AN3433 datasheet(22/34 Pages) STMICROELECTRONICS

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Tag detection principle

AN3433

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Doc ID 019046 Rev 2

Appendix A

Tag detection principle

The Tag Detection process requires the temporary use of internal resources such as the

13.56 MHz HF oscillator or the DAC which are usually switched off in WFE mode and only

turned on during an RF burst emission to determine if tag entered the device operating

range.

To detect the presence of a tag, the device measures a portion of the device antenna’s

current using the DAC (DacLevel) and compares this value to a reference value (CalRef)

determined in the same way when the application is initialized.

To do this, once the HF oscillator is stabilized, the device emits an RF burst and monitors its

antenna current using its internal 64-bit DAC. This result is then compared to a reference

value established during the Tag Detection Calibration phase.

The device is initially calibrated by the user application in a free-air environment (no tags

present) in order to obtain a reference value (CalRef) that corresponds to the basic

threshold of its comparator.

When a new measurement of the antenna’s current results in a value different from the

CalRef value, a tag is probably present in the RF field.

To reduce the margin of error for false detections, in addition to comparing the value of the

measured antenna current (DacLevel) to the reference value (CalRef), the device checks

that the measured value is within a given range:

DacDataL < DacLevel < DacDataH

Using range limits ensures a stable Tag Detection process. We usually choose:

DacDataL = CalRef – 0x08 and DacDataH = CalRef + 0x08

The parameters that define measurement resources, oscillator and DAC setup times or RF

burst duration, can be individually adjusted within the Idle command parameters, but they

must remain unchanged between the calibration and tag detection periods.

We recommend for the HF oscillator and DAC stabilization times (OscStart and DacStart,

respectively) using a minimum value of 0x60 that corresponds to a duration of 3 ms

(96 * tLFO).

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