CLRC63201TSLASH0FE datasheet(120/163 Pages) PHILIPS

120 / 163 page

Philips Semiconductors

Product Specification Rev. 3.0; May 2003

Multiple Protocol Contactless Reader IC

CL RC632

120

Confidential

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

The collision position follows the table below:

Collision in Bit

Value of CollPos

SOF

LSBit of LSByte

…

MSBit of LSByte

LSBit of second Byte

…

MSBit of second Byte

LSBit of third Byte

…

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.

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