Module 5 : Experiments on Microcontrollers

Lecture 42 : Interfacing a Keypad with AT89C51

Aim

To detect which key is pressed on a 16-key keypad (interfaced with Port1) and display the corresponding key number on Port0.

Components/Software

  1. Atmel 89C51 microcontroller
  2. 8051 simulator- Win8051
  3. Universal SP3 Programmer
  4. Compiler and Software to load the code into Universal SP3 Programmer
  5. Computer System with Windows 98 or later operating system and RS-232 Cable
  6. +5V D.C Power Supply
  7. 16-key keypad with keys arranged in 4x4 array
  8. Resistors
  9. Capacitors
  10. 10 MHz crystal oscillator
  11. Signal Generator and CRO

Description

Keypad is an array of 4x4 keys with each key being a switch. The four rows are connected to pins 0-3 of Port1 and the four columns to pins 4-7 of Port1 as shown in the figure below. Each column is also connected to supply voltage through a pull up resistor (around 1k Ohms). To read a particular key, the corresponding row is driven to '0'. In a key is pressed and the corresponding row is driven '0' then current flows through the corresponding pull up resistor and the corresponding column reads '0'. Otherwise the corresponding column reads '1' (open circuit). For example if Key 5 is pressed -

  • R1 = 0, C1:C4 = 1111
  • R2 = 0, C1:C4 = 1011

Fig  42.1  Keypad schematic

 Procedure
  1. Write the assembly code in Win8051. Compile and simulate it.
  2. Once the code is error free, run it and check output with the Simulator
  3. After checking the code in the simulator, the code (file with .HEX extension) is loaded into Atmel 89C51 microcontroller using Universal SP3 Programmer.
  4. Now connections are made as shown in the circuit diagram.
  5. Switch on the supply and push Reset button
  6. Observe the results
  7. Switch off the supply
Assembly Code

Observation

Microcontroller takes few microseconds to scan all 16 keys on the keypad whereas human response is of the order of milliseconds. Hence contact time of a key is long enough for it to be scanned by the microcontroller.

Circuit Diagram
Fig  42.2 Circuit diagram for keypad interfacing