PIC16F627A_05 Datasheet, PDF (38/180 Pages) Microchip Technology – Flash-Based, 8-Bit CMOS Microcontrollers with nanoWatt Technology

PIC16F627A/628A/648A
5.2 PORTB and TRISB Registers
PORTB is an 8-bit wide bidirectional port. The
corresponding data direction register is TRISB. A ‘1’ in
the TRISB register puts the corresponding output driver
in a High-impedance mode. A ‘0’ in the TRISB register
puts the contents of the output latch on the selected
pin(s).
PORTB is multiplexed with the external interrupt,
USART, CCP module and the TMR1 clock input/output.
The standard port functions and the alternate port
functions are shown in Table 5-3. Alternate port
functions may override the TRIS setting when enabled.
Reading PORTB register reads the status of the pins,
whereas writing to it will write to the port latch. All write
operations are read-modify-write operations. So a write
to a port implies that the port pins are first read, then
this value is modified and written to the port data latch.
Each of the PORTB pins has a weak internal pull-up
(≈200 μA typical). A single control bit can turn on all the
pull-ups. This is done by clearing the RBPU
(OPTION<7>) bit. The weak pull-up is automatically
turned off when the port pin is configured as an output.
The pull-ups are disabled on Power-on Reset.
Four of PORTB’s pins, RB<7:4>, have an interrupt-on-
change feature. Only pins configured as inputs can
cause this interrupt to occur (i.e., any RB<7:4> pin
configured as an output is excluded from the interrupt-
on-change comparison). The input pins (of RB<7:4>)
are compared with the old value latched on the last
read of PORTB. The “mismatch” outputs of RB<7:4>
are OR’ed together to generate the RBIF interrupt (flag
latched in INTCON<0>).
This interrupt can wake the device from Sleep. The
user, in the interrupt service routine, can clear the
interrupt in the following manner:
a) Any read or write of PORTB. This will end the
mismatch condition.
b) Clear flag bit RBIF.
A mismatch condition will continue to set flag bit RBIF.
Reading PORTB will end the mismatch condition and
allow flag bit RBIF to be cleared.
This interrupt on mismatch feature, together with
software configurable pull-ups on these four pins allow
easy interface to a key pad and make it possible for
wake-up on key-depression (See Application Note
AN552 “Implementing Wake-up on Key Strokes”
(DS00552).
Note:
If a change on the I/O pin should occur
when a read operation is being executed
(start of the Q2 cycle), then the RBIF
interrupt flag may not get set.
The interrupt-on-change feature is recommended for
wake-up on key depression operation and operations
where PORTB is only used for the interrupt-on-change
feature. Polling of PORTB is not recommended while
using the interrupt-on-change feature.
FIGURE 5-8:
RBPU
BLOCK DIAGRAM OF
RB0/INT PIN
VDD
P Weak Pull-up
VDD
Data Bus
WR PORTB
D
Q
CK Q
Data Latch
RB0/INT
VSS
WR TRISB
D
Q
CK Q
TRIS Latch
RD TRISB
RD PORTB
INT
TTL
Input
Buffer
QD
E_EN_N
Schmitt
Trigger
DS40044D-page 36
© 2005 Microchip Technology Inc.