Microchip Technology MA160014 Data Sheet

 2010-2012 Microchip Technology Inc.

DS41412F-page 135

Depending on the device selected and features
enabled, there are up to five ports available. All pins of
the I/O ports are multiplexed with one or more alternate
functions from the peripheral features on the device. In
general, when a peripheral is enabled, that pin may not
be used as a general purpose I/O pin.

Each port has five registers for its operation. These
registers are:

• TRIS register (data direction register)

• PORT register (reads the levels on the pins of the 

device)

• LAT register (output latch)

• ANSEL register (analog input control)

• SLRCON register (port slew rate control)

The Data Latch (LAT register) is useful for read-modify-
write operations on the value that the I/O pins are
driving.

A simplified model of a generic I/O port, without the
interfaces to other peripherals, is shown in 

PORTA is an 8-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., enable the output driver and put the
contents of the output latch on the selected pin).

Reading the PORTA register reads the status of the
pins, whereas writing to it, will write to the PORT latch. 

The Data Latch (LATA) register is also memory mapped.
Read-modify-write operations on the LATA register read
and write the latched output value for PORTA.

The RA4 pin is multiplexed with the Timer0 module
clock input and one of the comparator outputs to
become the RA4/T0CKI/C1OUT pin. Pins RA6 and
RA7 are multiplexed with the main oscillator pins; they
are enabled as oscillator or I/O pins by the selection of
the main oscillator in the Configuration register (see

 for details). When

they are not used as port pins, RA6 and RA7 and their
associated TRIS and LAT bits are read as ‘0’.

The other PORTA pins are multiplexed with analog
inputs, the analog V

REF

+ and V

REF

- inputs, and the

comparator voltage reference output. The operation of
pins RA<3:0> and RA5 as analog is selected by setting
the ANSELA<5, 3:0> bits in the ANSELA register which
is the default setting after a Power-on Reset.

Pins RA0 through RA5 may also be used as comparator
inputs or outputs by setting the appropriate bits in the
CM1CON0 and CM2CON0 registers.

The RA4/T0CKI/C1OUT pin is a Schmitt Trigger input.
All other PORTA pins have TTL input levels and full
CMOS output drivers.

The TRISA register controls the drivers of the PORTA
pins, 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. 

Data
Bus

WR LAT

WR TRIS

RD Port

Data Latch

TRIS Latch

RD TRIS

Input

Buffer

I/O pin

Q

D

CK

Q

D

CK

EN

Q

D

EN

RD LAT

or Port

I/O pins have diode protection to V

DD

 and V

SS

.

TRISx

ANSELx

On a Power-on Reset, RA5 and RA<3:0>
are configured as analog inputs and read
as ‘0’. RA4 is configured as a digital input.

MOVLB

0xF

; Set BSR for banked SFRs

CLRF

PORTA

; Initialize PORTA by

; clearing output

; data latches

CLRF

LATA

; Alternate method

; to clear output

; data latches

MOVLW

E0h

; Configure I/O 

MOVWF

ANSELA

; for digital inputs

MOVLW 

0CFh

; Value used to 

; initialize data 

; direction

MOVWF 

TRISA

; Set RA<3:0> as inputs

; RA<5:4> as outputs