PIC16C77X Datasheet, PDF (27/200 Pages) Microchip Technology – 28/40-Pin, 8-Bit CMOS Microcontrollers w/ 12-Bit A/D

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 for the 40/44
pin devices and is 5-bits wide for the 28-pin devices.
PORTA<5> is not on the 28-pin devices. 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, this 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 RA port pins have TTL input levels and full
CMOS output drivers.
Other PORTA pins are multiplexed with analog inputs
and analog VREF inputs and precision on-board refer-
ences (VRL/VRH). 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 ;
CLRF PORTA
; Initialize PORTA by
; clearing output
; data latches
BSF STATUS, RP0 ; Select Bank 1
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’.
PIC16C77X
FIGURE 3-1: BLOCK DIAGRAM OF
RA3:RA2 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
VRH, VRL
VRHOEN, VRLOEN
Sense input for
VRO+, VRO- amplifier
Note 1: I/O pins have protection diodes to VDD and
VSS.
© 1999 Microchip Technology Inc.
Advance Information
DS30275A-page 27