Light Emitting Diodes (LEDs) are simple and most commonly used electronic components to display the digital signal states. The LED emits light when current is passed through it. It could blow up if we pass more current, hence we put a current limiting resistor. Usually, 220, 470 and 1K ohm resistors are commonly used as limiting resistors. You can use any of these depending on the required brightness. Lets, start blinking with LEDs and then generate the different patterns using the available LEDs.

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8051 Ports

The basic and important feature of any controllers is the number of GPIO's available for connecting the peripherals. 8051 has 32-gpio's grouped into 4-Ports namely P0-P3 as shown in the below table.

PORT	Number of Pins	Alternative Function
P0	8 (P0.0-P0.7)	AD0-AD7 (Address and Data bus)
P1	8 (P1.0-P1.7)	None
P2	8 (P2.0-P2.7)	A8-A15 (Higher Address Bus)
P3	8 (P3.0-P3.7)	UART, Interrupts, (T0/T1)Counters

As shown in the above table many I/O pins have multiple functions. If a pin is used for other function then it may not be used as a gpio.

Hardware Connections

Examples

Example 1

Program to demonstrate the LED blinking. LED's are connected to P2 as shown in the above image.
LEDs are turned ON by sending a high pulse (All Ones).
After some time the LEDs are turned OFF by sending the low pulse (All Zeros).

	#include <reg51.h>

	void DELAY_ms(unsigned int ms_Count)
	{
	unsigned int i,j;
	for(i=0;i<ms_Count;i++)
	{
	for(j=0;j<100;j++);
	}
	}
	int main()
	{
	while(1)
	{
	P2 = 0xff; /* Turn ON all the leds connected to P2 */
	DELAY_ms(500);

	P2 = 0x00; /* Turn OFF all the leds connected to P2 */
	DELAY_ms(500);
	}
	return (0);
	}

view raw 8051_LedBlinking1.c hosted with ❤ by GitHub

Example 2

In this example, we will see how to access a single port pin to blink the LED.

	#include <reg51.h>

	sbit LED = P2^0; // Bling the single LED connected to P2.0

	void DELAY_ms(unsigned int ms_Count)
	{
	unsigned int i,j;
	for(i=0;i<ms_Count;i++)
	{
	for(j=0;j<100;j++);
	}
	}

	int main()
	{
	while(1)
	{
	LED = 1; /* Turn ON the LED connected to P2.0 */
	DELAY_ms(500);

	LED = 0; /* Turn OFF the LED connected to P2.0 */
	DELAY_ms(500);
	}
	return (0);
	}