SLE66R01PC datasheet(34/55 Pages) INFINEON

34 / 55 page

my-d™ move / my-d™ move NFC

SLE 66R01P / SLE 66R01PN

16-bit Value Counter Functionality

Data Book

34 / 55

2011-11-24

6.3

Decrementing Value Counter and Anti-Tearing

The DCR16 command is used to decrement the Value Counter value. For more details refer to the command

description in the Chapter 8.2.8.

During the execution of the DCR16 command the my-d™ move and my-d™ move NFC performs following

actions:

•

Read both Value Counter blocks;

•

Determine the correct valid Value Counter state. Therefore the values stored in blocks 22

H and 23H are

compared.

– Normally one of the counter value blocks is erased or has an incorrect format and the other block holds the

valid counter value.

– If both counter values are correctly formatted, the higher value is chosen as the valid counter value.

Note that at least one of the counters must be formatted correctly. Otherwise the Value Counter block is

corrupted and no further decrement of the Value Counter is possible.

– If both blocks carry invalid values (incorrect format) no further decrement of Value Counter is possible. The

my-d™ move and my-d™ move NFC then responds with a NACK.

•

Compares the received parameter and the valid counter value.

– If the received parameter is equal or lower than the valid counter value the my-d™ move and my-d™ move

NFC decrements the valid value by the received parameter, programs this value to the previous invalid

value, erases the previous valid value and replies the new written value.

– If the received parameter is higher then the valid value no decrement is possible and the my-d™ move and

my-d™ move NFC responds with a NACK

6.4

Protection Mechanisms for the Value Counter

The my-d™ move and my-d™ move NFC offers some methods to protect the Value Counter. Following measures

should be considered to prevent unauthorized changes.

•

The Password:

– If a Write Password is configured i.e. the bit SP-W is set, then the execution of Write commands (WR1B,

WR2B or CPTWR) on Value Counter blocks (22

H and 23H) is possible only after password verification.

– If a Read/Write Password is configured i.e. the bit SP-WR is set, then the execution of Read commands

RD2B and RD4B and Decrement command DCR16 on Value Counter (blocks 22

H and 23H) is possible only

after password verification.

•

The Locking Mechanism for Value Counter:

– After the configuration of the Value Counter it is strongly recommended to lock both blocks 22

H and 23H in

order to prevent the any unauthorized changes. The locking of blocks 22

H and 23H is done by changing the

locking information of the LOCK4 byte. If the bits 2 and 3 of the Lock4 byte are set then both Value Counter

blocks are locked.

•

Writing of the Value Counter block:

– If blocks 22

H and 23H are locked then no further overwriting of their values with write commands is possible.

Note that if one of the blocks is locked and the other one is not, then it is possible to change the data of the

unlocked block by using WR1B command. For this reason it is important to lock both blocks in order to

prevent unintentional harm to Value Counter (i.e. unintentional overwriting or setting an incorrect value or a

value with an incorrect format).

•

Reading and Decrement of the Value Counter block:

– If blocks 22

H and 23H are locked then reading and decrementing is still possible. Note that depending on the

chip configuration, password verification may be required.

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