Part 2: Beginners guide to the PIC Microcontroller.

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Beginners guide to the PIC Microcontroller

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Part 2 :

I/O Ports - using microcontrollers to communicate with the 'real' world.

A microcontroller is not much use if you can't control or measure real-world-signals and you do this using an I/O port.

An I/O port is usually as wide as the CPU part of the microcontroller and for mid range PIC is 8 bits wide.

Each port is usually labeled as PORT followed by a capital letter starting at A. So the first port is PORTA and the second is PORTB etc.

Common Port Functions

Each port has a basic functionality that is the same whichever port you use and this is digital I/O.

Each pin of the port corresponds to a pin on the microcontroller which you can set to either digital input or digital output.

Digital input

So for each port pin set as digital input:

Any voltage below 0.8V is read as logic zero.
any voltage above 2.0V is read as logic one.

Note: This is for a 5V or 3V system (check the data sheet for exact levels when using other power supply voltages).

Digital output

For digital output the following is true (when not loaded)

logic zero output : output voltage <= 0.6V
logic one output : output voltage >= Vsupply - 0.7V

Note: The actual output voltage depends on the loading at the pin - if a high current is drawn the the output voltage will be lower (I/O pins can sink/source up to 25mA)

Specialized PORTs

Since devices have a small number of pins compared to the number of peripherals in the device each pin is usually multi functional. You can either use it in its basic digital I/O mode or you can use one of its shared functions.

Shared functions from internal modules have fixed positions on the device i.e. a USART receive pin in a 16F877 is on pin 26 - this pin is one of the following:

* PORTC bit 7 (RC7) - Digital I/O.
* USART Rx - RS232 Receiver input.
* DT - Data input: synchronous reception.

So you have to decide what you want to use the pin function to be before you wire up the circuit!

More Resources...

Analogue input

A common feature of the modern PIC microcontrollers is a built in 10 bit ADC - you can find this in the following devices (as well as many others):

* 12F675
* 16F88
* 16F877(A)

This internal peripheral lets you read an analogue input (as long as it is within the range of the microcontroller power supply) and not only that - it has a multiplexer that lets you read analogue voltages from different pins.

Note: The exact pins that are allowed for reading analogue voltages are device specific so you need to read the data sheet to find out that information.

One of the device I find amazing is the 8 pin 12F675 - it has a full 10bit adc built into it, multiplexed to four pins, so you can make it into a 4 channel volt meter. Or measure the output of an LM35 temperature sensor.

Note: With suitable opamp preconditioning circuitry you can extend the range of the voltage input.

Using the ADC

With a high level language such as 'C' reading the ADC is a trivial task as all the hard work is done for you: Here is the code to read an ADC using the MikroC compiler:

val = ADC_Read(0);

This reads a value from ADC input zero - and that's all the code you need to know about! - no register setup - no data sheet - totally easy.

Next time...

... Internal timers.

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