GPIO( General Purpose IO) is the most basic method of communication between
MCU and external world. These are done with what is called a PORT. Ports are
nothing but a set of bits physically connected to PINs of Microcontroller and
available outside the chip. As we are working on PIC micros and they are 8 bits
so the maximum size of a PORT is 8 bits. Some PORTs have fewer than 8 bits.
You can turn each bit in a PORT on and off under program control. The bits which
you set as 1 becomes HIGH and the physical PIN of Micro is at Vcc(supply voltage,
usually 3.3v or 5v). And the PINs which you clear (=0) becomes low and physical
level is 0v(GND).

PIC IO Port example

Fig.: PIC IO Port example, PORTB.

 

Fig.: PIC IO Port example, Using PORTB, Turning BITs on and off.

You can test the operation of an IO port by using LEDs as shown below.

led on pic portb

Fig.: When the bit is set to 0 the LED remains off.

When you write a value 1 to corresponding bit the LED starts glowing.

Fig.: When the bit is set to 1 the LED starts glowing.

PORTs are Named like PORTA,PORTB,PORTC etc. The PIC we are using PIC4550 has
the following PORTs

  • PORTA(8bit wide)
  • PORTB(8bit wide)
  • PORTC(7bit wide)
  • PORTD(8bit wide)
  • PORTE(4bit wide)

Inputs using PORTs

You have seen how ports can be used to control the outside word. In that we
have used the PORTs as Output. They can also be used to get input from outside.
In input mode we can apply either logic 0 (GND) or 1 (Vcc = 5v) and we can read
the status of PORT and get what input we have on ports. This can be used to
interface switches and sensor with MCU. Any value below Vcc/2 is taken as low
and any voltage above Vcc/2 is high.

A simple switch interface is shown below.

pic interface with switch

Fig.: A Simple switch as Input to PIC Micro.

The CPU can read the required port and take decisions accordingly.

Practical Stuff.

Having the knowledge about digital IO Ports on PIC micro we can begin some
practical experimentations. So how can we actually access the ports in C? Well
the answer follows.

Each PORT in PIC micro has 3 associated registers.

  • TRIS: This is the data direction register – As I have told
    you that port can be used as both input and output purpose, the direction
    control (input or output) is done using this register. Simply make the bit
    as 1 when you want its as input or 0 for output.

Example:

TRISB=0xFF; //1111-1111 All Inputs
TRISB=0x00; //0000-0000 All Outputs
TRISB=0x0F;//0000-1111 Bit 7,6,5,4, as Output and Bit 3,2,1,0 as Input

Note: On power up all PORTs are Input.

To Remember this note that 1 looks like ‘i’ therefore Input, 0 looks line
‘o’ so Output

  • LAT: (LATCH): This register is used to set output values.
    Anything you want output on related pins must be put on this.

Example

LATB=0xFF; //All PINs becomes high
LATB=0x00; //All pins becomes Low
LATB=0x03; //0000-0011 BIT 0 and 1 becomes high.

  • PORT: This is used to get input from the PORTs. Reading
    its value gives the actual levels applied on the PINs. It must be noted that
    PORT is first configured as input using the TRIS register. Suppose you connected
    a switch as shown above on PORTB-0 then if you read PORTB it will be 0000-0000
    if switch is pressed and 0000-0001 if switch is released.

If you have any doubts or just want to express your feedback please post a
comment here.