Some devices have the flexibility to use one of several communication channels or pins for programming and debugging. These channels are generally referred to in data sheets as PGCx/PGDx, where x is the channel number identifier. As mentioned earlier, these channels are often multiplexed with some peripherals (I2C™, SPI, A/D).
If your application uses those peripherals and pins which are common to the default PGC/PGD pins, you must select a different channel and make provisions for the required ICSP connections. Likewise, when using MPLAB X IDE to communicate with the target using this new channel pin assignment, you must ensure that the Configuration bits in MPLAB X IDE match the connection channel in your target.
For some devices programming speeds can be slower on PGC3/PGD3 because they cannot make use of certain programming enhancements (i.e., a programming executive). Consult any data sheet errata for details.
Microchip Technology acquires Atmel