Datasheet

PIC24FJ64GA004 FAMILY
DS39881E-page 110  2010-2013 Microchip Technology Inc.
10.4.4.3 Configuration Bit Pin Select Lock
As an additional level of safety, the device can be con-
figured to prevent more than one write session to the
RPINRx and RPORx registers. The IOL1WAY
(CW2<4>) Configuration bit blocks the IOLOCK bit
from being cleared after it has been set once. If
IOLOCK remains set, the register unlock procedure will
not execute and the Peripheral Pin Select Control reg-
isters cannot be written to. The only way to clear the bit
and re-enable peripheral remapping is to perform a
device Reset.
In the default (unprogrammed) state, IOL1WAY is set,
restricting users to one write session. Programming
IOL1WAY allows users unlimited access (with the
proper use of the unlock sequence) to the Peripheral
Pin Select registers.
10.4.5 CONSIDERATIONS FOR
PERIPHERAL PIN SELECTION
The ability to control Peripheral Pin Selection intro-
duces 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 ‘11111’ and all RPORx registers reset to ‘00000’, all
Peripheral Pin Select inputs are tied to RP31 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 fea-
ture on. The peripheral must be specifically configured
for operation and enabled, as if it were tied to a fixed pin.
Where this happens in the application code (immediately
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 a 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
RP31, RP31 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 10-2))
// Assign U1RX To Pin RP0
RPINR18bits.U1RXR = 0;
// Assign U1CTS To Pin RP1
RPINR18bits.U1CTSR = 1;
// Configure Output Functions (Table 10-3)
// Assign U1TX To Pin RP2
RPOR1bits.RP2R = 3;
// Assign U1RTS To Pin RP3
RPOR1bits.RP3R = 4;
// Lock Registers
__builtin_write_OSCCONL(OSCCON | 0x40);