Datasheet

 2002-2013 Microchip Technology Inc. DS30487D-page 51
PIC16F87/88
5.0 I/O PORTS
Some pins for these I/O ports are multiplexed with an
alternate function for 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.
Additional information on I/O ports may be found in the
“PIC
®
Mid-Range MCU Family Reference Manual”
(DS33023).
5.1 PORTA and the TRISA Register
PORTA is an 8-bit wide, bidirectional port. The corre-
sponding data direction register is TRISA. Setting a
TRISA bit (= 1) will make the corresponding PORTA
pin an input (i.e., put the corresponding output driver in
a high-impedance mode). Clearing a TRISA bit (= 0)
will make the corresponding PORTA pin an output (i.e.,
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. 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.
Pin RA4 is multiplexed with the Timer0 module clock
input. On PIC16F88 devices, it is also multiplexed with
an analog input to become the RA4/AN4/T0CKI/
C2OUT pin. The RA4/AN4/T0CKI/C2OUT pin is a
Schmitt Trigger input and full CMOS output driver.
Pin RA5 is multiplexed with the Master Clear module
input. The RA5/MCLR
/VPP pin is a Schmitt Trigger
input.
Pin RA6 is multiplexed with the oscillator module input
and external oscillator output. Pin RA7 is multiplexed
with the oscillator module input and external oscillator
input. Pin RA6/OSC2/CLKO and pin RA7/OSC1/CLKI
are Schmitt Trigger inputs and full CMOS output drivers.
Pins RA<1:0> are multiplexed with analog inputs. Pins
RA<3:2> are multiplexed with analog inputs and com-
parator outputs. On PIC16F88 devices, pins RA<3:2>
are also multiplexed with the V
REF inputs. Pins RA<3:0>
have TTL inputs and full CMOS output drivers.
EXAMPLE 5-1: INITIALIZING PORTA
TABLE 5-1: PORTA FUNCTIONS
Note: On a Power-on Reset, the pins
PORTA<4:0> are configured as analog
inputs and read as ‘0’.
BANKSEL PORTA ; select bank of PORTA
CLRF PORTA ; Initialize PORTA by
; clearing output
; data latches
BANKSEL ANSEL ; Select Bank of ANSEL
MOVLW 0x00 ; Configure all pins
MOVWF ANSEL ; as digital inputs
MOVLW 0xFF ; Value used to
; initialize data
; direction
MOVWF TRISA ; Set RA<7:0> as inputs
Name Bit# Buffer Function
RA0/AN0 bit 0 TTL Input/output or analog input.
RA1/AN1 bit 1 TTL Input/output or analog input.
RA2/AN2/CVREF/VREF-
(2)
bit 2 TTL Input/output, analog input, VREF- or comparator VREF
output.
RA3/AN3/V
REF+
(2)
/C1OUT bit 3 TTL Input/output, analog input, VREF+ or comparator output.
RA4/AN4
(2)
/T0CKI/C2OUT bit 4 ST Input/output, analog input, TMR0 external input or
comparator output.
RA5/MCLR/VPP bit 5 ST Input, Master Clear (Reset) or programming voltage input.
RA6/OSC2/CLKO bit 6 ST Input/output, connects to crystal or resonator, oscillator
output or 1/4 the frequency of OSC1 and denotes the
instruction cycle in RC mode.
RA7/OSC1/CLKI bit 7 ST/CMOS
(1)
Input/output, connects to crystal or resonator or oscillator
input.
Legend: TTL = TTL input, ST = Schmitt Trigger input
Note 1: This buffer is a Schmitt Trigger input when configured in RC Oscillator mode and a CMOS input otherwise.
2: PIC16F88 only.