ST25R200 datasheet(24/88 Pages) STMICROELECTRONICS

24 / 88 page

5.12

Wake-up mode

Once in PD mode, the bit wu_en sets the ST25R200 into wake-up mode (WU), which is used to perform low

power card detection. An integrated low-power 26.48 kHz RC oscillator and configurable wake-up timer (WUT)

are used to trigger periodic measurements.

Usually the presence of a card is detected by RF/NFC polling. In this process, the reader field is periodically

turned on and the host checks whether a card is present using RF commands. This procedure consumes

considerable energy since the reader emits a field for long periods before a command can be issued, due to

guard time fulfillment.

Low power detection of card presence is performed by detecting a change in the reader environment, produced

by an approaching detuning element. When a change is detected, an interrupt is sent to the host. The host can

then perform the regular RF/NFC polling afterwards.

In wake-up mode the ST25R200 performs periodic measurements and sends an IRQ to the host when a

difference to the configured reference value is detected.

The ST25R200 monitors the I and Q channels to assess a variation on the antenna surroundings.

The latest measurement value is shown in i_adc<7:0> and q_adc<7:0>, and the current reference is defined on

i_ref<7:0> and q_ref<7:0>. ADC values are provided in 8-bit signed values using two's complement

representation. The delta/difference to the reference that triggers a wake-up interrupt is defined in i_diff<5:0> and

q_diff<5:0>.

Card detection

The presence of a card close to the reader antenna coil produces a change of the antenna LC tank signal. The

reader field activation time required to perform the measurement is extremely short, compared to the activation

time required to send a protocol activation command.

The power level during the measurement can be lower than that during normal operation as the card does not

have to be powered to produce a coupling effect. The emitted power can be configured by changing the RFO

driver resistance.

An IRQ is sent when the difference between a measured value and reference value is larger than the configured

delta/diff value. There are two possibilities how to define the reference value, controlled by the bit iq_aaref:

•

The ST25R200 can automatically calculate the reference based on previous measurements (auto-

averaging)

•

The host determines the reference value(s) and sets it on i_ref<7:0> and q_ref<7:0>

Auto-averaging

In case auto-averaging is enabled (iq_aaref = 1) the reference value is recalculated after every measurement.

The last measurement value, the old reference value, and the weight are used in this calculation.

The following formula is used to calculate the new reference value:

new_reference = old_reference - (old_reference - measured_value) / weight

The bits i_iirqm and q_iirqm define whether a measurement that causes an interrupt is taken into account for the

average value calculation.

The influence that the new measurement has over the reference can be configured by a weight defined in bits

i_aaw<1:0> and q_aaw<1:0>

Calibration

The wake-up mode has an additional calibration step that can be executed manually or automatically. The bits

skip_cal and skip_recal control the automatic calibration behavior.

When starting the wake-up mode, if skip_cal = 0, a calibration step is performed automatically on the first WUT

timeout.

While the wake-up mode is running and skip_recal = 0, if reference ± delta/diff is larger than 63, a recalibration is

performed.

Wake-up IRQ

The device allows to configure the conditions when a wake-up IRQ is sent to the host.

This behaviour is defined by bits i_tdi_en<2:0> and q_tdi_en<2:0> and the following three conditions can be used:

ST25R200

Application information

DS13658 - Rev 2

page 24/88

ST25R200 数据表(PDF) 24 Page - STMicroelectronics

ST25R200 数据表(HTML) 24 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88

数据表下载