What PIC32MZ SPI data pins (SDI and SDO) can be used to achieve 50 MHz?

You can use the peripheral pin select (PPS) function on PIC32MZ devices to change which pins are used for the SPI data pins. The pins you choose will affect the maximum frequency (25 MHz vs 50 MHz) on the SPI bus.

The PIC32MZ EF family data sheet specifies which I/O pins can be used with the SPI peripheral and what their maximum speed is. From the PIC32MZ EF Family landing page, go to Documentation > Data Sheets > PIC32MZ Embedded Connectivity with Floating Point Unit (EF) Family. Refer to Table 37-30 and 37-31 in the datasheet for details.

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