PIC18F45J10 FAMILY
10.1.3
INTERFACING TO A 5V SYSTEM
10.2 PORTA, TRISA and LATA Registers
Though the VDDMAX of the PIC18F45J10 family is 3.6V,
these devices are still capable of interfacing with 5V
systems, even if the VIH of the target system is above
3.6V. This is accomplished by adding a pull-up resistor
to the port pin (Figure 10-2), clearing the LAT bit for that
pin and manipulating the corresponding TRIS bit
(Figure 10-1) to either allow the line to be pulled high or
to drive the pin low. Only port pins that are tolerant of
voltages up to 5.5V can be used for this type of
interface (refer to Section 10.1.2 “Input Pins and
Voltage Considerations”).
PORTA is a 5-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.
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.
FIGURE 10-2:
+5V SYSTEM HARDWARE
INTERFACE
The other PORTA pins are multiplexed with analog
inputs, the analog VREF+ and VREF- inputs and the com-
parator voltage reference output. The operation of pins
RA<3:0> and RA5 as A/D converter inputs is selected
by clearing or setting the control bits in the ADCON1
register (A/D Control Register 1).
+5V
+5V Device
PIC18F45J10
Pins RA0 and RA3 may also be used as comparator
inputs and RA5 may be used as the C2 comparator
output by setting the appropriate bits in the CMCON
register. To use RA<3:0> as digital inputs, it is also
necessary to turn off the comparators.
RD7
Note:
On a Power-on Reset, RA5 and RA<3:0>
are configured as analog inputs and read
as ‘0’.
EXAMPLE 10-1:
COMMUNICATING WITH
THE +5V SYSTEM
All PORTA pins have TTL input levels and full CMOS
output drivers.
BCF LATD, 7
; set up LAT register so
; changing TRIS bit will
; drive line low
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.
BCF TRISD, 7 ; send a 0 to the 5V system
BCF TRISD, 7 ; send a 1 to the 5V system
EXAMPLE 10-2:
INITIALIZING PORTA
CLRF
PORTA
; Initialize PORTA by
; clearing output
; data latches
CLRF
LATA
; Alternate method
; to clear output
; data latches
MOVLW
MOVWF
MOVWF
MOVWF
MOVLW
07h
ADCON1
07h
CMCON
0CFh
; Configure A/D
; for digital inputs
; Configure comparators
; for digital input
; Value used to
; initialize data
; direction
MOVWF
TRISA
; Set RA<3:0> as inputs
; RA<5:4> as outputs
DS39682E-page 98
© 2009 Microchip Technology Inc.
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