PIC32 can run at lower frequencies but not at high frequencies

Using a PIC32MX device, I can enter debug mode using an MPLAB® ICD3 when the device is operating at up to 40 MHz, but not when it is operating at a higher frequency. In Run mode, it is the same behavior, not running when the oscillator PLL is set for higher frequencies. Why is this happening?

First, check to make sure that you aren't exceeding the frequency of the PIC32. Some PIC32MX devices only operate up to 40 MHz. If the operating frequency is in range, then another cause for this problem is a missing capacitor on the Vcap pin or a capacitor value which is too small.

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