Peripheral Pin Select (PPS) for 8-Bit Devices

Peripheral Pin Select (PPS) enables flexible digital peripheral I/O pin mapping to a wide range of external pins. In older 8-bit devices, a peripheral was hard-wired to a specific pin (example: PWM1 output on pin RC5). PPS allows the developer to choose from a number of output and input pins to connect to the digital peripheral.

This can be extremely handy 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.

Some Microchip customers may be familiar with PPS from the PIC24 product line. PPS for the 8-bit family differs in that each digital peripheral input signal is connected to a pin by default. This stands in contrast to PPS for the PIC24 products, where peripherals are left unconnected by default. For 8-bit PPS, peripheral input and output signals need to be selected before the peripheral is enabled. Then the peripheral should be enabled after both input and output selections have been completed.

PPS has its limitations though. Only digital signals are included in the selections. All analog inputs and outputs remain fixed to their assigned pins due to the internal circuitry requirements.

PPS is available on a small selection of devices but that number will increase as new controllers are released. The PIC16F170x family has the PPS feature so we will use that as a reference in this training module.

Figure 1 shows a simplified block diagram of the PPS.

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PPS Video Tutorial

This video introduces the Peripheral Pin Select (PPS) for Microchip 8-bit MCU devices and show how to use it.

For iPad® users, the video quality may be poor. Use the direct link to YouTube for a higher resolution video.

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Input Selection

Each peripheral that can be rearranged with the PPS has a register to select the input source. The register selects which input pin to connect to the input circuitry. The register name will have a peripheral nickname followed by the letters PPS. For example, the USART Receive input uses the RXPPS register to select the input pin connection.

A generic register is shown below since all the registers share the same setup. XXX in the name represents the peripheral name. Five bits in the register select the input pin. On reset, the register defaults to a value that selects the default input pin. This will vary from peripheral to peripheral. Changing the five bits to the selected input will change the input pin for that particular peripheral. (The example here is for the PIC16F1708.)

Example

Here is the code in MPLAB® XC8 format for setting the USART RX input pin to RB4:

In MPASM Assembly format, the External Interrupt Pin has been connected to RA3

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Output Selection

Each Peripheral output can be connected to a selected I/O pin through the PPS. This is done in a similar fashion to the input selection except that selecting the output connection is opposite from the input selection. Instead of the register being named a peripheral nickname followed by PPS, the output is the pin name followed by PPS. Thus, the pin RB0 would have a RB0PPS register to select its connection.

There are five bits in the Output Source register used to choose the connection. These five bits select the Peripheral output that the pin will be connected to. (The example here is for the PIC16F1708.)

Example

Here is the code in MPLAB^® XC8 format for setting the USART TX output pin to RA0:

In MPASM Assembly format CLC1OUT Pin is connected to RA5

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PPS Lock

The PPS includes a mode in which all input and output selections can be locked to prevent inadvertent changes. PPS selections are locked by setting the PPSLOCKED bit of the PPSLOCK register. Setting and clearing this bit requires a special sequence as an extra precaution against inadvertent changes.

Assembly Code for PPS Lock Sequence

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PPS Permanent Lock

The PPS can be permanently locked by setting the PPS1WAY configuration bit. When this bit is set, the PPSLOCKED bit can only be cleared and set one time after a device Reset. This allows for clearing the PPSLOCKED bit so that the input and output selections can be made during initialization. When the PPSLOCKED bit is set after all selections have been made, it will remain set and cannot be cleared until after the next device Reset event.

For this reason, we recommend that all the PPS register selections be placed near the top of the project code so I/O is properly set right after reset. The configuration register that contains the PPS1WAY bit is shown in Figure 4 for the PIC16F1708.

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Sleep Mode

During Sleep mode the PPS input and output selections are unaffected. Consequently, entering sleep and returning from sleep will leave all the connection selections in place.

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Reset

When power is cycled on the device and it goes through a full power on reset, all the PPS input and output selections are reset to their default values. All other types of reset leave the selections unchanged.

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MPLAB Code Configurator

MPLAB® Code Configurator (MCC) makes creating the PPS code quite easy.

In Figure 5, a PWM3 peripheral is set up to connect to the RA1 pin (Green Lock Symbol). MCC will generate the PPS lock code and the register settings when the Generate Code button is clicked in the Composer Area of MCC. This creates a pin_manager.c file that contains the PPS code.

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