Datasheet

© 2008 Microchip Technology Inc. DS41262E-page 69
PIC16F631/677/685/687/689/690
4.3 PORTB and TRISB Registers
PORTB is a 4-bit wide, bidirectional port. The
corresponding data direction register is TRISB (Register
4-6). Setting a TRISB bit (= 1) will make the
corresponding PORTB pin an input (i.e., put the
corresponding output driver in a High-Impedance mode).
Clearing a TRISB bit (= 0) will make the corresponding
PORTB pin an output (i.e., enable the output driver and
put the contents of the output latch on the selected pin).
Example 4-3 shows how to initialize PORTB. Reading
the PORTB register (Register 4-5) reads the status of the
pins, whereas writing to it will write to the PORT latch. All
write operations are read-modify-write operations.
Therefore, a write to a port implies that the port pins are
read, this value is modified and then written to the PORT
data latch.
The TRISB register controls the PORTB pin output
drivers, even when they are being used as analog inputs.
The user should ensure the bits in the TRISB register are
maintained set when using them as analog inputs. I/O
pins configured as analog input always read ‘0’.
EXAMPLE 4-3: INITIALIZING PORTB
4.4 Additional PORTB Pin Functions
PORTB pins RB<7:4> on the device family device have
an interrupt-on-change option and a weak pull-up
option. The following three sections describe these
PORTB pin functions.
4.4.1 WEAK PULL-UPS
Each of the PORTB pins has an individually configurable
internal weak pull-up. Control bits WPUB<7:4> enable or
disable each pull-up (see Register 4-9). Each weak
pull up is automatically turned off when the port pin is
configured as an output. All pull-ups are disabled on a
Power-on Reset by the RABPU
bit of the OPTION
register.
4.4.2 INTERRUPT-ON-CHANGE
Four of the PORTB pins are individually configurable as
an interrupt-on-change pin. Control bits IOCB<7:4>
enable or disable the interrupt function for each pin.
Refer to Register 4-10. The interrupt-on-change feature
is disabled on a Power-on Reset.
For enabled interrupt-on-change pins, the present
value is compared with the old value latched on the last
read of PORTB to determine which bits have changed
or mismatch the old value. The ‘mismatch’ outputs are
OR’d together to set the PORTB Change Interrupt flag
bit (RABIF) in the INTCON register (Register 2-3).
This interrupt can wake the device from Sleep. The user,
in the Interrupt Service Routine, clears the interrupt by:
a) Any read or write of PORTB. This will end the
mismatch condition.
b) Clear the flag bit RABIF.
A mismatch condition will continue to set flag bit RABIF.
Reading or writing PORTB will end the mismatch
condition and allow flag bit RABIF to be cleared. The latch
holding the last read value is not affected by a MCLR
nor
Brown-out Reset. After these Resets, the RABIF flag will
continue to be set if a mismatch is present.
Note: The ANSELH register must be initialized
to configure an analog channel as a digital
input. Pins configured as analog inputs will
read ‘0’.
BCF STATUS,RP0 ;Bank 0
BCF STATUS,RP1 ;
CLRF PORTB ;Init PORTB
BSF STATUS,RP0 ;Bank 1
MOVLW FFh ;Set RB<7:4> as inputs
MOVWF TRISB ;
BCF STATUS,RP0 ;Bank 0
Note: If a change on the I/O pin should occur when
the read operation is being executed (start of
the Q2 cycle), then the RABIF interrupt flag
may not get set. Furthermore, since a read
or write on a port affects all bits of that port,
care must be taken when using multiple pins
in Interrupt-on-Change mode. Changes on
one pin may not be seen while servicing
changes on another pin.
REGISTER 4-7: PORTB: PORTB REGISTER
R/W-x R/W-x R/W-x R/W-x U-0 U-0 U-0 U-0
RB7 RB6 RB5 RB4
— — — —
bit 7 bit 0
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown
bit 7-4 RB<7:4>: PORTB I/O Pin bit
1 = Port pin is > V
IH
0 = Port pin is < VIL
bit 3-0 Unimplemented: Read as ‘0’