ST AN3433 APPLICATION NOTE

AN3433

Application note

Optimizing wakeup time and power consumption

in CR95HF and STRFNFCA devices

Introduction

This document describes several tips for application engineers to effectively manage CR95HF and STRFNFCA operating states and modes to define an adapted wakeup condition to return to its Active state to start communication with the external world in order to reduce power consumption.

The CR95HF and STRFNFCA have 2 main functional domains. In the first one, Active mode, the devices are able to communicate with a host using one of the serial interfaces (UART or SPI) or with RFID tags using its RF capabilities. In this mode, the device consumption is only a few milliamps, and when it emits RF signals, its field consumption becomes tens of milliamps. In the second domain, Wait for Event (WFE) mode, the device remains quiescent while waiting for an external event. In this mode, a very low level of power consumption (only a few microamps) may be achieved.

An application engineer is able to manage the wakeup conditions in order to adapt his strategy to optimize wakeup time or system power consumption.

This application note will provide guidelines for choosing the best implementation.

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Contents AN3433

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 Reference information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1 Reference documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Terms and acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3 Notation conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Management of CR95HF or STRFNFCA activity . . . . . . . . . . . . . . . . . . . 6

2.1 Overview of operating modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Idle command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 Idle command parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4 Using LFO frequency setting to reduce power consumption . . . . . . . . . . 10

2.5 Optimizing wake-up conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.5.1 Wake-up source register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.6 Using various techniques to return to Ready state . . . . . . . . . . . . . . . . . 12

2.6.1 Default setting: from POR to Ready state . . . . . . . . . . . . . . . . . . . . . . . 12

2.6.2 From Ready state to Hibernate state and back to Ready state . . . . . . . 12

2.6.3 From Ready state to Sleep state and back to Ready state . . . . . . . . . . 12

2.7 Tag detection calibration procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3 Power consumption considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1 Transition from Ready to Hibernate state . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2 Transition from Ready to Sleep state . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.3 Wake-up by Tag Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Appendix A Tag detection principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

A.1 Tag detection sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Appendix B Tag Detection Calibration process. . . . . . . . . . . . . . . . . . . . . . . . . . 26

B.1 Tag Detection Calibration algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

B.2 Tag Detection calibration command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Appendix C Example of CR95HF tag detection calibration process . . . . . . . . . 29

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Appendix D Example of CR95HF tag detection command using results of tag

detection calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

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Reference information AN3433

1 Reference information

1.1 Reference documents

CR95HF: 13.56 MHz Multi-protocol Contactless Transceiver IC with SPI and UART serial access datasheet

STRFNFCA: Near field communication transceiver datasheet

1.2 Terms and acronyms

Table 1. List of terms and acronyms

Term Meaning

AFE Analog Front End

DAC Digital-to-analog converter

DFT Design for Test

NVM Non Volatile Memory

RFID Radio frequency identification

RFU Reserved for Future Use

WFE Wait For Event

1.3 Notation conventions

The following symbols correspond to:

>>>: Frame sent by the host to CR95HF or STRFNFCA

<<<: Frame sent by the CR95HF or STRFNFCA to the host

Table 2. List of characteristic values

Symbol Definition Value

CalRef

DacDataH High Level detection limit

DacDataL Low Level detection limit

MaxSleep Number of inactivity periods (t

SwingCnt Number of 13.56 MHz periods in the RF burst

DAC

HFO

INACTIVE

LFO

Tag Calibration Reference level and DAC threshold level for device setup

DAC set up time

Period of RF carrier (13.56 MHz) 73 ns

Inactivity period, delay between two wakeups for RF burst emissions in a Tag Detection sequence

Period of device Low frequency oscillator (32 kHz) 31.25 µs

INACTIVE

) before a timeout.

INACTIVE

(WuPeriod+1)*t

REF

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AN3433 Reference information

Table 2. List of characteristic values (continued)

Symbol Definition Value

OSC

REF

SWING

WuPeriod

Oscillator set-up time

Reference time

RF Burst duration in Tag detection t

Timeout delay after which the device will automatically leave WFE mode.

Number of reference times (RT) in an inactivity period (t

INACTIVE

)

REF

= 256*t

SWING

INACTIVE

= 8 ms

LFO

= Swing * t

= (MaxSleep) *

HFO

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2 Management of CR95HF or STRFNFCA activity

2.1 Overview of operating modes

The CR95HF and STRFNFCA have 2 operating modes: Wait for Event (WFE) and Active. In Active mode, the device communicates actively with a tag or an external host (MCU). WFE mode includes four low consumption states: Power-up, Hibernate, Sleep and Tag Detector.

The CR95HF or STRFNFCA can switch from one mode to another.

Table 3. CR95HF or STRFNFCA operating modes and states

Mode State Description

This mode is accessible directly after POR.

Wait For Event (WFE)

Active

Power-up

Hibernate

Sleep

Tag D e t e c t o r

Power-up

Hibernate

Low level on IRQ_IN source. LFO (low-frequency oscillator) is running in this state.

Lowest power consumption state. The device has to be woken-up in order to communicate. Low level on IRQ_IN pin (longer than 10 µs) is the only wakeup source.

Low power consumption state. Wakeup source is configurable: –Timer –IRQ_IN – SPI_SS pin LFO (low-frequency oscillator) is running in this state.

Low power consumption state with tag detection. Wakeup source is configurable:

–Timer –IRQ_IN – SPI_SS – Tag detector LFO (low-frequency oscillator) is running in this state.

This mode is accessible directly after POR. Low level on IRQ_IN

source. LFO (low-frequency oscillator) is running in this state.

Lowest power consumption state. The device has to be woken-up in order to communicate. Low level on IRQ_IN the only wakeup source.

pin (longer than 10 µs) is the only wakeup

pin (longer than 10 µs) is

Hibernate, Sleep and Tag Detector states can only be activated by a command from the external host (MCU). As soon as any of these three states are activated, the device can no longer communicate with the external host. It can only be woken up.

The behavior of the device in 'Tag Detector' state is defined by the Idle command.

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Figure 1. CR95HF or STRFNFCA initialization and operating state change

2.2 Idle command

The Idle command switches the device into WFE mode (low consumption) and defines the way it returns to Ready state.

In WFE mode, the device has two low consumption states: Hibernate and Sleep/TagDetector.

In Hibernate state, the device power consumption is extremely low (a few microamps) because most of its resources are switched off. When it receives an external event (pin IRQ_IN

In Sleep/Tag-Detector state, the device power consumption is low (tens of microamps) depending on the selected wakeup source. The wakeup source can be an internal timer, an external interrupt or the presence of a tag detected by the device.

In Tag-Detector state, after executing an initial Tag Detection Calibration procedure to set-up the device, the application engineer must adjust command parameters to select the best trade-off between the wakeup delay and the power budget.

), it will return to Ready mode in only a few milliseconds.

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Table 4. Idle command description

Direction Data Comments Example

07 Command code Example of switch from Active

mode to Hibernate state:

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

Example of switch from Active to WFE mode (wakeup by low pulse on IRQ_IN

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

Example of switch from Active to WFE mode (wakeup by low pulse on SPI_SS

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

Example of wakeup by timeout

(7 seconds): Duration before Timeout = 256

* (WU period + 2) *

* t

(MaxSleep + 1)

>>>0x07 0E 01 21 00 38 00 18 00 60 60 00 00 00 00 08

Example of switch from Active to Tag Detector mode (wakeup by tag detection or low pulse on IRQ_IN

pin) (32 kHz, inactivity duration = 272 ms, DAC oscillator = 3 ms, Swing = 63 pulses of 13.56 MHz), 0A = Infinite sequence, 0B = Until TimeOut:

>>>0x07 0E 0A 21 00 79 01 18 00 20 60 60 64 74 3F 08

Example of a basic Idle command used during the Tag Detection Calibration process:

>>>0x07 0E 03 A1 00 F8 01 18 00 20 60 60 00 xx 3F 01

where xx is the DacDataH value.

pin):

Host to CR95HF or STRFNFCA

0E Length of data

EnterCtrlL

EnterCtrlH

WUCtrlL

WUCtrlH

LeaveCtrlL

LeaveCtrlH

Specifies authorized wakeup sources and the LFO frequency

Settings to enter WFE mode

Settings to wake up from WFE mode

Settings to leave WFE mode (Default value = 0x1800)

Period of time between two tag

detection bursts. Also used to specify the duration before timeout.

Defines the Wait time for HFO to

stabilize: <OscStart> * t (Default value = 0x60)

Defines the Wait time for DAC to

stabilize: <DacStart> * t (Default value = 0x60)

Lower compare value for tag detection

This value must be set to 0x00

(1)

during Tag Detection Calibration.

Higher compare value for tag detection

This is a variable used during

(1)

Tag Detection Calibration.

Number of swings HF during tag detection (Default value = 0x3F)

Max. number of tag detection trials before timeout

This value must be set to 0x01 during Tag Detection

Calibration. Also used to specify duration

before timeout. MaxSleep must be:

0x00 < MaxSleep < 0x1F

(1)

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Table 4. Idle command description (continued)

Direction Data Comments Example

0x00 Result code This response is sent only

when CR95HF or STRFNFCA exits WFE mode.

<<<0x000101 Wakeup by timeout

<<<0x000102 Wakeup by tag detect

CR95HF or STRFNFCA to Host

0x01 Length of data

<Data> Data (Wakeup source)

<<<0x000108 Wakeup by

low pulse on IRQ_IN

CR95HF or STRFNFCA to Host

1. An initial calibration is necessary to determine DacDataL and DacDataH values required for leaving Tag Detector state. For more information, contact your ST sales office for the corresponding application note.

0x82 Error code

0x00 Length of data

<<<0x8200 Invalid command

length

2.3 Idle command parameters

The Idle command (Host to device) has the following structure (all values are hexadecimal):

Table 5. Idle command structure

07 0E xx yy zz yy zz yy zz aa bb cc dd ee ff gg

Comma nd code

Data

length

source

Enter

Control

Leave

Control

Period

Osc

Start

DAC Start

DAC Data

Swing Count

Max

Sleep

Table 6. Summary of parameters

Parameter Description

Command code

Data length

This byte is the command code. ‘07’ represents the Idle command. This command switches the device from Active mode to WFE mode.

This byte is the length of the command in bytes. Its value depends on the following parameter values.

This byte defines the authorized wakeup sources in the wakeup source

WU Source

01: Time out 02: Tag Detection 08: Low pulse on IRQ_IN

These two bytes (EnterCtrlL and EnterCtrlH) define the resources when entering WFE mode.

Enter Control

0x0400: Hibernate 0x0100: Sleep (or 0x2100 if Timer source is enabled) 0xA100: Tag Detector Calibration 0x2100: Tag detection

These two bytes (WuCtrlL and WuCtrlH) define the wakeup resources.

WU Control

0x0400: Hibernate 0x3800: Sleep 0xF801: Tag Detector Calibration 0x7901: Tag detection

10: Low pulse on SPI_SS

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Table 6. Summary of parameters (continued)

Parameter Description

These two bytes (LeaveCtrlL and LeaveCtrlH) define the resources when

Leave Control

WU Period

Osc Start

DAC Start

DAC Data

returning to Ready state.

0x1800: Hibernate 0x1800: Sleep 0x1800: Tag Detector Calibration 0x1800: Tag detection

This byte is the coefficient used to adjust the time allowed between two tag detections. Also used to specify the duration before timeout. (Typical value: 0x20)

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

This byte defines the delay for HFO stabilization. (Recommended value: 0x60)

Defines the Wait time for HFO to stabilize: <OscStart> * t

This byte defines the delay for DAC stabilization. (Recommended value: 0x60)

Defines the Wait time for DAC to stabilize: <DacStart> * t

These two bytes (DacDataL and DacDataH) define the lower and higher comparator values, respectively. These values are determined by a calibration process.

When using the device demoboard, these values should be set to approximately 0x64 and 0x74, respectively.

Swing Count

Max Sleep

This byte defines the number of HF swings allowed during Tag Detection. (Recommended value: 0x3F)

This byte defines the maximum number of tag detection trials or the coefficient to adjust the maximum inactivity duration before timeout.

MaxSleep must be: 0x00 < MaxSleep < 0x1F This value must be set to 0x01 during the Tag Detection Calibration

process. Also used to specify duration before timeout. Duration before Timeout = 256 * t

(Typical value: 0x28)

* (WU period + 2) * (MaxSleep + 1)

2.4 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 CR95HF or STRFNFCA power consumption in WFE mode, the slower the LFO frequency, the lower the power consumption.

Example 1: Setting a lower LFO frequency

The following equation defines a basic timing reference:

= 256*tL ms (where tL = 1/f

REF

= 8 ms (when bits [7:6] are set to ‘00’, or 32 kHz)

REF

= 64 ms (when bits [7:6] are set to ‘11’, or 4 kHz)

REF

LFO

)

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2.5 Optimizing wakeup conditions

Using the Wakeup source register, it is possible to cumulate sources for a wakeup event. It is strongly recommended to always set an external event as a possible wakeup source.

To cumulate wakeup sources, simply set the corresponding bits in the wakeup source register. For example, to enable a wakeup when a tag is detected (bit 1 set to ‘1’) or on a low pulse on pin IRQ_IN

2.5.1 Wakeup source register

Wakeup conditions are defined in the 8-bit wakeup condition register. These bits select one or several external conditions to be evaluated by the device in order to exit Idle mode.

Bit 0: When set, the device will wake up and return to Ready state at the end of a predefined cycle. The Time Out (TO) value is defined by the Max sleep and Wake Up period:

TO = (MaxSleep *(WuPeriod+1)*RT or RT= 256*t

This bit must be set when using the timer as a possible wakeup source. It must be set during Tag Detection Calibration to force a wakeup after the first Tag Detection trial.

Bit 1: When set, the device will wake up when a Tag is detected in the RF field. This bit must also be set during Tag Detection Calibration or during Tag Detection (when the RF field is active).

(bit 3 set to ‘1’), set the register to 0x0A.

= 8 ms

LFO

Bit 2: This bit is RFU.

Bit 3: When set, the device will wake up when an external interrupt (low level on pin IRQ_IN

is detected. This is useful for SPI communications.

It is recommended to set this bit to ‘1’ in order to recover in the event of a system crash.

Bit 4: When set, the device will wake up when an external interrupt (low level on pin |SPI_SS

Bits [7:5] These bits are RFU.

) is detected. This is useful for UART communication.

Cumulation of wakeup conditions

The wakeup conditions selected in the Wakeup condition register can be cumulated.

When the device returns to Ready state, it is possible to use the RdReg command to read the Wakeup condition register to know which event caused the device to wake up from Idle mode.

Read Wakeup condition register: 08 03 62 01 00

Reply (‘xx’ represent the Wakeup condition register bits): 00 01 xx

This information is used after each iteration during the Tag Detection Calibration process to determine the CalRef level. This level corresponds to the switching of the bits in the Wakeup condition register from 0x02 (Tag Detector) to 0x01 (TimeOut) when using the device previously calibrated in a free-air environment (no tags present).

)

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2.6 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 CR95HF or STRFNFCA 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.

2.6.1 Default setting: from POR to Ready state

After power-on, the CR95HF or STRFNFCA enters Power-up state.

To wake up the device and set it to Ready state, the user must send a low pulse on the IRQ_IN enters Ready state and is able to accept commands after a delay of approximately 3 ms.

2.6.2 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 CR95HF or STRFNFCA can wake up from Hibernate state is by an external event (low pulse on pin IRQ_IN

pin. The device then automatically selects the external interface (SPI or UART) and

A basic Idle command is:

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

Note: The Wakeup flag value is NOT significant when returning to Ready state from Hibernate

state or after a POR.

2.6.3 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 wakeup source to achieve a moderate power consumption level.

An Idle command example when wakeup 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

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

Wakeup 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.

as a wakeup source:

Duration before Timeout = 256 * t

Note: Note that: 0x00 < MaxSleep < 0x1F.

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AN3433 Management of CR95HF or STRFNFCA activity

An Idle command example when wakeup source is timer (0x01) when f

= 32 kHz (mean

LFO

power consumption is 25 µA)

>>>0x07 0E 01 21 00 38 00 18 00 60 60 00 00 00 00 08

An Idle command example when wakeup source is timer (0xC1) when f

= 4 kHz (mean

LFO

power consumption is 20 µA):

>>>0x07 0E C1 21 00 38 00 18 00 60 60 00 00 00 00 08

The same command can be used mixing a timer and the IRQ_IN

pin (0xC9) as a wakeup

source:

>>>0x07 0E C9 21 00 38 00 18 00 60 60 00 00 00 00 08

Wakeup by Tag Detection

In this mode, the typical consumption can greatly vary in function of parameter settings (WU period without RF activity and Swing Count defining the RF burst duration). Using default settings, consumption in the range of 100 µA can be achieved.

Tag Detector is a state where the CR95HF or STRFNFCA is able to detect an RF event and a wakeup will occur when a tag sufficiently modifies the antenna load and is detected by the device.

An Idle command example when wakeup source is Tag Detection (0x02):

>>>0x07 0E 02 21 00 79 01 18 00 20 60 60 64 74 3F 08

The same command can be used mixing Tag Detection and the IRQ_IN wakeup source:

>>>0x07 0E 0A 21 00 79 01 18 00 20 60 60 64 74 3F 08

pin (0x0A) as a

The tag detection sequence is defined by dedicated parameters:

● WU source (Byte 3)

– The Timeout bit (bit 0) must be set to ‘1’ in order to manage a certain number of

emitted bursts. Otherwise, bursts will be sent indefinitely until a stop event occurs (for example, tag detection or a low pulse on pin IRQ_IN

– The Tag Detect bit (bit 1) must be set to ‘1’ to enable RF burst emissions.

– It is recommended to also set Bits 3 or 4 to ‘1’ to ensure that it is possible to leave

Tag Detect mode via an external event (for example, a low pulse on pin IRQ_IN

● WU period (Byte 10): Defines the period of inactivity (t

INACTIVE

●

OscStart, DacStart (Bytes 11 and 12): Define the set-up time of the HFO and Digital

= (WuPeriod + 2) * t

REF

INACTIVE

) between two RF bursts:

Analog Converter, respectively. In general, 3 ms is used both set-up times.

HFO | DAC set-up time = (OscStart | DacStart) * t

● DacDataL, DacDataH (Bytes 13 and 14): Reference level for Tag Detection (calculated

during the tag detection calibration process).

● SwingsCnt (Byte 15): Represents the number of 13.56-MHz swings allowed during a

Tag Detection burst. We recommend using 0x3F.

● MaxSleep (Byte 16): The CR95HF or STRFNFCA emits (MaxSleep +1) bursts before

leaving Tag Detection mode if bit 0 (Timer Out) of the WU source register is set to ‘1’. Otherwise, when this bit is set to ‘0’, a burst is emitted indefinitely.

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Note: Bytes 4 to 9 should be used as shown in the examples in Section 2.3: Idle command

parameters.

Note that the MaxSleep value is coded on the 5 least significant bits, thus: 0x00 < MaxSleep < 0x1F.

All the previously described command parameters must be chosen accordingly for the initial tag detection calibration when setting up the CR95HF or STRFNFCA.

Their value will impact tag detection efficiency and CR95HF or STRFNFCA power consumption during Tag Detection periods.

2.7 Tag detection calibration procedure

The Idle command allows the use of a tag detection as a wakeup event. Certain parameters of the Idle command are dedicated to setting the conditions of a tag detection sequence.

During the tag detection sequence, the CR95HF or STRFNFCA regularly emits RF bursts and measures the current in the antenna driver I

When a tag enters the device antenna RF operating volume, it modifies the antenna loading characteristics and induces a change in I

, and consequently, the DAC data register

DRIVE

reports a new value.

using the internal 6-bit DAC.

DRIVE

This value is then compared to the reference value established during the tag detection calibration process. This enables the device to decide if a tag has entered or not its operating volume.

The reference value (DacDataRef) is established during a tag detection calibration process using the device application setting with no tag in its environment.

The calibration process consists in executing a tag detection sequence using a well-known configuration, with no tag within the antenna RF operating volume, to determine a specific reference value (DacDataRef) that will be reused by the host to define the tag detection parameters (DacDataL and DacDataH).

During the calibration process, DacDataL is forced to 0x00 and the software successively varies the DacDataH value from its maximum value (0xFE) to its minimum value (0x00). At the end of the calibration process, DacDataRef will correspond to the value of DacDataH for which the wakeup event switches from timeout (no tag in the RF field) to tag detected.

To avoid too much sensitivity of the tag detection process, we recommend using a guard band. This value corresponds to 2 DAC steps (0x08).

Recommended guard band value:

DacDataL = DacDataRef – Guard and DacDataH = DacDataRef + Guard

The parameters used to define the tag detection calibration sequence (clocking, set-up time, burst duration, etc.) must be the same as those used for the future tag detection sequences.

When executing a tag detection sequence, the device compares the DAC data register value to the DAC Data parameter values (DacDataL and DacDataH) included in the Idle command. The device will exit WFE mode through a Tag Detection event if the DAC data register value is greater than the DAC Data parameter high value (DacDataH) or less than the DAC Data parameter low value (DacDataL). Otherwise, it will return to Ready state after a timeout.

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An efficient 8-step calibration algorithm is described in Example of CR95HF tag detection

calibration process on page 29.

An example of a basic Idle command used during the Tag Detection Calibration process:

>>>0x07 0E 03 A1 00 F8 01 18 00 20 60 60 00 xx 3F 01

where xx is the DacDataH value.

An example of a tag detection sequence is provided in Example of CR95HF tag detection

command using results of tag detection calibration on page 32.

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Power consumption considerations AN3433

3 Power consumption considerations

This chapter describes the advantages and benefits for using the various Idle command parameters to select the best wakeup configuration for your device to ensure optimal power consumption for your application.

The previous chapter describes how to use the Idle command to set the device from Ready to one of the three WFE modes (Hibernate, Sleep, and Tag Detector). The following sections describe the various wakeup processes for the selected WFE mode.

3.1 Transition from Ready to Hibernate state

Hibernate state consumes the least amount of power of all the possible device states.

Only an Idle command can set the device from Ready state to Hibernate state. After receiving the Hibernate command via the SPI bus, the device (Figure 2) stops the oscillator and analog resources and the device enters Hibernate state.

A basic Idle command to set the device from Ready state to Hibernate state is:

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

In Hibernate state, the device minimizes its power consumption by disabling most of its resources before stopping the external oscillator. In this state, power consumption will decrease from approximately 2.5 mA to less than 2 µA.

The only way the device can wakeup from Hibernate state is by an external event (pin IRQ_IN

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 Hibernate state (Figure 3), 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.

goes low).

16/34 Doc ID 019046 Rev 2

AN3433 Power consumption considerations

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

Figure 2. Transition from Ready to Hibernate state

1. In the above figure, yellow represents the 27.12-MHz (HFO) oscillator and the SPI_SCK signal is in green.

Figure 3 shows the device waking up from Hibernate state when pin IRQ_IN (green) goes

low. When the device returns to Ready state, the HF oscillator (yellow) restarts analog resources.

Figure 3. Wakeup from Hibernate state

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Power consumption considerations AN3433

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

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*t

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

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

or SPI_SS goes low).

*WuPeriod*MaxSleep

LFO

) is:

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

>>> 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 (t

INACTIVE

WuPeriod parameter.

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

principle.

INACTIVE

In the example shown in Figure 4:

INACTIVE

) between two RF burst emissions is defined in the Idle command by the

= (WuPeriod+1)*t

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

) is:

REF

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AN3433 Power consumption considerations

0x07 0E 0A 21 00 79 01 18 00 20 60 60 64 74 3F 08

Figure 4 illustrates a Tag Detection cycle.

Figure 4. Typical cycle in Tag Detection mode

1. In the above figure, yellow represents the 27.12-MHz (HFO) oscillator and the RF emission is in green.

Figure 5 shows a zoomed image of Figure 4 including the HFO stabilization time (OscStart)

and the DAC initialization time (DacStart).

Figure 5. Restart of resources in Tag Detection mode

1. In the above figure, yellow represents the 27.12-MHz (HFO) oscillator and the RF emission is in green.

Doc ID 019046 Rev 2 19/34

Power consumption considerations AN3433

The duration of the RF burst (t

) is defined by SwingsCnt, the number of 13.56-MHz

SWING

pulses emitted. This value must be adjusted initially in the application environment to obtain the best measurement stability.

SWING

= Swing * t

= 64 * 73 ns = 4.5 µs

HFO

Increasing the SwingsCnt value improves DAC accuracy, but also increases the power consumption of the RF drivers (pin VPS_TX).

When using the CR95HF or STRFNFCA demoboard, we recommend a SwingsCnt value of 0x3F. This value corresponds to 64 RF pulses per burst (in blue) for a duration of 4.7 µs as shown in Figure 6. This results in a mean power consumption of 100 µA.

Figure 6. RF burst in Tag Detection mode

1. In the above figure, yellow represents the 27.12-MHz (HFO) oscillator and the RF emission is in green.

20/34 Doc ID 019046 Rev 2

AN3433 Power consumption considerations

The device consumes the most power during RF emissions in Tag Detection mode (Figure 7). The User can regulate the mean consumption by modifying the period between bursts (Wuperiod).

Figure 7. Power consumption in Tag Detection mode

1. In the above figure, yellow represents the 27.12-MHz (HFO) oscillator and the device power consumption is in green.

Doc ID 019046 Rev 2 21/34

Tag detection principle AN3433

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 * t

LFO

22/34 Doc ID 019046 Rev 2

AN3433 Tag detection principle

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A.1 Tag detection sequence

Several parameters of the tag detection sequence can be managed to optimize the wakeup time or power budget.

By adjusting the inactivity time (t

INACTIVE

), the time between two tag detection RF bursts, the power consumption can be modified through the WuPeriod parameter in the Idle command (usually set to 0x20).

INACTIVE

= (WuPeriod+1)*t

REF

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A minimum duration of 3 ms must be set for the DAC and HFO setup times to obtain a good accuracy of measure. This phase is a high consumption period, so we recommend keeping it short. These setup times (DacStart and OscStart, respectively) are defined in the Idle command and are both usually set to 0x60.

The RF burst length (SwingCnt) corresponds to the acquisition time necessary for the DAC. It must be sufficient to obtain a reproducible measurement, but it means turning on the RF driver which consumes the most energy. The SwingCnt parameter value is defined in the Idle command and is usually set to 0x3F.

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Tag detection principle AN3433

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Figure 9 illustrates a time-limited tag detection sequence where no tag is present in the

operating volume, none is detected and the device wakes up after a timeout. (The lower part of the figure illustrates the power consumption profile.)

A basic Idle command used for a time-limited tag detection sequence is:

>>> 0x 07 0E 0B 21 00 79 01 18 00 20 60 60 54 64 3F 08

Figure 9. Time-limited tag detection sequence (display of device consumption)

24/34 Doc ID 019046 Rev 2

AN3433 Tag detection principle

Figure 10 illustrates an unlimited tag detection sequence interrupted by a tag entering the

device operating volume. (The lower part of the figure illustrates the power consumption profile during tag detection and after the device automatically returns to Ready state.)

A basic Idle command used for an unlimited tag detection sequence is:

>>> 0x 07 0E 0A 21 00 79 01 18 00 20 60 60 54 64 3F 08

Figure 10. Tag detection sequence, Wakeup after tag entry in operating volume

(display of device consumption)

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Doc ID 019046 Rev 2 25/34

Tag Detection Calibration process AN3433

Appendix B Tag Detection Calibration process

The Tag Detection Calibration process must be performed when the application is initialized.

A fast and efficient dichotomist flow in 8 steps is proposed to achieve best results.

Before starting this flow, the user must define its basic setup which consists in defining the following parameters using the Idle command. These parameters are described in

Section 2.3: Idle command parameters on page 9.

When using the CR95HF or STRFNFCA demoboard and development kit, the following values are recommended:

● Wakeup condition: 0x03 (tag detection or timeout)

● EnterCtrlL/EnterCtrlH, WUCtrlL/WUCtrlH, and LeaveCtlL/LeaveCtrlH values define

internal parameters settings during the tag detection process. We recommend:

>>> 0xA1 00 F8 01 18 00

● MaxSleep: MUST be set to ‘01’ in Tag Detection Calibration process for immediate

wakeup. This means that only one detection impulse will be sent and immediately afterwards the device will exit Tag Detection Calibration mode.

● Oscillator and DAC stabilization times. We recommend: 0x60

● Number of 13.56 MHz swings during tag detection (SwingsCnt), we recommend using

0x3F to obtain a good reproducibility of measurements.

● WakeUp period. This parameter is not used during the Tag Detection Calibration

process.

● The Host can then use the RdReg command to read the Wakeup condition register to

determine the cause of the wakeup or check the reply code.

A basic Tag Detection Calibration command is:

>>> 0x 07 0E 03 A1 00 F8 01 18 00 20 60 60 00 xx 3F 01

B.1 Tag Detection Calibration algorithm

To determine the RF Field reference level (CalRef) corresponding to your application setup, you must use the Tag Detection Calibration function (TagCal).

For calibration purposes, parameters are set to execute only one RF field evaluation before leaving Tag Detection state.

This is achieved by setting the WU condition parameter to 0x03 (wakeup upon timeout or tag detection) and the MaxSleep value to 0x01 forcing the CR95HF or STRFNFCA to leave the Tag Detection state after only one attempt.

The value of the Wakeup flag is updated to the following value each time the device leaves Tag Detection state.

Here, the WakeUp_n variable corresponds to the third byte of the device Idle command reply.

If the measured antenna current is between DacDataL and DacDataH, the wakeup event will be a timeout: WakeUp_n = 1

If the measured antenna current is less than DacDataL or greater than DacDataH, the wakeup event will be a tag detection: WakeUp_n = 2

26/34 Doc ID 019046 Rev 2

AN3433 Tag Detection Calibration process

During the calibration process, DacDataL is set to 0x00 and the reference level is determined changing only the DacDataH parameter.

The reference level corresponds to the switching level between wakeup sources. It is determined using the fast dichotomist process in 8 steps;

The Tag Detection Calibration command is formatted as follows:

TagCal (Ref_H) = 0x07 0E 03 A1 00 F8 01 18 00 20 60 60 00 Ref_H 3F 01

All the parameters are predefined to fit applicative requests in terms of wakeup sources and power consumption. A compromise must be established between the detection period and power consumption.

TagDet (64,74, or DacDataL and DacDataH, respectively) is defined using the command:

>>> 0x07 0E 0B 21 00 79 01 18 00 20 60 60 64 74 3F 00

B.2 Tag Detection calibration command

The Tag Detection Calibration command (TagCal) is a special setting of the Tag Detection command (TagDet) for which the only variable is Ref_H.

>>> 0x07 0E 03 A1 00 F8 01 18 00 01 60 60 00 XX 3F 00

<<< 0x00 01 01 (in the event of a timeout)

<<< 0x00 01 02 (in the event of a tag detection in the RF field)

Here, the third byte (the WakeUp_n variable) defines the Wakeup event.

It is also possible to directly use the Wakeup Flag command: 0x08 03 62 01 00

Read Wakeup Flag reply:

– In case of timeout: 0x00 01 01 (WakeUp_n = 0x01)

– In case of Tag Detection: 0x00 01 02 (WakeUp_n = 0x02)

In Figure 11, the function TagCal (0xRef_n) corresponds to the execution of the following device Idle command:

>>> 0x07 0E 03 A1 00 F8 01 18 00 20 60 60 00 0xRef_n 3F 01

<<< 0x00 01 01 (in the event of a timeout)

<<< 0x00 01 02 (in the event of a tag detection in the RF field)

Here, the third byte (the WakeUp_n variable) defines the Wakeup event.

Doc ID 019046 Rev 2 27/34

Tag Detection Calibration process AN3433

Figure 11. Dichotomist algorithm for tag detection calibration

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28/34 Doc ID 019046 Rev 2

AN3433 Example of CR95HF tag detection calibration process

Appendix C Example of CR95HF tag detection calibration

process

From this Revision 1.1, we can directly use the CR95HF reply during Tag Detection Calibration or Tag Detection sequences and avoid using the R

This is a dichotomic approach to quickly converge to the DacDataRef value for which a wakeup event switches from tag detection to timeout. In this process, only the DacDataH parameter is changed in successive Idle commands. And we look at the wakeup event reply to decide the next step.

<<< 00 01 02 corresponds to a Tag Detection (WakeUp_n = 2) <<< 00 01 01 corresponds to a Timeout (WakeUp_n = 1)

REM, Tag Detection Calibration Test

REM, Sequence: Power-up Tag Detect Wake-up by Tag Detect (1 try measurement greater or equal to DacDataH) or Timeout

REM, CMD 07 0E 03 A100 F801 1800 20 60 60 00 XX 3F 01

REM, 03 WU source = TagDet or Timeout

DREG command.

REM, A100 Initial Dac Compare

REM, F801 Initial Dac Compare

REM, 1800 HFO

REM, 20 WuPeriod = 32, Inactivity period = 256ms (LFO @ 32kHz)

REM, 60 Osc 3ms (LFO @ 32kHz)

REM, 60 Dac 3ms (LFO @ 32kHz)

REM, 00 DacDataL = minimum level (floor)

REM, xx DacDataH 00 = minimum level (ceiling)

REM, 3F Swing 13.56 4.6 us

REM, 01 Maximum number of Sleep before Wakeup 2

REM, Tag Detection Calibration Test

REM, During tag detection calibration process DacDataL = 0x00

REM, We execute several tag detection commands with different DacDataH values to determine DacDataRef level corresponding to CR95HF application set-up

REM, DacDataReg value corresponds to DacDataH value for which Wakeup event switches from Timeout (0x01) to Tag Detect (0x02)

REM, Wake-up event = Timeout when DacDataRef is between DacDataL and DacDataH

Doc ID 019046 Rev 2 29/34

Example of CR95HF tag detection calibration process AN3433

REM, Search DacDataref value corresponding to value of DacDataH for which Wake-up event switches from Tag Detect (0x02) to Timeout (0x01)

REM, Step 0: force wake-up event to Tag Detect (set DacDataH = 0x00)

REM, With these conditions Wake-Up event must be Tag Detect

>>> CR95HFDLL_STCMD, 01 070E03A100F801180020606000003F01

<<< 00 01 02

REM, Read Wake-up event = Tag Detect (0x02); if not, error .

REM, Step 1: force Wake-up event to Timeout (set DacDataH = 0xFC

REM, With these conditions, Wake-Up event must be Timeout

>>> CR95HFDLL_STCMD, 01 070E03A100F801180020606000FC3F01

<<< 00 01 01

REM, Read Wake-up event = Timeout (0x01); if not, error.

REM, Step 2: new DacDataH value = previous DacDataH +/- 0x80

REM, If previous Wake-up event was Timeout (0x01) we must decrease DacDataH (-0x80)

>>> CR95HFDLL_STCMD, 01 070E03A100F8011800206060007C3F01

<<< 00 01 01

REM, Read Wake-up event = Timeout (0x01) or Wake-up event = Tag Detect (0x02)

REM, Step 3: new DacDataH value = previous DacDataH +/- 0x40

REM, If previous Wake-up event was Timeout (0x01), we must decrease DacDataH (-0x40); else, we increase DacDataH (+ 0x40)

>>> CR95HFDLL_STCMD, 01 070E03A100F8011800206060003C3F01

<<< 00 01 02

REM, Read Wake-up event = Timeout (0x01) or Wake-up event = Tag Detect (0x02)

REM, Step 4: new DacDataH value = previous DacDataH +/- 0x20

REM, If previous Wake-up event was Timeout (0x01), we must decrease DacDataH (-0x20); else, we increase DacDataH (+ 0x20)

>>> CR95HFDLL_STCMD, 01 070E03A100F8011800206060005C3F01

<<< 00 01 02

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AN3433 Example of CR95HF tag detection calibration process

REM, Read Wake-up event = Timeout (0x01) or Wake-up event = Tag Detect (0x02)

REM, Step 5: new DacDataH value = previous DacDataH +/- 0x10

REM, If previous Wake-up event was Timeout (0x01), we must decrease DacdataH (-0x10); else, we increase DacDataH (+ 0x10)

>>> CR95HFDLL_STCMD, 01 070E03A100F8011800206060006C3F01

<<< 00 01 02

REM, Read Wake-up event = Timeout (0x01) or Wake-up event = Tag Detect (0x02)

REM, Step 6: new DacDataH value = previous DacDataH +/- 0x08

REM, If previous Wake-up event was Timeout (0x01), we must decrease DacDataH (-0x08); else, we increase DacDataH (+ 0x08)

>>> CR95HFDLL_STCMD, 01 070E03A100F801180020606000743F01

<<< 00 01 01

REM, Read Wake-up event = Timeout (0x01) or Wake-up event = Tag Detect (0x02)

REM, Step 7: new DacDataH value = previous DacDataH +/- 0x04

REM, If previous Wake-up event was Timeout (0x01), we must decrease DacDataH (-0x04); else, we increase DacDataH (+ 0x04)

>>> CR95HFDLL_STCMD, 0 1070E03A100F801180020606000703F01

<<< 00 01 01

REM, Read Wake-up event = Timeout (0x01) or Wake-up event = Tag Detect (0x02)

REM, If last Wake-up event = Tag Detect (0x02), search DacDataRef = last DacDataH value

REM, If last Wake-up event = Timeout (0x01), search DacDataRef = last DacDataH value -4

REM, For tag detection usage, we recommend setting DacDataL = DacDataRef -8 and DacDataH = DacDataRef +8

>>> CR95HFDLL_STCMD, 01 070E0B21007901180020606064743F01

<<< 00 01 01

Doc ID 019046 Rev 2 31/34

Example of CR95HF tag detection command using results of tag detection calibration AN3433

Appendix D Example of CR95HF tag detection command

using results of tag detection calibration

This is an example of a Tag Detection command when a tag is not present in the RF operating volume using CR95HF revision 1.1.:

>>> CR95HFDll_STCmd, 01 070E0B21007901180020606064743F01

<<< 00 01 01 Wake-up event = Timeout (0x01)

>>> CR95HFDll_STCmd, 01 0803620100

<<< 00 01 01

This is an example of a Tag Detection command when a tag is present in the RF operating volume using CR95HF revision 1.1.:

>>> CR95HFDll_STCmd, 01 070E0B21007901180020606064743F01

<<< 00 01 02 Wake-up event = Tag Detect (0x02)

>>> CR95HFDll_STCmd, 01 0803620100

<<< 00 01 02

32/34 Doc ID 019046 Rev 2

AN3433 Revision history

Revision history

Table 7. Document revision history

Date Revision Changes

05-Dec-2011 1 Initial release.

23-Feb-2012 2 Updated the document throughout to include the STRFNFCA device.

Doc ID 019046 Rev 2 33/34

AN3433

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34/34 Doc ID 019046 Rev 2

ST AN3433 APPLICATION NOTE

Specifications and Main Features

Frequently Asked Questions

User Manual

Introduction

1 Reference information

1.1 Reference documents

1.2 Terms and acronyms

Table 1. List of terms and acronyms

1.3 Notation conventions

2 Management of CR95HF or STRFNFCA activity

2.1 Overview of operating modes

Table 3. CR95HF or STRFNFCA operating modes and states

Figure 1. CR95HF or STRFNFCA initialization and operating state change

2.2 Idle command

2.3 Idle command parameters

Table 5. Idle command structure

Table 6. Summary of parameters

2.4 Using LFO frequency setting to reduce power consumption

2.5 Optimizing wakeup conditions

2.5.1 Wakeup source register

2.6 Using various techniques to return to Ready state

2.6.1 Default setting: from POR to Ready state

2.6.2 From Ready state to Hibernate state and back to Ready state

2.6.3 From Ready state to Sleep state and back to Ready state

2.7 Tag detection calibration procedure

3 Power consumption considerations

3.1 Transition from Ready to Hibernate state

Figure 2. Transition from Ready to Hibernate state

Figure 3. Wakeup from Hibernate state

3.2 Transition from Ready to Sleep state

3.3 Wakeup by Tag Detection

Figure 4. Typical cycle in Tag Detection mode

Figure 5. Restart of resources in Tag Detection mode

Figure 6. RF burst in Tag Detection mode

Figure 7. Power consumption in Tag Detection mode

Appendix A Tag detection principle

Figure 8. Tag detection principle (levels of antenna current)

A.1 Tag detection sequence

Figure 9. Time-limited tag detection sequence (display of device consumption)

Appendix B Tag Detection Calibration process

B.1 Tag Detection Calibration algorithm

B.2 Tag Detection calibration command

Figure 11. Dichotomist algorithm for tag detection calibration

Revision history

Table 7. Document revision history