16F876 / 16F873 / 16F872 Physical Input and Output

The 16F876, 16F873 and 16F872 have the same pin layouts. These chips only differ internally by the amount of memory they contain, otherwise they function identically. The description of the pins and port access discussed below for the 16F876 also applies to the 16F873 and 16F872.

The 16F876 has 22 I/O pins

These pins are accessed via port A, B and C

To identify which bit within which port is responsible for driving a particular PIC pin, look at the pin diagram below (this diagram is only good for the 16F876, refer to other pin diagrams for other PICs). Locate the pin you are interested in and read off the label.

The label will be of the form Rxj

where x will be A for PORTA, B for PORTB etc., and j will be the bit number within the port (in the range 0 to 7)

On the 16F876 port A bit 0 connects to the pin labelled RA0 which is pin 2, port A bit 3 connects to the pin labelled RA3 which is pin 5

NOTE: most of the I/O pins on the 16F876 have multiple functions and multiple labels to indicate these functions. When using a pin it must be configured to perform the required function. This is done internally by the program when it starts running. The program writes to special function registers which are responsible for configuring the pins.

LABEL PIN   PIN DIAGRAM
 
RA0 2            16f876 pin diagram
RA13
RA24
RA35
RA46
RA57
  
RB021
RB122
RB223
RB324
RB425
RB526
RB627
RB728
  
RC011
RC112
RC213
RC314
RC415
RC516
RC617
RC718
sample projects (ciruits, sample code and chip programming)

XCSB compiler

Writing to a specific pin

To write to pin 5 of a 16F876 lookup the label on pin 5 in the pin diagram. The label is RA3. This translates to PORTA bit 3.

BIT NUMBER    7 6 5 4 3 2 1 0
PORTA x x x x x x x x
TRISA x x x x x x x x
PIN LABEL      RA7     RA6     RA5     RA4     RA3     RA2     RA1     RA0  
PIN NUMBER N/A N/A 7 6 5 4 3 2

Clear the corresponding bit in the PORTA data direction register (called TRISA) to 0, that is clear TRISA bit 3 to 0. This configures pin 5 as an output.

BIT NUMBER    7 6 5 4 3 2 1 0
PORTA x x x x x x x x
TRISA x x x x 0 x x x
PIN LABEL      RA7     RA6     RA5     RA4     RA3     RA2     RA1     RA0  
PIN NUMBER N/A N/A 7 6 5 4 3 2

Then write the required value (0 or 1) to PORTA bit 3

BIT NUMBER    7 6 5 4 3 2 1 0
PORTA x x x x 0 or 1 x x x
TRISA x x x x 0 x x x
PIN LABEL      RA7     RA6     RA5     RA4     RA3     RA2     RA1     RA0  
PIN NUMBER N/A N/A 7 6 5 4 3 2

This can be done in XCSB as: 

// configure pin 5 as output by writing 0 to TRISA bit 3
TRISA = TRISA & ~0x08

// set pin 5 low (to 0) by writing 0 to PORTA bit 3
PORTA = PORTA & ~0x08

// set pin 5 high (to 1) by writing 1 to PORTA bit 3
PORTA = PORTA | 0x08
The 0x08 value is a hex mask (the equivalent of binary 00001000 see binary to hex).

A simpler way to access a bit within a byte is to use a constant expression of the form 

	(1 << n)
where n is the bit number and << is the left shift operator

To access multiple bits within a byte, combine them with the bit wise OR operator
e.g. 

	(1 << n) | (1 << m) | (1 << j)
where n, m and j are the bit numbers

To set pins 1 and 17 to 1 use 

TRISA = TRISA & ~((1 << 2) | (1 << 0))
PORTA = PORTA | ((1 << 2) | (1 << 0))
More about bit manipulation and binary

Reading from a specific pin

To read from pin 5 of a 16F876 lookup the label on pin 5 in the pin diagram. The label is RA3. This translates to PORTA bit 3.

BIT NUMBER    7 6 5 4 3 2 1 0
PORTA x x x x x x x x
TRISA x x x x x x x x
PIN LABEL      RA7     RA6     RA5     RA4     RA3     RA2     RA1     RA0  
PIN NUMBER N/A N/A 7 6 5 4 3 2

Set the corresponding bit in the PORTA data direction register (called TRISA) to 1, that is set TRISA bit 3 to 1. This configures pin 5 as an input.

BIT NUMBER    7 6 5 4 3 2 1 0
PORTA x x x x x x x x
TRISA x x x x 1 x x x
PIN LABEL      RA7     RA6     RA5     RA4     RA3     RA2     RA1     RA0  
PIN NUMBER N/A N/A 7 6 5 4 3 2

Then read from PORTA and mask out bit 3

This can be done in XCSB as: 

// configure pin 5 as input by writing 1 to TRISA bit 3
TRISA = TRISA | 0x08

// read 0 or 1 into the variable called result
result = (PORTA & 0x08) != 0

// NOTE: the expression (PORTA & 0x08) return 0x00 or 0x08
// the expression X != 0 return 0 if X is 0x00 or 1 if X is 0x08
The 0x08 value is a hex mask (the equivalent of binary 00001000 see binary to hex).

A simpler way to access a bit within a byte is to use a constant expression of the form 

	(1 << n)
where n is the bit number and << is the left shift operator

The above example could then be re-written as: 

// configure pin 5 as input by writing 1 to TRISA bit 3
TRISA = TRISA | (1 << 3)

// read 0 or 1 into the variable called result
result = (PORTA & (1 << 3)) != 0

// NOTE: the expression (PORTA & (1 << 3)) return 0x00 or 0x08
// the expression X != 0 return 0 if X is 0x00 or 1 if X is 0x08

More about bit manipulation and binary