PIC16F872 Datasheet, PDF (23/160 Pages) Microchip Technology – 28-Pin, 8-Bit CMOS FLASH Microcontroller

3.0 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
PICmicro™ Mid-Range Reference Manual,
(DS33023).
3.1 PORTA and the TRISA Register
PORTA is a 6-bit wide, bi-directional 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
hi-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, the value is modified and then written to the port
data latch.
Pin RA4 is multiplexed with the Timer0 module clock
input to become the RA4/T0CKI pin. The RA4/T0CKI
pin is a Schmitt Trigger input and an open drain output.
All other PORTA pins have TTL input levels and full
CMOS output drivers.
Other PORTA pins are multiplexed with analog inputs
and analog VREF input. The operation of each pin is
selected by clearing/setting the control bits in the
ADCON1 register (A/D Control Register1).
Note: On a Power-on Reset, these pins are con-
figured as analog inputs and read as '0'.
The TRISA register controls the direction of the RA
pins, even when they are being used as analog inputs.
The user must ensure the bits in the TRISA register are
maintained set when using them as analog inputs.
EXAMPLE 3-1: INITIALIZING PORTA
BCF
STATUS, RP0 ;
BCF
STATUS, RP1 ; Bank0
CLRF PORTA
; Initialize PORTA by
; clearing output
; data latches
BSF
STATUS, RP0 ; Select Bank 1
MOVLW 0x06
; Configure all pins
MOVWF ADCON1
; as digital inputs
MOVLW 0xCF
; Value used to
; initialize data
; direction
MOVWF TRISA
; Set RA<3:0> as inputs
; RA<5:4> as outputs
; TRISA<7:6> are always
; read as ’0’.
PIC16F872
FIGURE 3-1: BLOCK DIAGRAM OF
RA<3:0> AND RA5 PINS
Data
Bus
WR
Port
D
Q
CK
Q
Data Latch
D
Q
VDD
P
N
I/O pin⁽¹⁾
WR
TRIS
CK
Q
TRIS Latch
VSS
Analog
Input
Mode
RD TRIS
Q
D
TTL
Input
Buffer
EN
RD Port
To A/D Converter
Note 1: I/O pins have protection diodes to VDD and VSS.
FIGURE 3-2: BLOCK DIAGRAM OF RA4/
T0CKI PIN
Data
Bus
WR
Port
WR
TRIS
DQ
CK Q
Data Latch
DQ
CK Q
TRIS Latch
N
I/O pin⁽¹⁾
VSS
Schmitt
Trigger
Input
Buffer
RD Port
RD TRIS
QD
ENEN
TMR0 clock input
Note 1: I/O pin has protection diodes to VSS only.
© 1999 Microchip Technology Inc.
Preliminary
DS30221A-page 23