64GA106 datasheet(134/330 Pages) MICROCHIP

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PIC24FJ256GA110 FAMILY

DS39905E-page 134

 2010 Microchip Technology Inc.

10.4.5

CONSIDERATIONS FOR

PERIPHERAL PIN SELECTION

The ability to control Peripheral Pin Select options

introduces several considerations into application

design that could be overlooked. This is particularly

true for several common peripherals that are available

only as remappable peripherals.

The main consideration is that the Peripheral Pin

Selects are not available on default pins in the device’s

default (Reset) state. Since all RPINRx registers reset

to ‘111111’ and all RPORx registers reset to ‘000000’,

all Peripheral Pin Select inputs are tied to VSS and all

Peripheral Pin Select outputs are disconnected.

This situation requires the user to initialize the device

with the proper peripheral configuration before any

other application code is executed. Since the IOLOCK

bit resets in the unlocked state, it is not necessary to

execute the unlock sequence after the device has

come out of Reset. For application safety, however, it is

best to set IOLOCK and lock the configuration after

writing to the control registers.

Because the unlock sequence is timing critical, it must

be executed as an assembly language routine in the

same manner as changes to the oscillator configura-

tion. If the bulk of the application is written in C or

another high-level language, the unlock sequence

should be performed by writing in-line assembly.

Choosing the configuration requires the review of all

Peripheral Pin Selects and their pin assignments,

especially those that will not be used in the application.

In all cases, unused pin-selectable peripherals should

be disabled completely. Unused peripherals should

have their inputs assigned to an unused RPn pin

function. I/O pins with unused RPn functions should be

configured with the null peripheral output.

The assignment of a peripheral to a particular pin does

not automatically perform any other configuration of the

pin’s I/O circuitry. In theory, this means adding a

pin-selectable output to a pin may mean inadvertently

driving an existing peripheral input when the output is

driven. Users must be familiar with the behavior of

other fixed peripherals that share a remappable pin and

know when to enable or disable them. To be safe, fixed

digital peripherals that share the same pin should be

disabled when not in use.

Along these lines, configuring a remappable pin for a

specific peripheral does not automatically turn that

feature on. The peripheral must be specifically config-

ured for operation and enabled, as if it were tied to a fixed

pin. Where this happens in the application code (immedi-

ately following device Reset and peripheral configuration

or inside the main application routine) depends on the

peripheral and its use in the application.

A final consideration is that Peripheral Pin Select func-

tions neither override analog inputs, nor reconfigure

pins with analog functions for digital I/O. If a pin is

configured as an analog input on device Reset, it must

be explicitly reconfigured as digital I/O when used with

a Peripheral Pin Select.

Example 10-2 shows a configuration for bidirectional

communication with flow control using UART1. The

following input and output functions are used:

• Input Functions: U1RX, U1CTS

• Output Functions: U1TX, U1RTS

EXAMPLE 10-2:

CONFIGURING UART1

INPUT AND OUTPUT

FUNCTIONS

Note:

In tying Peripheral Pin Select inputs to

RP63, RP63 does not have to exist on a

device for the registers to be reset to it.

// Unlock Registers

__builtin_write_OSCCONL(OSCCON & 0xBF);

// Configure Input Functions (Table 9-1))

// Assign U1RX To Pin RP0

RPINR18bits.U1RXR = 0;

// Assign U1CTS To Pin RP1

RPINR18bits.U1CTSR = 1;

// Configure Output Functions (Table 9-2)

// Assign U1TX To Pin RP2

RPOR1bits.RP2R = 3;

// Assign U1RTS To Pin RP3

RPOR1bits.RP3R = 4;

// Lock Registers

__builtin_write_OSCCONL(OSCCON | 0x40);

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