AN3433 datasheet(18/34 Pages) STMICROELECTRONICS

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Power consumption considerations

AN3433

18/34

Doc ID 019046 Rev 2

3.2

Transition from Ready to Sleep state

This transition ensures low power consumption as well as a fast recovery (switch to Ready

state).

When the CR95HF or STRFNFCA receives an Idle command with the Sleep option, it only

partially disables its resources. In this state, power consumption will decrease from 2.5 mA

to approximately 20 µA.

The device can wake up from Idle state only at the end of pre-defined timeout or by an

external event (pin IRQ_IN or SPI_SS goes low).

A basic Idle command used to wake up the device at the end of pre-defined timeout is:

>>> 0x07 0E 01 01 00 38

00 18 00 60 00 00 00 00 00 00

In this example, the timer uses only the LF oscillator. The timeout period is defined by the

following command parameters:

TO = 256*tLFO*WuPeriod*MaxSleep

A basic Idle command used to wake up the device by an external event (pin IRQ_IN) is:

>>> 0x07 0E 08 01 00 38

00 18 00 60 00 00 00 00 00 00

A basic Idle command used to wake up the device by an external event (pin SPI_SS) is:

>>> 0x07 0E 10 01 00 38

00 18 00 60 00 00 00 00 00 00

The device will only send its response after having returned to Ready state after an external

event is detected.

When the device wakes up from Sleep state, it restarts the external oscillator and all other

resources, including the selected communication interface, before returning to Ready state

and waiting for a new command.

In certain cases (mainly when using the UART), it may be necessary to use an Echo

command to re-synchronize the host and CR95HF or STRFNFCA device.

3.3

Wakeup by Tag Detection

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.

The delay (tINACTIVE) between two RF burst emissions is defined in the Idle command by the

WuPeriod parameter.

For a complete description of the Tag Detection process, see Appendix A: Tag detection

principle.

tINACTIVE = (WuPeriod+1)*tREF

In the example shown in Figure 4:

tINACTIVE = (32+1)*8 ms = 264 ms

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