mfrc531 NXP Semiconductors, mfrc531 Datasheet - Page 81

• Manufacturer Part Number: mfrc531
• Description: Iso/iec 14443 Reader Ic
• Manufacturer: NXP Semiconductors

NXP Semiconductors
MFRC531_34
Product data sheet
PUBLIC
11.2.2.4 Receiving bit oriented frames
11.2.2.5 Communication errors
Table 135. Return values for bit-collision positions
Parity bits are not counted in the CollPos register because bit-collisions in parity bit occur
after bit-collisions in the data bits. If a collision is detected in the SOF, a frame error is
flagged and no data is sent to the FIFO buffer. In this case, the receiver continues to
monitor the incoming signal. It generates the correct notifications to the microprocessor
when the end of the faulty input stream is detected. This helps the microprocessor to
determine when it is next allowed to send data to the card.
The receiver can manage byte streams with incomplete bytes which result in bit-oriented
frames. To support this, the following values may be used:
RxLastBits[2:0] is only valid if a frame error is not indicated by the FramingErr flag. If
RxAlign[2:0] is not zero and ParityEn is active, the first parity bit is ignored and not
checked.
The events which can set error flags are shown in
Table 136. Communication error table
Collision in bit
SOF
Least Significant Bit (LSB) of the Least Significant Byte (LSByte)
…
Most Significant Bit (MSB) of the LSByte
LSB of second byte
…
MSB of second byte
LSB of third byte
…
Cause
Received data did not start with the SOF pattern
CRC block is not equal to the expected value
Received data is shorter than the CRC block
The parity bit is not equal to the expected value (i.e. a bit-collision, not parity)
A bit-collision is detected
•
•
BitFraming register’s RxAlign[2:0] bits select a bit offset for the first incoming byte. For
example, if RxAlign[2:0] = 3, the first 5 bits received are forwarded to the FIFO buffer.
Further bits are packed into bytes and forwarded. After reception, RxAlign[2:0] is
automatically cleared. If RxAlign[2:0] = logic 0, all incoming bits are packed into one
byte.
RxLastBits[2:0] returns the number of bits valid in the last received byte. For example,
if RxLastBits[2:0] evaluates to 5 bits at the end of the received command, the 5 least
significant bits are valid. If the last byte is complete, RxLastBits[2:0] evaluates to zero.
Rev. 3.4 — 26 January 2010
056634
Table
136.
ISO/IEC 14443 reader IC
CollPos register value
(Decimal)
0
1
…
9
…
16
17
…
8
MFRC531
© NXP B.V. 2010. All rights reserved.
Flag bit
FramingErr
CRCErr
CRCErr
ParityErr
CollErr
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