Microchip Technology MCP3421DM-WS Data Sheet
PIC18F2455/2550/4455/4550
DS39632E-page 116
© 2009 Microchip Technology Inc.
10.2
PORTB, TRISB and LATB
Registers
Registers
PORTB is an 8-bit wide, bidirectional port. The corre-
sponding Data Direction register is TRISB. 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.,
put the contents of the output latch on the selected pin).
The Data Latch register (LATB) is also memory
mapped. Read-modify-write operations on the LATB
register read and write the latched output value for
PORTB.
Each of the PORTB pins has a weak internal pull-up. A
single control bit can turn on all the pull-ups. This is
performed by clearing bit, RBPU (INTCON2<7>). The
weak pull-up is automatically turned off when the port
pin is configured as an output. The pull-ups are
disabled on a Power-on Reset.
sponding Data Direction register is TRISB. 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.,
put the contents of the output latch on the selected pin).
The Data Latch register (LATB) is also memory
mapped. Read-modify-write operations on the LATB
register read and write the latched output value for
PORTB.
Each of the PORTB pins has a weak internal pull-up. A
single control bit can turn on all the pull-ups. This is
performed by clearing bit, RBPU (INTCON2<7>). The
weak pull-up is automatically turned off when the port
pin is configured as an output. The pull-ups are
disabled on a Power-on Reset.
Four of the PORTB pins (RB7:RB4) have an interrupt-
on-change feature. Only pins configured as inputs can
cause this interrupt to occur. Any RB7:RB4 pin
configured as an output is excluded from the interrupt-
on-change comparison. The pins are compared with
the old value latched on the last read of PORTB. The
“mismatch” outputs of RB7:RB4 are ORed together to
generate the RB Port Change Interrupt with Flag bit,
RBIF (INTCON<0>).
The interrupt-on-change can be used to wake the
device from Sleep. The user, in the Interrupt Service
Routine, can clear the interrupt in the following manner:
a)
on-change feature. Only pins configured as inputs can
cause this interrupt to occur. Any RB7:RB4 pin
configured as an output is excluded from the interrupt-
on-change comparison. The pins are compared with
the old value latched on the last read of PORTB. The
“mismatch” outputs of RB7:RB4 are ORed together to
generate the RB Port Change Interrupt with Flag bit,
RBIF (INTCON<0>).
The interrupt-on-change can be used to 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 (except with the
MOVFF (ANY), PORTB instruction). This will
end the mismatch condition.
MOVFF (ANY), PORTB instruction). This will
end the mismatch condition.
b)
Wait one T
CY
delay (for example, execute one
NOP instruction).
c)
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 after a one T
Reading PORTB will end the mismatch condition and
allow flag bit, RBIF, to be cleared after a one T
CY
delay.
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.
Pins, RB2 and RB3, are multiplexed with the USB
peripheral and serve as the differential signal outputs
for an external USB transceiver (TRIS configuration).
Refer to Section 17.2.2.2 “External Transceiver” for
additional information on configuring the USB module
for operation with an external transceiver.
RB4 is multiplexed with CSSPP, the chip select
function for the Streaming Parallel Port (SPP) – TRIS
setting. Details of its operation are discussed in
Section 18.0 “Streaming Parallel Port”.
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.
Pins, RB2 and RB3, are multiplexed with the USB
peripheral and serve as the differential signal outputs
for an external USB transceiver (TRIS configuration).
Refer to Section 17.2.2.2 “External Transceiver” for
additional information on configuring the USB module
for operation with an external transceiver.
RB4 is multiplexed with CSSPP, the chip select
function for the Streaming Parallel Port (SPP) – TRIS
setting. Details of its operation are discussed in
Section 18.0 “Streaming Parallel Port”.
EXAMPLE 10-2:
INITIALIZING PORTB
Note:
On a Power-on Reset, RB4:RB0 are
configured as analog inputs by default and
read as ‘0’; RB7:RB5 are configured as
digital inputs.
By programming the Configuration bit,
PBADEN (CONFIG3H<1>), RB4:RB0 will
alternatively be configured as digital inputs
on POR.
configured as analog inputs by default and
read as ‘0’; RB7:RB5 are configured as
digital inputs.
By programming the Configuration bit,
PBADEN (CONFIG3H<1>), RB4:RB0 will
alternatively be configured as digital inputs
on POR.
CLRF PORTB
; Initialize PORTB by
; clearing output
; data latches
; clearing output
; data latches
CLRF
LATB
; Alternate method
; to clear output
; data latches
; to clear output
; data latches
MOVLW 0Eh
; Set RB<4:0> as
MOVWF
ADCON1 ; digital I/O pins
; (required if config bit
; PBADEN is set)
; PBADEN is set)
MOVLW
0CFh
; Value used to
; initialize data
; direction
; initialize data
; direction
MOVWF
TRISB
; Set RB<3:0> as inputs
; RB<5:4> as outputs
; RB<7:6> as inputs
; RB<5:4> as outputs
; RB<7:6> as inputs