ST25R95 datasheet(32/61 Pages) STMICROELECTRONICS

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5.9.2

Using LFO frequency setting to reduce power consumption

In WFE mode, the high frequency oscillator (HFO) is stopped and most processes being executed are clocked by

the low frequency oscillator (LFO). To minimize the ST25R95 power consumption in WFE mode select the lowest

LFO frequency.

Example 1: Setting a lower LFO frequency

The following equation defines a basic timing reference:

tREF = 256*tL ms (where tL = 1/fLFO)

tREF = 8 ms (when <WU Source> bits [7:6] are set to “00”, or 32 kHz)

tREF = 64 ms (when <WU Source> bits [7:6] are set to “11”, or 4 kHz)

5.9.3

Optimizing wake-up conditions

Using the Wake-up source register, it is possible to accumulate sources for a wake-up event. It is strongly

recommended to always set an external event as a possible wake-up source.

To accumulate wake-up sources, simply set the corresponding bits in the Wake-up source register. For example,

to enable a wake-up when a tag is detected (bit 1 set to ‘1’) or on a low pulse on pin IRQ_IN (bit 3 set to ‘1’), set

the register to 0x0A.

5.9.4

Using various techniques to return to ready state

The Idle command and reply set offers several benefits to users by enabling various methods to return the

ST25R95 to Ready state. Some methods are nearly automatic, such as waiting for a timer overflow or a tag

detection, but others consume more power compared to the ones requesting a host action. A description of each

method follows below.

Default setting: from POR to ready state

After power-on, the ST25R95 enters Power-up state.

To wake up the ST25R95 and set it to Ready state, the user must send a low pulse on the IRQ_IN pin. The

ST25R95 then automatically selects the external interface (SPI) and enters ready state and is able to accept

commands after a maximum delay of 10 ms (t3).

From ready state to hibernate state and back to ready state

In Hibernate state, most resources are switched off to achieve an ultra-low power consumption.

The only way the ST25R95 can wake-up from Hibernate state is by an external event (low pulse on pin IRQ_IN).

A basic Idle command is:

>>>0x07 0E 08 04 00 04 00 18 00 00 00 00 00 00 00 00

Note:

The wake-up flag value is NOT significant when returning to Ready state from hibernate state or after a POR.

From ready state to sleep state and back to ready state

Wake-up by external event (low pulse on IRQ_IN or SPI_SS pin)

In sleep or power-up states, operating resources are limited in function of the selected wake-up source to achieve

a moderate power consumption level.

An Idle command example when wake-up source is pin IRQ_IN:

>>>0x07 0E 08 01 00 38 00 18 00 00 60 00 00 00 00 00

A similar command can be implemented using pin SPI_SS as a wake-up source:

>>>0x07 0E 10 01 00 38 00 18 00 00 60 00 00 00 00 00

Wake-up by timeout

The LFO is required to use the timer. However, this increases the typical power consumption by 80 µA. Several

parameters can be modified to reduce power consumption as much as possible.

The duration before timeout is defined by parameters WU period and MaxSleep, respectively 0x60 and 0x08 in

the following example.

Duration before timeout = 256 * tL * (WU period + 2) * (MaxSleep + 1)

Note:

0x00 < MaxSleep < 0x1F.

ST25R95

Idle command (0x07) description

DS12807 - Rev 4

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