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Replying to @josecastillo

there is a subtle difference in pinout between the D21 and L21: on the D21, VDDCORE is on pin 29, whereas on the L21 it’s on pin 27 (which you connect to pin 29 via an inductor). Since the D21 doesn’t have a switching regulator, it helpfully offers an extra GPIO, PA28, on pin 27.

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