Smart Clocking Techniques Extend Battery Life of Wearables

Dec 2, 2015

Jehangir Parvereshi Sr. Manager, Customer Engineering

SiTime Corporation

Today’s Wearables: Space Sensitive & Battery Driven

Wearable electronics are designed to: – Fit in ever-shrinking PCB real-estate – Collect and send data in short bursts – Return to the lowest power state – Stay in the lowest power state as long as feasible – Run for days on a small capacity, small footprint battery – Optimize current drain in a cyclic sleep scenario

Battery life is dictated by the coulombs consumed per cycle – Coulombs = Iactive.TON + Isleep.Tsleep

– Expressed in µC or mAH

Active State

Sleep State

Active State

Wake-up

TimeShut-down

Time

Tsleep TON

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Wearable Architecture

Major power consumption contributors – MCU – Wireless radio

Sleep Clock

RTC/WD Clock

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Which RF Protocol is a Power-miser?

From: A. Dementeyev, S. Hodges, S. Taylor and J. Smith, "Power Consumption Analysis of Bluetooth Low Energy, ZigBee, and ANT Sensor

Nodes in Cyclic Sleep Scenario," in IWIS, Austin, 2013.

The Bluetooth Low Energy (BLE) protocol has the lowest current drain • BLE has optimized radio power, bit-rate and connection time

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BLE Power Savings Model

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Sleep Clock Accuracy (SCA) & BLE Power

Master Slave

Initiating_LL_PDU SlaveSCA

setting:

0 to 500 ppm

Bluetooth Core 4.0, Vol 6 Specification Conn_Update_PDU

Slave negotiates link

parameters to (1)

determine Sleep Time,

and (2) to receive Master

SCA setting

Slave determines wake

up time based on the

combined MasterSCA

and SlaveSCA settings

and Sleep Time

• Average power is directly proportional to the ratio of “ON” time to “Sleep” time

• Early ON time (ΔT) to accommodate inaccurate sleep clock causes power penalty

• ΔT = (SLEEP CLOCK ACCURACY) * (SLEEP TIME)

Early ON Time

SLEEP ON

SLEEP TIME TON

ΔT

ON

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SCA & Window Widening in BLE Core 4.0

ΔT = (SLEEP CLOCK ACCURACY) * (SLEEP TIME)

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BLE Power Savings Evaluation System

32768 Crystal

USB Powered CC2540 BLE

MASTER SLAVE

Current waveform reflects Connection event profile

PC

USB

CC2541 Key FOB Slave

TI CC2540 TI CC2541

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BLE Window Widening Details

Sleeping Sleeping

Wake up Tx

ON Time

RX

Window Post-process Pre-process

Rx window width is proportional to (masterSCA + slaveSCA)

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Measured ‘ON Time’ vs. Sleep Time

masterSCA + slave SCA= 80 ppm For Sleep Time = 4s; ON Time = 3.6 ms

ON Time = 5.43 ms

Sleep Time (ms)

ON Time (ms)

100 2.9

2000 3.2

4000 3.6

8000 4.3

16000 5.2 ON Time = 3.6 ms

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BLE Power Savings with a 32 kHz TCXO

200 ppm quartz crystal

5 ppm TCXO

30% power savings going from ±200 ppm to ±5 ppm SCA

Quartz RX widening unacceptable at sleep time > 8s

2 µA Sleep current

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BLE Power Savings Test Results

µPower 32 kHz TCXO vs. 32 kHz crystal – µPower TCXO supports tighter SCA of < 5 ppm for maximum

power savings across operating temperature

• Typical 32 kHz crystal solution can only support SCA > ±200 ppm

– The actual frequency stability of Master and Slave sleep clocks must match the tighter programmed SCA settings to avoid link time-out

– A 32 kHz TCXO supports longer sleep times necessary to maximize power savings

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Overview of Clock Sources

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Characteristics of Clock Sources in Wearables

Device Today’s Typical

Clock Sources

Frequency

Range (MHz)

Current

(uA)

Resume

Time (ms)

Frequency

Stability (ppm)

MCU

RC MHz on-chip XO 0.1 to 48 5/MHz** 0.02 ±2500*

RC kHz on-chip XO 0.032768 0.12 0.15 ±2500*

Ext XTAL + on-chip MHz XO 4 to 48 34/MHz** 3 ±40

Ext XTAL + on-chip kHz XO 0.032768 0.5** 2000 ±200

BLE

RC MHz on-chip XO 16 3/MHz** 0.02 ±10000*

Ext XTAL + on-chip MHz XO 16 400** 3 ±40

Ext XTAL + on-chip kHz XO 0.032768 2** 0.5** ±250

* Calibrated with 32 kHz crystal oscillator; drifts with VDD, and temperature; CPU calibration power additive ** At 25⁰C; increases with temperature

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Clocking Characteristics for Next Gen Wearables

Device Next Gen

Clock Source

Frequency

Range (MHz)

Current (uA) Resume

Time (ms)

Frequency

Stability (ppm)

MCU

High Speed 4 to 24 10/MHz* 2 ±50*

Low Speed 0.032768 1* 200 ±5*

BLE

High Speed 16, 24 10/MHz* < 1 ±40*

Low Speed 0.032768 1* 200 ±5*

* Across operating temperature, VDD, and aging

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µPower MEMS Clocks

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Architecture of a µPower MEMS Oscillator

• < 100 ppm XO, ± 5 ppm TCXO frequency stability over temp • 1 Hz to 26 MHz programmable clock output • Environmental resiliency 30x better than quartz

MEMS Oscillator

Sustaining

Circuit

Frac-N

PLL

Charge

Pump

Temperature

To

Digital

OTP

Memory

Temp

Comp

Dividers,

Drivers,

I/O

CTRL

CLK

Programmable Analog IC

MEMS Resonator

1.5 mm

0.8

mm

524 kHz

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32kHz MEMS XO & TCXO vs. Quartz XTAL

Features SiT1532 / 52 Quartz XTAL

Package Footprint w/ Load Caps

1.2mm2 (80% smaller)

5.5mm2

Load Capacitors No Yes

Load Dependent Start-up No Yes

Bypass Caps No NA

32kHz CLK

XIN

XO

MCU, BLE, or Chipset PMIC

SiT1532 XO / SiT1552 TCXO

Quartz XTAL Solution

XIN

XO

MCU, BLE, or Chipset PMIC

Osc I/O Osc I/O

Vdd

XTAL+Caps

1.2mm2 total footprint 5.5mm2 total

footprint

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Industrial Temp Specification

MEMS XO 100 PPM Over Temp

2x More accurate than quartz XTAL

MEMS TCXO ±5 PPM Over Temp

30x – 40x more accurate than quartz XTAL

Quartz XTAL -160 to -200 ppm

Over Temp

Measured Measured

µPower MEMS 32 kHz XO & TCXO for Wearables

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µPower MHz MEMS Oscillator for Wearables

MEMS core power: 60 uA

Smallest footprint: 1.2mm2

Any frequency: 1 to 26 MHz

MCU Operation Mode

Current Consumption (mA)

MHz MEMS + MCU

MHz XTAL + MCU

Operating Mode 5.2 (7% lower) 5.6

Sleep Mode 2.1 (18% lower) 2.6

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µPower MEMS Clock Characteristics

SiT8021 - MHz clocks for MCU – Frequency range: 1 MHz to 26 MHz – Frequency stability: ± 50 ppm across -20⁰C to 70⁰C, VDD, aging – Core current = 60 uA – Standby current = 0.7 uA – Resume time = 2 ms – Footprint: 1.2mm2 (1508) – Drives multiple loads

SiT1552 – 32.768 kHz clocks for RTC/WD & BLE sleep clocks – Frequency range: 32.768 kHz – Frequency stability: ±5 ppm across temp, VDD, aging – Active current = 1 uA – Footprint: 1.2mm2 (1508)

– Drives multiple loads

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Smart Clocking Techniques

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Wearable Using µPower MEMS Clocks

• Eliminates 2x 32.768 kHz crystals & 4 load caps • Eliminates bulky MHz crystal & 2 load caps • Saves power and PCB real-estate

Synchronize GPIO with BLE radio state Optional

Low-Power

MCU

BLE SoC24 MHz

32 kHz 32 kHz

CLK

μPower

24 MHz

STB

VDD

GND

1.8V

μPower

32.768 kHz

CLKVDD

GND

1.8V

SPI/I2C

Antenna

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BLE: Power Saving Clocking Techniques

• Replace 32 kHz XTAL with µPower 5 ppm TCXO • Shut down on-chip XTAL oscillator

Shut-down RCX

• Shut-down 32 kHz RC (500 ppm) • Save CPU cycles used for calibration

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Dialog Semiconductor BLE: DA14580

MCU: Power Saving Clocking Techniques

• Replace with µpower MEMS oscillator

• Shut-down on-chip crystal oscillator

• MEMS reduces wake-up time

• Shut-down RC oscillator • Save CPU cycles used

for calibration

• Set RC oscillator to 2 MHz

• Disable calibration; save CPU cycles

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STMicroelectronics MCU: STM32L476G

Conclusion

µPower MEMS 32.768 kHz XO reduces BLE current drain by 30% – 32 kHZ TCXO optimizes BLE RX radio ON time

– Supports extended sleep time

µPower MHz MEMS XO reduce MCU power consumption by 17% – Radio synchronized turn-on/off achieves lower active and sleep current

Added benefits of using µPower MEMS oscillators – Accurate time-keeping

– Reduces BOM cost

– Saves PCB real-estate

– Achieves an environmentally resilient product

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Power Savings Demonstration: Dialog Semi BLE Platform

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Demonstration HW

BLE Master

32 kHz MEMS TCXO

DA14580 BLE Slave

HDR Current Monitor

TCXO adapter board

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Dialog Semi Pro kit

Demo Software: SmartSnippets-1

Sleep Time = 1.2s

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Demo Software: SmartSnippets-2

ON Time = 7.63 ms

Coulombs consumed

• SlaveSCA = 200 ppm • Coulombs consumed during ON time = 6.6 µC

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Demo Software: SmartSnippets-2

ON Time = 7 ms

Coulombs consumed

• SlaveSCA = 5 ppm • Coulombs consumed during ON time = 4.98 µC ; Power savings = 32%

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Appendix

White Paper Resources at: http://www.sitime.com/support/sitime-university

Extending Battery Life of Wireless Medical Devices

Abstract: To continuously monitor and upload vital data, wireless medical devices need long battery life and long-term connectivity to the cloud. This paper discusses the advantages of Bluetooth® low energy (BLE) and a new architecture using an ultra-small, high-accuracy sleep clock that wakes up only when vitals must be updated, thus allowing the medical device to stay in sleep mode longer to reduce power consumption and extend battery life. http://www.sitime.com/images/stories/applications/SiTime-Extending-Battery-Life-of-Wireless-Medical-Devices.pdf

MEMS Timekeeper Extends Standby Life of Mobile Devices Abstract: This paper discusses how to extend battery life with techniques such as shutting down functional blocks with the highest current drain and switching to suspend or sleep states, and use of unique power saving strategies – such as programmable output frequencies and programmable output drive swing levels –

that reduce power consumption of always ON clocks used in mobile devices. http://www.sitime.com/images/stories/applications/SiTime-MEMS-TimeKeeper-Extends-Life-in-Mobile-Devices-2013.pdf

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