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dsPIC33A Oscillator Module Deep Dive

Reference Clocks

Last modified by Microchip on 2026/10/19 09:35
   Clock Monitor Example Project  Oscillators   

Overview

The dsPIC33AK512MPS512 family provides one or more Reference Clock Input (REFIx) and Output (REFOx) functions.

In simple terms, REFO allows an internal clock source to be routed out to a device pin for external use, debugging, synchronization, or clock measurement.

Ref Clocks Vverview

Each REFOx module is driven by an assigned Clock Generator (CLKGEN). As a result, the REFO output frequency is based on the clock source selected for that CLKGEN, which can be any supported internal or external clock source.

Key points:

  • REFI1 and REFI2 are pin-remappable using Peripheral Pin Select (PPS)
  • REFO1 and REFO2 are also pin-remappable through PPS, but each is associated with a specific CLKGEN

Two clock generators are reserved specifically for REFO operation. These REFO-dedicated CLKGENs operate similarly to the other clock generators and can also serve as clock sources for other CLKGEN modules.

REFIx and REFOx Application Examples

Refi Refo App Example

REFIx can be used to:

  • Bring an external reference clock into the device
  • Synchronize with another system clock
  • Support clock monitoring and measurement applications

REFOx can be used to:

  • Output an internal clock to a device pin
  • Observe or validate system clock behavior
  • Provide a clock source to external devices
  • Support debugging and timing analysis

Together, REFI and REFO provide flexible clock input and output capabilities for system synchronization, monitoring, and validation. Potential applications could range from a unique clock for a Smart Card clock to a Master Clock (MCLK) for an audio reference.

Reference Clocks: Example PPS Configuration

REFI1: Pin Remap

In this example, we want to map the REFI1 input function to re-mappable pin RP103/RG6 on the 128-pin version of dsPIC33AK512MPS512.

REFI1 function mapping is controlled by SFR RPINR18 (Peripheral Pin Select Input Register 18):

RPINR18bits.REFI1R = RPn; // where n is the RP number of the selected pin

For this example, the source code to complete the mapping looks like this:

  RPINR18bits.REFI1R = 103; // map REFI1 to RP103 (pin "RG6" - 128-pin TQFP)
  _TRISG6 = 1;               // set pin "RG6" as an input

This allows an external reference clock to be routed into the device for clock generation, monitoring or synchronization applications.

REFO2: Pin Remap

In this example, we want to map the REFO2 output function to re-mappable pin RP38/RC5 on the 128-pin version of dsPIC33AK512MPS512.

REFO2 function mapping is controlled by SFR RPOR9 (Peripheral Pin Select Output Register 9, which sets the output pin function for pin RP38):

RPOR9bits.RP38R = RPn; // where n is the RP number of the selected pin

For this example, the source code to complete the mapping looks like this:

RPOR9bits.RP38R = 38; // map REFO2 to RP38 (pin "RC5" - 128-pin TQFP)
_TRISC5 = 0;          // set pin "RC5" as output
CLK14CONbits.OE = 1;  // CLKGEN14 enable onto pin

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