PIC16F882/883/884/886/887
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.
3.0
I/O PORTS
There are as many as thirty-five general purpose I/O
pins available. Depending on which peripherals are
enabled, some or all of the pins may not be available as
general purpose I/O. In general, when a peripheral is
enabled, the associated pin may not be used as a
general purpose I/O pin.
The TRISA register (Register 3-2) controls the PORTA
pin output drivers, even when they are being used as
analog inputs. The user should ensure the bits in the
TRISA register are maintained set when using them as
analog inputs. I/O pins configured as analog input always
read ‘0’.
3.1
PORTA and the TRISA Registers
Note:
The ANSEL register must be initialized to
configure an analog channel as a digital
input. Pins configured as analog inputs will
read ‘0’.
PORTA is a 8-bit wide, bidirectional port. The
corresponding data direction register is TRISA
(Register 3-2). Setting a TRISA bit (= 1) will make the
corresponding PORTA pin an input (i.e., disable the
output driver). Clearing a TRISA bit (= 0) will make the
corresponding PORTA pin an output (i.e., enables
output driver and puts the contents of the output latch
on the selected pin). Example 3-1 shows how to
initialize PORTA.
EXAMPLE 3-1:
INITIALIZING PORTA
BANKSELPORTA
;
CLRF
BANKSELANSEL
PORTA
;Init PORTA
;
CLRF
BCF
ANSEL
STATUS,RP1 ;Bank 1
;digital I/O
Reading the PORTA register (Register 3-1) reads the
status of the pins, whereas writing to it will write to the
PORT latch. All write operations are read-modify-write
BANKSELTRISA
;
MOVLW
MOVWF
0Ch
TRISA
;Set RA<3:2> as inputs
;and set RA<5:4,1:0>
;as outputs
REGISTER 3-1:
PORTA: PORTA REGISTER
R/W-x
RA7
R/W-x
RA6
R/W-x
RA5
R/W-x
RA4
R/W-x
RA3
R/W-x
RA2
R/W-x
RA1
R/W-x
RA0
bit 7
bit 0
Legend:
R = Readable bit
-n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared
x = Bit is unknown
bit 7-0
RA<7:0>: PORTA I/O Pin bit
1= Port pin is > VIH
0= Port pin is < VIL
REGISTER 3-2:
TRISA: PORTA TRI-STATE REGISTER
(1)
(1)
R/W-1
R/W-1
R/W-1
R/W-1
R/W-1
R/W-1
R/W-1
R/W-1
TRISA7
bit 7
TRISA6
TRISA5
TRISA4
TRISA3
TRISA2
TRISA1
TRISA0
bit 0
Legend:
R = Readable bit
-n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared x = Bit is unknown
bit 7-0
TRISA<7:0>: PORTA Tri-State Control bit
1= PORTA pin configured as an input (tri-stated)
0= PORTA pin configured as an output
Note 1: TRISA<7:6> always reads ‘1’ in XT, HS and LP Oscillator modes.
© 2007 Microchip Technology Inc.
Preliminary
DS41291D-page 39
MICROCHIP [ MICROCHIP ]
