AN3433 datasheet(14/34 Pages) STMICROELECTRONICS

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Management of CR95HF or STRFNFCA activity

AN3433

14/34

Doc ID 019046 Rev 2

Note:

Bytes 4 to 9 should be used as shown in the examples in Section 2.3: Idle command

parameters.

Note that the MaxSleep value is coded on the 5 least significant bits, thus:

0x00 < MaxSleep < 0x1F.

All the previously described command parameters must be chosen accordingly for the initial

tag detection calibration when setting up the CR95HF or STRFNFCA.

Their value will impact tag detection efficiency and CR95HF or STRFNFCA power

consumption during Tag Detection periods.

2.7

Tag detection calibration procedure

The Idle command allows the use of a tag detection as a wakeup event. Certain parameters

of the Idle command are dedicated to setting the conditions of a tag detection sequence.

During the tag detection sequence, the CR95HF or STRFNFCA regularly emits RF bursts

and measures the current in the antenna driver IDRIVE using the internal 6-bit DAC.

When a tag enters the device antenna RF operating volume, it modifies the antenna loading

characteristics and induces a change in IDRIVE, and consequently, the DAC data register

reports a new value.

This value is then compared to the reference value established during the tag detection

calibration process. This enables the device to decide if a tag has entered or not its

operating volume.

The reference value (DacDataRef) is established during a tag detection calibration process

using the device application setting with no tag in its environment.

The calibration process consists in executing a tag detection sequence using a well-known

configuration, with no tag within the antenna RF operating volume, to determine a specific

reference value (DacDataRef) that will be reused by the host to define the tag detection

parameters (DacDataL and DacDataH).

During the calibration process, DacDataL is forced to 0x00 and the software successively

varies the DacDataH value from its maximum value (0xFE) to its minimum value (0x00). At

the end of the calibration process, DacDataRef will correspond to the value of DacDataH for

which the wakeup event switches from timeout (no tag in the RF field) to tag detected.

To avoid too much sensitivity of the tag detection process, we recommend using a guard

band. This value corresponds to 2 DAC steps (0x08).

Recommended guard band value:

DacDataL = DacDataRef – Guard and DacDataH = DacDataRef + Guard

The parameters used to define the tag detection calibration sequence (clocking, set-up time,

burst duration, etc.) must be the same as those used for the future tag detection sequences.

When executing a tag detection sequence, the device compares the DAC data register value

to the DAC Data parameter values (DacDataL and DacDataH) included in the Idle

command. The device will exit WFE mode through a Tag Detection event if the DAC data

the DAC Data parameter low value (DacDataL). Otherwise, it will return to Ready state after

a timeout.

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