12-03-2012, 02:23 PM
Interfacing of 8051 with displays
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Interfacing of 8051 with LED
forward voltage drop of LED is from 1.7 to 2.5
forward current of LED is 20mA
89C51(8051) comes under TTL logic families
TTL can sink up to 16mA and source 250uA
The 8051 port pin can sink 1.6mA (3.2mA for port 0) and source 60uA.
To drive significant current, we should arrange our circuits to use current sinking.
Design calculation
The resistor limits the current.
Assume voltage is about 2.5V and the TTL output is 0.9V. For 2.2V LED
1.9V(2.5-0.9)V is the voltage across the resistor
R=316Ω (1.9V/6mA).
The resistor should not less than 100ohm or the LED would fail.
Types of seven segment displays.
Common anode type
common cathode type
in common anode all the anodes of 7 LEDs are shorted and their cathode terminals are open to connect 8051
where as in common cathode the case is reversed.
Interfacing using 7447 decoder
We connect first four pins of the microcontroller Port 2 to the 7447 and the Output 8 pins of 7447 to the 8 legs of the LED as shown in the figure.
The number required to display is sent as the lower nibble of the Port 2 of the Microcontroller.
The 7447 converts the four input bits (BCD) to their corresponding 7-segment codes.
