4×4 Keypad Interfacing with 8051 + Display

Hardware: Rows R1-R4 → P1.0-P1.3 (output), Columns C1-C4 → P1.4-P1.7 (input with pull-ups) or via P2. Keypad matrix 4×4=16 keys (0-9,A-F or 1-9,* #). Supply decoupling, debounce capacitors.

Scanning Algorithm:

1. Drive all rows high.
2. Set one row low at a time, read columns. Column low = key pressed at (row, col).
3. Debounce 20 ms delay, re-read.
4. Compute key code = row*4+col, look up ASCII in table.
5. Display on 7-seg/LCD at P0.

Program sketch (assembly):

SCAN: MOV P1,#0F0H ; rows outputs pull?
ROW_LOOP: MOV A,#0FEH ; row0 low
        MOV P1,A
        MOV A,P1
        ANL A,#0F0H
        CJNE A,#0F0H,FOUND
        ; next row 0FDH etc
FOUND: ; compute
        MOV DPTR,#KEYMAP
        MOVC A,@A+DPTR
        MOV P2,A ; display
        ACALL DEBOUNCE

Display: Send decoded hex to LCD via P0 data + P2 control (RS,RW,EN) or to 7-seg via P0 segments + P2 digit select as in Q38. Multiplex display continuously.
Circuit: P1.0-3 via 470Ω to row lines, P1.4-7 via 10k pull-up to Vcc + column lines, optional 74C922 encoder alternative.