PIC16F631/677/685/687/689/690
port pins are read, this value is modified and then
written to the PORT data latch. RA3 reads ‘0’ when
4.0
I/O PORTS
There are as many as eighteen 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.
MCLRE = 1.
The TRISA register 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’.
4.1
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 6-bit wide, bidirectional port. The
corresponding data direction register is TRISA
(Register 4-2). 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). The exception is RA3, which is
input only and its TRIS bit will always read as ‘1’.
Example 4-1 shows how to initialize PORTA.
EXAMPLE 4-1:
INITIALIZING PORTA
BCF
BCF
STATUS,RP0 ;Bank 0
STATUS,RP1
;
CLRF PORTA
;Init PORTA
BSF
STATUS,RP1 ;Bank 2
CLRF ANSEL
;digital I/O
BSF
BCF
STATUS,RP0 ;Bank 1
STATUS,RP1
Reading the PORTA register (Register 4-1) 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
;
MOVLW 0Ch
MOVWF TRISA
;Set RA<3:2> as inputs
;and set RA<5:4,1:0>
;as outputs
BCF
STATUS,RP0 ;Bank 0
REGISTER 4-1:
PORTA: PORTA REGISTER
U-0
—
U-0
—
R/W-x
RA5
R/W-x
RA4
R-x
R/W-x
RA2
R/W-x
RA1
R/W-x
RA3
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 7-6
bit 5-0
Unimplemented: Read as ‘0’
RA<5:0>: PORTA I/O Pin bit
1= Port pin is > VIH
0= Port pin is < VIL
REGISTER 4-2:
TRISA: PORTA TRI-STATE REGISTER
U-0
—
U-0
—
R/W-1
R/W-1
R-1
R/W-1
R/W-1
R/W-1
TRISA5
TRISA4
TRISA3
TRISA2
TRISA1
TRISA0
bit 0
bit 7
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 7-6
bit 5-0
Unimplemented: Read as ‘0’
TRISA<5:0>: PORTA Tri-State Control bit
1= PORTA pin configured as an input (tri-stated)
0= PORTA pin configured as an output
Note 1: TRISA<3> always reads ‘1’.
2: TRISA<5:4> always reads ‘1’ in XT, HS and LP Oscillator modes.
© 2007 Microchip Technology Inc.
DS41262D-page 59
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