CLRC632 Datasheet, PDF (120/163 Pages) NXP Semiconductors – Multiple Protocol Contactless Reader IC

Philips Semiconductors
Multiple Protocol Contactless Reader IC
Product Specification Rev. 3.0; May 2003
CL RC632
18.4.2.3 Collision Detection
If more than one card is within the RF-field during the card selection phase, they will respond simultaneously.
The CL RC632 supports the algorithm defined in ISO14443-A to resolve data-collisions of cards serial
numbers by doing the so-called anti-collision procedure. The basis for this is the ability to detect bit-
collisions.
Bit-collision detection is supported by the used bit-coding scheme, namely the Manchester-coding. If in the
first and second half-bit of a bit a sub-carrier modulation is detected, instead of forwarding a 1 or a 0 a bit
collision will be signalled. To distinguish a 1 or 0-bit from a bit-collision, the CL RC632 uses the setting of
CollLevel. If the amplitude of the half-bit with smaller amplitude is larger than defined by CollLevel, the CL
RC632 indicates a bit-collision.
If a bit-collision is detected, the error flag CollErr is set. If a bit-collision is detected in a parity bit, the flag
ParityErr is set indicating a parity error.
Independent from the detected collision the receiver continues receiving the incoming data stream. In case of
a bit-collision, the decoder forwards 1 at the collision position.
Note: As an exception, if bit ZeroAfterColl is set, all bits received after the first bit-collision are forced to zero,
regardless whether a bit-collision or an unequivocal state has been detected. This feature eases for the
software to carry out the anti-collision procedure defined in ISO14443-A.
When the first bit collision in a frame is detected, the bit position of this collision is stored in the CollPos
Register.
The collision position follows the table below:
Collision in Bit
SOF
LSBit of LSByte
…
MSBit of LSByte
LSBit of second Byte
…
MSBit of second Byte
LSBit of third Byte
…
Value of CollPos
0
1
…
8
9
…
16
17
…
Table 18-2: Returned Values for Bit Collision Positions
The parity bits are not counted in CollPos, since a bit-collision in a parity bit per definition succeeds a bit-
collision in the data bits. If a collision is detected in the SOF a frame error is reported and no data is
forwarded to the FIFO buffer. In this case the receiver continues to monitor the incoming signal and
generates the correct notifications to the µ-Processor when the ending of the faulty input stream is detected.
This helps the µ-Processor to determine the time when it is allowed next to send anything to the card.
120
Confidential