GCBASIC documentation
Port control

Introduction:

Peripheral Pin Select (PPS) enables the digital peripheral I/O pins to be changed to support mapping of external pins to different pins.

In older 8-bit Microchip devices, a peripheral was hard-wired to a specific pin (example: PWM1 output on pin RC5).

PPS allows you to choose from a number of output and input pins to connect to the digital peripheral.

This can be extremely useful for routing circuit boards.

There are cases where a change of I/O position can make a circuit board easier to route. Sometimes mistakes are found too late to fix, so having the option to change a pinout mapping in software rather than creating a new printed circuit board can be very helpful.

You must use the command UnLockPPS to enable setting of the PPS if the PPS have been previously locked, and, you can, optionally, use LockPPS to prevent unintentional change to PPS settings afterward.

GCBASIC includes these two macros to ensure this process is handled correctly.

Also, see microchip.wikidot.com for more information.

Why the priority value of 85 is important:

InitPPS is registered with #startup using an explicit priority, for example #startup InitPPS, 85. This priority number is not arbitrary.

GCBASIC’s own system initialisation, InitSys, always runs first, at priority 80. Low-level hardware communication libraries — the hardware USART used by HSerPrint and friends, and the hardware I2C module — register their own startup initialisation at priority 90. Any subroutine that does not specify a priority defaults to 100.

Startup subroutines run in order from the smallest priority number to the largest, so a priority of 85 places InitPPS after InitSys (80) but before any hardware communication library’s own startup code (90).

This ordering matters because the hardware USART and hardware I2C modules are physically tied to whichever pins the PPS registers currently point at. If a project uses the serial library and its startup routine were to run before the PPS pins are remapped, the USART would still be wired to its default pins rather than the ones the program actually uses — so HSerPrint output would go to the wrong physical pin, or nowhere at all, with no error raised anywhere. Keeping InitPPS at priority 85 guarantees the correct pin mapping is already in place before the serial or I2C library’s own startup code runs.

If you copy this pattern into your own code, always keep the priority between 80 and 90 (85 is the conventional value), and never remove it — without an explicit priority, InitPPS would default to 100 and run after the communication libraries have already initialised on the wrong pins.

Example:

    'Please check configuration before using on an alternative microcontroller.

    #chip 16f18855,32
    #option explicit

    'Set the PPS of the I2C and the RS232 ports.
    #startup InitPPS, 85
    Sub InitPPS
      UNLOCKPPS          ' <<< the UnLockPPS instruction
        RC0PPS = 0x0010       'RC0->EUSART:TX;
        RXPPS  = 0x0011       'RC1->EUSART:RX;

        SSP1CLKPPS = 0x14     'RC3->MSSP1:SCL1;
        SSP1DATPPS = 0x13     'RC4->MSSP1:SDA1;
        RC3PPS = 0x15         'RC3->MSSP1:SCL1;
        RC4PPS = 0x14         'RC4->MSSP1:SDA1;
      LOCKPPS
    End Sub

Key line: #startup InitPPS, 85 — the priority value of 85 is what guarantees this routine runs after InitSys (priority 80) but before the hardware serial and I2C libraries initialise (priority 90), so the USART and MSSP pins are already remapped by the time those libraries start using them.

See Also: