PIC16F877A Pin out & Descriptions
There are 5 ports that provide parallel I/O interfaces to outside world PORTA, PORTB, PORTC, PORTD, PORTE,
Each port provides 8 bidirectional digital I/O lines which are conneected to PIC16F877A pins provided that alternate functions are not selected on that port.
Eventhough Bidirectional at any time the I/O line can either be INPUT or OUTPUT
The Directions of each I/O lines must be configured (INPUT or OUTPUT) before they are used.
I/O Registers related to Ports
By clearing some bit of the TRISx register (bit=0), the corresponding port pin is configured as output. Similarly, by setting some bit of the TRISx register (bit=1), the corresponding port pin is configured as input. This rule is easy to remember 0=Output, 1= Input.
Role of Pull up resistors at the port pins
Let us say you have configured a port pin as input pin.
If there is nothing connected with the pin and your program reads the state of the pin, will it be high or low??? It will be difficult to tell
This phenomena is referred to as floating state.
In order to prevent this unknown state , pull up resistor is used.
A switch connected at the input pin without pull up resistor
A switch connected at the input pin with pull up resistor
Summary Action During Output
Step1: If we
want to send (out) data through a pin of a port, the corresponding bit of TRISx Register
has to be set to ‘0’.
Step
2: Then if we want to send ‘1’ write ‘1’ to the
corresponding bit of PORTx Register and ‘0’ if we want to send ‘0’.
Note: We can
not send data (either ‘1’ or ‘0’) if we do not complete step 1. At START UP or
UPON RESET TRISx register contains 0x00, that is all pins remain at output state.
Summary Action During Input
Step1: If we
want to read (in) data through a pin of a port, the corresponding bit of TRISx Register
has to be set to ‘1’.
Step
2: Whatever data is in that pin of the port the data will
appear in the corresponding bit of TRISx
register.
Step
3: Each pin has a provision of connecting a pull up
resistor internally. If we want to connect that resistor, we have to write ‘1’
to the corresponding bit of PORTx register.
Note: At START UP or UPON RESET, TRISx register
contains 0x11. That is the port remains at Input state. But if it is used as
OUTPUT at one stage of program, afterwards, we can not read data from a pin if
we do not complete step 1.
A Practical example of Outputting Data through a Port
Let us assume that 8 LEDs are connected to 8-pins of PORTD of an PIC16F877A chip.
We want to glow eight LEDs alternately [if '1' is seent, the corresponding LED glows ]
And they will toggle at every 1 sec and it will be repeated continuously.
What is an LED?
Light Emitting Diode (LED) is a special diode that emits light when electric voltage is applied to it. It is a common electronic equipment used in many devices for indication purpose. There are two leads of an LED that are used to supply input voltage. The longer lead is positive and known as "Post", the smaller is negative known as "Anvil"
Connection of LEDs with a Port
Proteus Design:
PIC Simulator IDE:
PIC Simulator IDE is powerful application that supplies Microchip Microcontroller users with user-friendly graphical development environment for windows with integrated simulator (emulator), pic basic compiler, assembler, disassembler and debugger. PIC simulator IDE supports the extensive number of microcontrollers (MCUs) from the Microchip 8-bit PIC Mid-Range architecture product line (selected PIC16F,PIC12F,PIC10F).
Basic Code:
Define CONF_WORD = 0x3f72 ' Configuration of PIC16F877A
Define
CLOCK_FREQUENCY = 12 ' Clock frequency 12MHz
AllDigital
' All Ports are Digital
Dim i As Byte ' i is a variables, data types is Byte
TRISD = 0x00 ' PORTD is output
PORTD= 0x00 ' PORTD is zero or off state
loop: ' loop call
For i = 0 To
255 ' counting 0 to 255 by the for loop
PORTD = i
' PORTD output will depend i variable when i=1,PORTD will be 1
WaitMs 100 ' delay 100ms
Next i
' count next number
Goto loop ' finishing for loop then goto again "loop "
C Code
int i:
void main (){
TRISD=0x00;
PORTD= 0x00;
while(1){
for(i=0; i<255; i++){
PORTD=i;
delay_ms(100);
}
}
}
