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PIC16(L)F1508/9
DS40001609E-page 108
2011-2015 Microchip Technology Inc.
11.3
PORTA Registers
11.3.1
DATA REGISTER
PORTA is a 6-bit wide, bidirectional port. The
corresponding data direction register is TRISA
(
). 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
’.
shows how to initialize an I/O port.
Reading the PORTA 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. 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 (LATA).
11.3.2
DIRECTION CONTROL
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
’.
11.3.3
ANALOG CONTROL
The ANSELA register (
) is used to
configure the Input mode of an I/O pin to analog.
Setting the appropriate ANSELA bit high will cause all
digital reads on the pin to be read as ‘
0
’ and allow
analog functions on the pin to operate correctly.
The state of the ANSELA bits has no effect on digital
output functions. A pin with TRIS clear and ANSEL set
will still operate as a digital output, but the Input mode
will be analog. This can cause unexpected behavior
when executing read-modify-write instructions on the
affected port.
EXAMPLE 11-1:
INITIALIZING PORTA
11.3.4
PORTA FUNCTIONS AND OUTPUT
PRIORITIES
Each PORTA pin is multiplexed with other functions. The
pins, their combined functions and their output priorities
are shown in
When multiple outputs are enabled, the actual pin
control goes to the peripheral with the highest priority.
Analog input functions, such as ADC and comparator
inputs, are not shown in the priority lists. These inputs
are active when the I/O pin is set for Analog mode using
the ANSELx registers. Digital output functions may
control the pin when it is in Analog mode with the
priority shown below in
.
Note:
The ANSELA bits default to the Analog
mode after Reset. To use any pins as
digital general purpose or peripheral
inputs, the corresponding ANSEL bits
must be initialized to ‘
0
’ by user software.
BANKSEL
PORTA
;
CLRF
PORTA
;Init PORTA
BANKSEL
LATA
;Data Latch
CLRF
LATA
;
BANKSEL
ANSELA
;
CLRF ANSELA
;digital
I/O
BANKSEL
TRISA
;
MOVLW
B'00111000' ;Set RA<5:3> as inputs
MOVWF
TRISA
;and set RA<2:0> as
;outputs
TABLE 11-2:
PORTA OUTPUT PRIORITY
Pin Name
Function Priority
(1)
RA0
ICSPDAT
DAC1OUT1
RA0
RA1
RA1
RA2
DAC1OUT2
CLC1
(2)
C1OUT
PWM3
RA2
RA3
None
RA4
CLKOUT
SOSCO
RA4
RA5
SOSCI
RA5
Note
1:
Priority listed from highest to lowest.
2:
Default pin (see APFCON register).
3:
Alternate pin (see APFCON register).
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