16F688 Physical Input and Output

The 16F688 has 12 I/O pins

These pins are accessed via port A 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 16F688, 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 and C for PORTC, and j will be the bit number within the port (in the range 0 to 7)

On the 16F688 port A bit 0 connects to the pin labelled RA0 which is pin 13, port A bit 3 connects to the pin labelled RA3 which is pin 4

NOTE: most of the I/O pins on the 16F688 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 13            16f688 pin diagram
RA112
RA211
RA34
RA43
RA52
  
RC010
RC19
RC28
RC37
RC46
RC55
sample projects (ciruits, sample code and chip programming)

XCSB compiler

Writing to a specific pin

To write to pin 11 of a 16F688 lookup the label on pin 11 in the pin diagram. The label is RA2. This translates to PORTA bit 2.

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 2 3 4 11 12 13

Clear the corresponding bit in the PORTA data direction register (called TRISA) to 0, that is clear TRISA bit 2 to 0. This configures pin 11 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 x 0 x x
PIN LABEL      RA7     RA6     RA5     RA4     RA3     RA2     RA1     RA0  
PIN NUMBER N/A N/A 2 3 4 11 12 13

Then write the required value (0 or 1) to PORTA bit 2

BIT NUMBER    7 6 5 4 3 2 1 0
PORTA x x x x x 0 or 1 x x
TRISA x x x x x 0 x x
PIN LABEL      RA7     RA6     RA5     RA4     RA3     RA2     RA1     RA0  
PIN NUMBER N/A N/A 2 3 4 11 12 13

This can be done in XCSB as: 

// configure pin 11 as output by writing 0 to TRISA bit 2
TRISA = TRISA & ~0x04

// set pin 11 low (to 0) by writing 0 to PORTA bit 2
PORTA = PORTA & ~0x04

// set pin 11 high (to 1) by writing 1 to PORTA bit 2
PORTA = PORTA | 0x04
The 0x04 value is a hex mask (the equivalent of binary 00000100 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 13 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 11 of a 16F688 lookup the label on pin 11 in the pin diagram. The label is RA2. This translates to PORTA bit 2.

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 2 3 4 11 12 13

Set the corresponding bit in the PORTA data direction register (called TRISA) to 1, that is set TRISA bit 2 to 1. This configures pin 11 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 x 1 x x
PIN LABEL      RA7     RA6     RA5     RA4     RA3     RA2     RA1     RA0  
PIN NUMBER N/A N/A 2 3 4 11 12 13

Then read from PORTA and mask out bit 2

This can be done in XCSB as: 

// configure pin 11 as input by writing 1 to TRISA bit 2
TRISA = TRISA | 0x04

// read 0 or 1 into the variable called result
result = (PORTA & 0x04) != 0

// NOTE: the expression (PORTA & 0x04) return 0x00 or 0x04
// the expression X != 0 return 0 if X is 0x00 or 1 if X is 0x04
The 0x04 value is a hex mask (the equivalent of binary 00000100 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 11 as input by writing 1 to TRISA bit 2
TRISA = TRISA | (1 << 2)

// read 0 or 1 into the variable called result
result = (PORTA & (1 << 2)) != 0

// NOTE: the expression (PORTA & (1 << 2)) return 0x00 or 0x04
// the expression X != 0 return 0 if X is 0x00 or 1 if X is 0x04

More about bit manipulation and binary