Keypad Source: http://www.edsim51.com/8051Notes/interfacing.html#keypad under http://www.edsim51.com/8051Notes/interfacing.html under http://www.edsim51.com/8051Notes/index.html

4 X 4 keypad The keypad is a matrix of switches. In the default state (all switches open), there is no connection between the rows and columns. When a switch is pressed, a connection between the switch's row and the switch's column is made.

Hardware Decoder for Keypad The decoder scans the keypad. When a key is pressed, the 4-bit code for the key appears on the four output lines and the data available line (DA) goes LOW. The microcontroller will be interrupted by DA. The ISR could then read the 4 LSBs of P1 and process the data. Key pressed O1~O4 DA (Data available)

Software Decoder for Keypad Scanning a row is achieved by applying 0 V to the port pin for that row and 5 V to the other three rows, then scanning each individual column to see if one of them is LOW. If it is, then the key at the junction between the current row and column being scanned is the pressed key. Sequence of scanning: Clear row 1, set other 3 Scan column 1 Scan column 2 Scan column 3 Scan column 4 Clear row 2, set other 3 Clear row 3, set other 3 Clear row 4, set other 3

No key is pressed: A key is pressed: Sequence of scanning: Clear row 0, set other 3 Scan column 0 Scan column 1 Scan column 2 Scan column 3 Clear row 1, set other 3 Clear row 2, set other 3 Clear row 3, set other 3 Col. 3 Col. 2 Col. 1 Col. 0 Row 3 Row 2 Row 1 Row 0 1 1 1 1 1 Col. 3 Col. 2 Col. 1 Col. 0 Row 3 Row 2 Row 1 Row 0 1 1 0 1 1

Key numbers in edsim51 Key numbers

Keypad connection in edsim51 8051 Resistor AND gate Col. 3 Col. 2 P0.6 Pull up AND in Col. 1 P0.5 Col. 0 P0.4 Row 3 P0.0 Row 2 P0.1 Row 1 P0.2 Row 0 P0.3

Finding the pressed key: row scan & col. scan Check Row 3 P0[3:0] = 0111 Check P0[6:4] P0[6:4] = 011 P0[6:4] = 101 P0[6:4] = 110 “1” “2” “3” Check Row 2 P0[3:0] = 1011 “4” “5” “6” Check Row 1 P0[3:0] = 1101 “7” “8” “9” Check Row 0 P0[3:0] = 1110 “*” “0” “#”

-