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I have a prototype of NFC reader which is attached to my Board. But it seem it doesn't work ... the only change I made was the Xtal because I don't find 27.12MHZ.

Some people told me I can replace it if the hz was a multiple. The fact it don't work as expect. Is it normal ?

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    \$\begingroup\$ Go find the "some people" and tell them to fix it. Otherwise you'll have to provide the right crystal actually specified for the part. \$\endgroup\$ – Olin Lathrop Dec 21 '15 at 15:27
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    \$\begingroup\$ It is not beyond the realm of possibility that adjustments to the circuit could get a crystal of 1/3 of the needed frequency to oscillate on an overtone. But 1/2 won't work as it must be an odd multiple. 1/2 would only work if there were normally an internal divider which could be disabled, or else an internal multiplier. \$\endgroup\$ – Chris Stratton Dec 21 '15 at 16:32
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Some people told me I can replace it if the hz was a multiple. The fact it don't work as expect. Is it normal ?

Next time, better take a look at the data-sheet. The PN532 wants to see a 27.12MHZ xtal or external clock of the same frequency. You can't just pick a different clock-frequency.

You shouldn't have any problem finding the correct xtal online. They're cheap.

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