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.

February 2012

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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

1.1Reference documents

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

STRFNFCA: Near field communication transceiver datasheet

1.2Terms 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

1.3Notation 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		Tag Calibration Reference level and
		DAC threshold level for device setup
DacDataH		High Level detection limit
DacDataL		Low Level detection limit
MaxSleep		Number of inactivity periods (tINACTIVE) before a timeout.
SwingCnt		Number of 13.56 MHz periods in the RF burst
tDAC		DAC set up time
tHFO		Period of RF carrier (13.56 MHz)	73 ns
tINACTIVE		Inactivity period, delay between two wakeups for RF burst	tINACTIVE =
		emissions in a Tag Detection sequence	(WuPeriod+1)*tREF
tLFO		Period of device Low frequency oscillator (32 kHz)	31.25 µs

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	Table 2.	List of characteristic values (continued)
	Symbol		Definition		Value
	tOSC		Oscillator set-up time
	tREF		Reference time		tREF
					= 256*tLFO = 8 ms
	tSWING		RF Burst duration in Tag detection		tSWING = Swing * tHFO
	tTO		Timeout delay after which the device will automatically		tTO = (MaxSleep) *
			leave WFE mode.		tINACTIVE
	WuPeriod		Number of reference times (RT) in an inactivity period
			(tINACTIVE)

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2 Management of CR95HF or STRFNFCA activity

2.1Overview 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.
		Power-up	Low level on					pin (longer than 10 µs) is the only wakeup
							IRQ_IN
			source. LFO (low-frequency oscillator) is running in this state.
			Lowest power consumption state. The device has to be woken-up in
		Hibernate	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:
	Wait For		– Timer
		Sleep	– IRQ_IN pin
	Event
			– SPI_SS pin
	(WFE)
			LFO (low-frequency oscillator) is running in this state.
			Low power consumption state with tag detection. Wakeup source is
			configurable:
			– Timer
			–		pin
		Tag Detector		IRQ_IN
			–			pin
				SPI_SS
			– Tag detector
			LFO (low-frequency oscillator) is running in this state.
			This mode is accessible directly after POR.
		Power-up	Low level on					pin (longer than 10 µs) is the only wakeup
							IRQ_IN
	Active		source. LFO (low-frequency oscillator) is running in this state.
			Lowest power consumption state. The device has to be woken-up in
		Hibernate	order to communicate. Low level on						IRQ_IN	pin (longer than 10 µs) is
			the only wakeup source.

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

2.2Idle 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), it will return to Ready mode in only a few milliseconds.

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.

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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:
			0E	Length of data
					>>>0x07 0E 08 04 00 04
				Specifies authorized wakeup
			<WU Source>		00 18 00 00 00 00 00 00
				sources and the LFO frequency
					00 00
			EnterCtrlL	Settings to enter WFE mode	Example of switch from Active
					to WFE mode (wakeup by low
			EnterCtrlH
					pulse on		IRQ_IN		pin):
			WUCtrlL	Settings to wake up from WFE
					>>>0x07 0E 08 01 00 38
			WUCtrlH	mode
					00 18 00 00 00 00 00 00
			LeaveCtrlL	Settings to leave WFE mode	00 00
				(Default value = 0x1800)	Example of switch from Active
			LeaveCtrlH
					to WFE mode (wakeup by low
				Period of time between two tag
					pulse on SPI_SS pin):
				detection bursts. Also used to
			<WUPeriod>		>>>0x07 0E 10 01 00 38
				specify the duration before
					00 18 00 00 00 00 00 00
				timeout.
					00 00
				Defines the Wait time for HFO to	Example of wakeup by timeout
			<OscStart>	stabilize: <OscStart> * tL	(7 seconds):
				(Default value = 0x60)	Duration before Timeout = 256
					* tL * (WU period + 2) *
				Defines the Wait time for DAC to
	Host to		<DacStart>	stabilize: <DacStart> * tL	(MaxSleep + 1)
					>>>0x07 0E 01 21 00 38
	CR95HF or			(Default value = 0x60)
	STRFNFCA				00 18 00 60 60 00 00 00
				Lower compare value for tag
					00 08
				detection (1).
					Example of switch from Active
			<DacDataL>	This value must be set to 0x00
					to Tag Detector mode (wakeup
				during Tag Detection	by tag detection or low pulse
				Calibration.
					on IRQ_IN pin) (32 kHz,
				Higher compare value for tag	inactivity duration = 272 ms,
			<DacDataH>	detection (1).	DAC oscillator = 3 ms, Swing
				This is a variable used during	= 63 pulses of 13.56 MHz), 0A
				Tag Detection Calibration.	= Infinite sequence, 0B = Until
					TimeOut:
			<SwingsCnt>	Number of swings HF during tag
				detection (Default value = 0x3F)	>>>0x07 0E 0A 21 00 79
					01 18 00 20 60 60 64 74
				Max. number of tag detection	3F 08
				trials before timeout (1).
				This value must be set to 0x01	Example of a basic Idle
					command used during the Tag
				during Tag Detection
			<MaxSleep>	Calibration.	Detection Calibration process:
				Also used to specify duration	>>>0x07 0E 03 A1 00 F8
				before timeout.	01 18 00 20 60 60 00 xx
				MaxSleep must be:	3F 01
				0x00 < MaxSleep < 0x1F	where xx is the DacDataH
					value.

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	Table 4.	Idle command description (continued)
	Direction		Data	Comments	Example
			0x00	Result code	This response is sent only
			0x01	Length of data	when CR95HF or STRFNFCA
					exits WFE mode.
	CR95HF or				<<<0x000101 Wakeup by
	STRFNFCA		<Data>	Data (Wakeup source)	timeout
	to Host				<<<0x000102 Wakeup by
					tag detect
					<<<0x000108 Wakeup by
					low pulse on	IRQ_IN	pin
	CR95HF or		0x82	Error code	<<<0x8200 Invalid command
	STRFNFCA
			0x00	Length of data	length
	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.

2.3Idle 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	Data	WU	Enter	WU	Leave	WU	Osc	DAC	DAC	Swing	Max
nd code	length	source	Control	Control	Control	Period	Start	Start	Data	Count	Sleep

	Table 6.	Summary of parameters
	Parameter					Description
	Command code		This byte is the command code. ‘07’ represents the Idle command. This
			command switches the device from Active mode to WFE mode.
	Data length		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		register. Predefined values are:
			01: Time out			02: Tag Detection
			08: Low pulse on	IRQ_IN		10: Low pulse on	SPI_SS
			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

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	Table 6.	Summary of parameters (continued)
	Parameter		Description
			These two bytes (LeaveCtrlL and LeaveCtrlH) define the resources when
	Leave Control		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
	WU Period		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:
	Osc Start		0x60)
			Defines the Wait time for HFO to stabilize: <OscStart> * tL
			This byte defines the delay for DAC stabilization. (Recommended value:
	DAC Start		0x60)
			Defines the Wait time for DAC to stabilize: <DacStart> * tL
			These two bytes (DacDataL and DacDataH) define the lower and higher
			comparator values, respectively. These values are determined by a
	DAC Data		calibration process.
			When using the device demoboard, these values should be set to
			approximately 0x64 and 0x74, respectively.
	Swing Count		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
	Max Sleep		This value must be set to 0x01 during the Tag Detection Calibration
			process.
			Also used to specify duration before timeout.
			Duration before Timeout = 256 * tL * (WU period + 2) * (MaxSleep + 1)
			(Typical value: 0x28)

2.4Using 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:

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

tREF = 8 ms (when bits [7:6] are set to ‘00’, or 32 kHz) tREF = 64 ms (when bits [7:6] are set to ‘11’, or 4 kHz)

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2.5Optimizing 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 (bit 3 set to ‘1’), set the register to 0x0A.

2.5.1Wakeup 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*tLFO = 8 ms

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 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) is detected. This is useful for UART communication.

Bits [7:5] These bits are RFU.

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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