How to Interface Peripherals to Microprocessors

 SWITCH AND LED INTERFACING WITH THE MICROCONTROLLER:

              Switches and LEDs are the most widely used input/output devices of the 8052.

 SWITCH INTERFACING:

              CPU accesses the switches through ports. Therefore these switches are connected to a microcontroller. This switch is connected between the supply and ground terminals. A single microcontroller (consisting of a microprocessor, RAM and EEPROM and several ports all on a single chip) takes care of hardware and software interfacing of the switch.

These switches are connected to an input port. When no switch is pressed, reading the input port will yield 1s since they are all connected to high (Vcc). But if any switch is pressed, one of the input port pins will have 0 since the switch pressed provides the path to ground. It is the function of the microcontroller to scan the switches continuously to detect and identify the switch pressed. The switches that we are using in our project are 4 leg micro switches of momentary type.

Thus now the two conditions are to be remembered:

1. When the switch is open, the total supply i.e., Vcc appears at the port pin P2.0

P2.0 = 1

When the switch is closed i.e., when it is pressed, the total supply path is provided to   ground. Thus the voltage value at the port pin P2.0 will be zero.

P2.0 = 0         

By reading the pin status, the microcontroller identifies whether the switch is pressed or not. When the switch is pressed, the corresponding related to this switch press written in the program will be executed.

LED INTERFACING:

LED stands for Light Emitting Diode.

Microcontroller port pins cannot drive these LEDs as these require high currents to switch on. Thus the positive terminal of LED is directly connected to Vcc, power supply and the negative terminal is connected to port pin through a current limiting resistor. This current limiting resistor is connected to protect the port pins from sudden flow of high currents from the power supply.

Thus in order to glow the LED, first there should be a current flow through the LED. In order to have a current flow, a voltage difference should exist between the LED terminals. To ensure the voltage difference between the terminals and as the positive terminal of LED is connected to power supply Vcc, the negative terminal has to be connected to ground. Thus this ground value is provided by the microcontroller port pin. This can be achieved by writing an instruction “CLR P1.0”. With this, the port pin P1.0 is initialized to zero and thus now a voltage difference is established between the LED terminals and accordingly, current flows and therefore the LED glows. LED and switches can be connected to any one of the four port pins.