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Would anyone have an idea of the modulation used by this type of card (125kHz): would this be some sort of ASK ?
The tag sends the same thing all the time: it has a 4096 samples period at 125kHz.
The obvious would be to decode 'long' pulses as Ones and 'short' as zeroes by timing the zero crossings, but it does not give me any interesting bitstream value when matching it to the serial number printed on the card. Would someone have hints on another possible approach ?
I have an EM4102 that looks pretty much the same.
Indeed! I did some more work yesterday evening, and this is an EM chip, no doubt about it, everything matches the specs.
I'm currently writing a few extra routines for the host client to make analysis of this type of signals easier. I'm finishing up the manchester decoder at the moment (if anyone's got efficient algorithms, let me know). Next step will be to print our the EM mapping properly including the parity checking bits, and then moving it into the ARM code.
My view is that the host client should contain the signal analysis routines for unknown signals: autocorrelation, extraction, generic manchester & other decoding, whereas it makes more sense to include more mature tag decoding routines into the ARM itself.
Ed
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