MLX90132 Datasheet, PDF (32/44 Pages) Melexis Microelectronic Systems – 13.56MHz RFID / NFC Transceiver

MLX90132
13.56MHz RFID / NFC Transceiver
8 Tag Detector
8.1 Operating Principle
The objective of the TAG detector function is to be able to detect the presence of an RFID label/tag or an
NFC device in front of the reader’s antenna, with reduced power consumption.
The TAG detector function is based on the detection of any variation of the HF field. If an RFID transponder
or an NFC device approaches the reader’s antenna, it influences the amplitude of the generated HF by a
loading effect. This variation can be monitored by the MLX90132 to inform the external host microcontroller
that an RFID transponder or an NFC device is approaching the antenna.
When set in TAG detector state, the MLX90132 periodically generates a few periods (pulses) of HF carrier
frequency and measures the amplitude’s field. This value is then compared to reference levels
DacDataH[7:0] and DacDataL[7:0] defined by the user.
If the measured level is above DacDataH[7:0] or below DacDataL[7:0], - i.e. a change in the amplitude of the
HF field occurs - the MLX90132 automatically informs the external application MCU by: either generating an
IRQ on the pin IRQOUT (SPI interface), or directly sending the WUFlag register value (UART interface).In the
same time it enters to Ready state and waits for a Protocol select command (0x02) command. Therefore,
either the application MCU takes the control of the MLX90132 by sending a command Protocol select
command (0x02) and can decide to start communicating with the TAG, or the MLX90132 goes back in idle
(tag detector) mode. This mechanism is repeated until a new object is detected in the field or another kind of
event appears (e.g. max number of trials reached, wake-up from host MCU …).
Before using this feature it is necessary to perform a calibration by using the MCU
The TAG detector state is entered using the Idle command (0x07) command. The values of
DacDataH/DacDataL[7:0] are defined in this command, as well as the number of HF pulses and the time
between two HF bursts with respectively the bytes SwingsCnt[7:0] and WUPeriod[7:0]. The MLX90132 can
be forced to wake-up after a certain number of trials, even if no TAG has been detected. This number of trials
is set using the bits MaxSleep[4:0].
The bit “initial DAC compare index” in register EnterCtrlL is used to select the first comparison to be
performed when starting the TAG detector state. When set to ‘0’, the TAG detector feature is started with a
comparison to DacDataL[7:0].If set to ‘1’, the TAG detector feature is started with a comparison to
DacDataH[7:0]. Please note that the IREF bit in EnterCtrlH byte has to be set to allow a proper functionality of
the TAG detector feature.
The following picture illustrates the TAG detector operation described above.
TAG detection function
DacDataH
DacDataL
New-calibration
WUPeriod
If the field is detected out of the
Device wakes-up again if detected
Very short impulses of HF field are
generated (defined by the byte
SwingsCnt in IDLE command)
range defined by DacDataL
and DacDataH, the device
wakes-up
out of the new range defined by
DacDataL and DacDataH
Then the host MCU can take the control of the
communication, trying to detect any TAG. If no TAG is
detected, a new calibration might be done to compensate any
change of environment
Figure 11: MLX90132 TAG detection principle
3901090132
Rev. 009
Page 32 of 44
Jan-2014