Rainbow Electronics AT88RF1354 - Manual (Page 30)

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13.56 MHz Type B RF Reader
8547A −RFID−10/08
A.7. CRC Error Detection
A 2 byte CRC_B is required in each frame transmitted by the PICC or PCD to permit transmission error detection.
The CRC_B is calculated on all of the command and data bytes in the frame. The SOF, EOF, start bits, stop bits, and
EGT are not included in the CRC_B calculation. The two byte CRC_B follows the data bytes in the frame.
Figure A-6. Location of the two CRC_B bytes within a frame.
SOF K Data Bytes CRC1 CRC2 EOF
The CRC_B polynomial is defined in ISO/IEC 14443 and ISO/IEC 13239 as x 16 + x 12 + x 5 + x 0 . This is a hex
polynomial of $1021. The initial value of the register used for the CRC_B calculation is all ones ($FFFF). When
receiving information from the PICC, the AT88RF1354 reader automatically computes the CRC on the incoming
command, data, and CRC bytes. When transmitting data the AT88RF1354 reader automatically computes the CRC
on the outgoing data packet, and inserts it prior to the end of frame. Any CRC error detected by AT88RF1354 is
reported to the host microcontroller.
A.8. Modulation Index
The Modulation Index of the PCD generated magnetic field is measured by placing a calibration coil or wire loop near
the PCD antenna. Connect this loop to a high impedance oscilloscope probe and measure the amplitude modulation
(ASK) waveform as shown in Figure A-7. The PCD amplitude Modulation Index is defined in ISO/IEC 14443 part 2
as the M.I. = (A - B) / (A + B). For Type B operation the PCD modulation index is required to be between 8 % and 14
%.
If the PCD modulation is less than 8 % then the PICC receiver will not successfully decode the transmissions.
Excessive modulation reduces the power available to the PICC and may cause it to reset.
Figure A-7. Measurement of the PCD Amplitude Modulation Index.
A
B
Modulation Index =
Modulation Depth =
(A - B)
(A + B)
B
A
where:
A = Unmodulated Signal Amplitude
B = Modulated Signal Amplitude