Datasheet

PIC16F913/914/916/917/946
DS41250F-page 44 © 2007 Microchip Technology Inc.
3.2 PORTA and TRISA Registers
PORTA is a 8-bit wide, bidirectional port. The
corresponding data direction register is TRISA
(Register 3-3). 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). Example 3-1 shows how to
initialize PORTA.
Five of the pins of PORTA can be configured as analog
inputs. These pins, RA5 and RA<3:0>, are configured
as analog inputs on device power-up and must be
reconfigured by the user to be used as I/O’s. This is
done by writing the appropriate values to the CMCON0
and ANSEL registers (see Example 3-1).
Reading the PORTA register (Register 3-2) 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 means that the
port pins are read, this value is modified and then written
to the PORT data latch.
The TRISA register controls the direction of the PORTA
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. I/O
pins configured as analog inputs always read ‘0’.
EXAMPLE 3-1: INITIALIZING PORTA
Note 1: The CMCON0 and ANSEL registers must
be initialized to configure an analog
channel as a digital input. Pins configured
as analog inputs will read ‘0’.
BANKSEL PORTA ;
CLRF PORTA ;Init PORTA
BANKSEL TRISA ;
MOVLW 07h ;Set RA<2:0> to
MOVWF CMCON0 ;digital I/O
CLRF ANSEL ;Make all PORTA digital I/O
MOVLW 0F0h ;Set RA<7:4> as inputs
MOVWF TRISA ;and set RA<3:0> as outputs
REGISTER 3-2: PORTA: PORTA REGISTER
R/W-x R/W-x R/W-x R/W-x R/W-x R/W-x R/W-x R/W-x
RA7 RA6 RA5 RA4 RA3 RA2 RA1 RA0
bit 7 bit 0
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown
bit 7-0 RA<7:0>: PORTA I/O Pin bits
1 = Port pin is >V
IH min.
0 = Port pin is <V
IL max.
REGISTER 3-3: TRISA: PORTA TRI-STATE REGISTER
R/W-1 R/W-1 R/W-1 R/W-1 R/W-1 R/W-1 R/W-1 R/W-1
TRISA7 TRISA6 TRISA5 TRISA4 TRISA3 TRISA2 TRISA1 TRISA0
bit 7 bit 0
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown
bit 7-0 TRISA<7:0>: PORTA Tri-State Control bits
1 = PORTA pin configured as an input (tri-stated)
0 = PORTA pin configured as an output
Note 1: TRISA<7:6> always reads ‘1’ in XT, HS and LP Oscillator modes.