PORTA is a 6-bit wide, bidirectional port. The
corresponding data direction register is TRISA
11-3). 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
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).
The TRISA register
11-3) 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’.
The ANSELA register
11-5) 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
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.
B'00111000' ;Set RA<5:3> as inputs
;and set RA<2:0> as
PORTA FUNCTIONS AND OUTPUT
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
11-2) reads the
control the pin when it is in Analog mode with the
priority shown below in
Priority listed from highest to lowest.
Default pin (see APFCON register).
PORTA OUTPUT PRIORITY
2011 Microchip Technology Inc.