SWITCH AND LED INTERFACING WITH MICROCONTROLLER

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.

     Interfacing switch with the microcontroller

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

2.    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.

Light-emitting diode (LED)

Light-emitting diodes are elements for light signalization in electronics. They are manufactured in different shapes, colors and sizes. For their low price, low consumption and simple use, they have almost completely pushed aside other light sources- bulbs at first place. They perform similar to common diodes with the difference that they emit light when current flows through them.

                                

It is important to know that each diode will be immediately destroyed unless its current is limited. This means that a conductor must be connected in parallel to a diode. In order to correctly determine value of this conductor, it is necessary to know diode’s voltage drop in forward direction, which depends on what material a diode is made of and what colour it is. Values typical for the most frequently used diodes are shown in table below: As seen, there are three main types of LEDs.

Standard ones get ful brightness at current of 20mA. Low Current diodes get ful brightness at ten times lower current while Super Bright diodes produce more intensive light than Standard ones.

Since the 8051 microcontrollers can provide only low input current and since their pins are configured as outputs when voltage level on them is equal to 0, direct connectining to LEDs is carried out as it is shown on figure (Low current LED, cathode is connected to output pin).

Switches and Pushbuttons

There is nothing simpler than this! This is the simplest way of controlling appearance of some voltage on microcontroller’s input pin. There is also no need for additional explanation of how these components operate.

                        

Fig 4.3 :  Switches and Pushbuttons

Nevertheless, it is not so simple in practice... This is about something commonly unnoticeable when using these components in everyday life. It is about contact bounce- a common problem with m e c h a n i c a l switches. If contact switching does not happen so quickly, several consecutive bounces can be noticed prior to maintain stable state. The reasons for this are: vibrations, slight rough spots and dirt. Anyway, whole this process does not last long (a few micro- or miliseconds), but long enough to be registered by the microcontroller. Concerning pulse counter, error occurs in almost 100% of cases!

The simplest solution is to connect simple RC circuit which will “suppress” each quick voltage change. Since the bouncing time is not defined, the values of elements are not strictly determined. In the most cases, the values shown on figure are sufficient.

If complete safety is needed, radical measures should be taken! The circuit, shown on the figure (RS flip-flop), changes logic state on its output with the first pulse triggered by contact bounce. Even though this is more expensive solution (SPDT switch), the problem is definitely resolved! Besides, since the condensator is not used, very short pulses can be also registered in this way. In addition to these hardware solutions, a simple software solution is commonly applied too: when a program tests the state of some input pin and finds changes, the check should be done one more time after certain time delay. If the change is confirmed it means that switch (or pushbutton) has changed its position. The advantages of such solution are obvious: it is free of charge, effects of disturbances are eliminated too and it can be adjusted to the worst-quality contacts.