Fastest external oscillator for SAMD21

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I'd like to use an external I/O pin as a source for one of the generic clocks on the SAMD21 (Arduino Nano 33 IoT), but can't find the maximum allowed frequency in the datasheet.  I've got GCLK_IO selected as the source for the generic clock generator4 (connected to the 32-bit timer/counter) with a 40 MHz TCXO driving the pin.  When I check the clock count in 1 second intervals, it has only counted about 10 million counts, and varies quite a bit, leading me to believe that it can't handle 40 MHz when using the GCLK_IO as an input.

 

The datasheet (https://ww1.microchip.com/downlo...) on page 876 lists the max rise and fall time for a GPIO pin at 15nS.  So 30nS per cycle = about 33 MHz.

And page 866/867 lists the maximum frequency for the GCLK generator at 48 MHz, but doesn't specifically list the limits when running with GCLK_IO (external pin) as a source.

 

10 MHz would be fast enough for my project, but I'd like to know what the actual pin/generator/clock is rated to run at before ordering more parts!  Does anyone know what the limiting factor would be when driving the clock/counter via an external oscillator?

Last Edited: Fri. Feb 12, 2021 - 10:19 AM