time which row and column are being accessed.
The CPU accesses both rows and columns through ports; therefore, with two 8-bit ports, an 8 x 8 matrix of keys can be connected to a microprocessor. The rows are connected to an output port and the columns are connected to an input port.
To detect a pressed key, the microcontroller grounds all rows by providing 0 to the output latch, then it reads the columns. If the data read from the columns is D3 – DO = 1111, no key has been pressed and the process continues until a key press is detected. However, if one of the column bits has a zero, this means that a key press has occurred. For example, if D3 – DO = 1101, this means that a key in the Dl column has been pressed. After a key press is detected, the microcontroller will go through the process of identifying the key. Starting with the top row, the microcontroller grounds it by providing a low to row DO only; then it reads the columns. If the data read is all Is, no key in that row is activated and the process is moved to the next row. It grounds the next row, reads the columns, and checks for any zero. This process continues until the row is identified. After identification of the row in which the key has been pressed, the next task is to find out which column the pressed key belongs to. This should be easy since the microcontroller knows at any time which row and column are being accessed.
ORG 0H MOV P2, #0FFH ; Make P2 an input port KEYS1 : MOV P1, #0 ; make p1 as output port MOV A, P2 ; read all columns ensure all keys are open ANL A, #00001111B ; mask unused bits CJNE A, #00001111B, KEYS1 ; check till all keys are released KEYS2 : ACALL DELAY ; call 20ms delay MOV A, P2 ; see if any key is pressed ANL A, #00001111B ; mask unused bits CJNE A, #00001111B, OVER ; key pressed, await closure SJMP KEYS2 OVER : ACALL DELAY ; wait 20ms debounce time MOV A, P2 ; check key closure ANL A, #00001111B ; mask unused bits CJNE A, #00001111B, OVER1 ; key pressed find row SJMP KEYS2 ; if none, keep polling OVER1 : MOV P1, #11111110B ; ground row 0 MOV A, P2 ; read all columns ANL A, #00001111B ; mask unused bits CJNE A, #00001111B, ROW_0 ; key row 0, find the column MOV P1, #11111101B ; ground row 1 MOV A, P2 ; read all columns ANL A, #00001111B ; mask unused bits CJNE A, #00001111B, ROW_1 ; key row 1, find the column MOV P1, #11111011B ; ground row 2 MOV A, P2 ; read all columns ANL A, #00001111B ; mask unused bits CJNE A, #00001111B, ROW_2 ; key row 2, find the column MOV P1, #11110111B ; ground row 3 MOV A, P2 ; read all columns ANL A, #00001111B ; mask unused bits CJNE A, #00001111B, ROW_3 ; key row 3, find the column LJMP KEYS2 ; if none, false input, repeat ROW_0 : MOV DPTR, #KCODE0 ; set DPTR = start of row 0 SJMP FIND ; find column key belongs to ROW_1 : MOV DPTR, #KCODE1 ; set DPTR = start of row 1 SJMP FIND ; find column key belongs to ROW_2 : MOV DPTR, #KCODE2 ; set DPTR = start of row 2 SJMP FIND ; find column key belongs to ROW_3 : MOV DPTR, #KCODE3 ; set DPTR = start of row 3 FIND : RRC A ; see if any CY bit is low JNC MATCH ; if zero, get the ASCII code INC DPTR ; point to next column address SJMP FIND ; keep searching MATCH : CLR A ; set A = 0, match is found MOVC A, @A+DPTR ; get ASCII code from table MOV P0, A ; display pressed key LJMP KEYS1 ; some delay DELAY : MOV R3, #50 HERE2 : MOV R4, #255 HERE : DJNZ R4, HERE ; stay until R4 = 0 DJNZ R3, HERE2 ; stay until R3 = 0 RET ; ASCII look-up table for each row ORG 300H KCODE0 : DB '0','1','2','3' ; ROW 0 KCODE1 : DB '4','5','6','7' ; ROW 1 KCODE2 : DB '8','9','A','B' ; ROW 2 KCODE3 : DB 'C','D','E','F' ; ROW 3 END
Related topics:
8051 External Program Memory Interfacing | 8051 External Data Memory Interfacing | 8051 Memory Mapped IO | 8051 LED Interfacing | 8051 Switch Interfacing | 8051 7-Segment Display Interfacing | 8051 LCD Interfacing | 8051 ADC Interfacing | 8051 DAC Interfacing | 8051 Relay Interfacing | 8051 Sensor Interfacing | 8051 Stepper Motor Interfacing | 8051 DC Motor Interfacing
List of topics: 8051
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