High-Resolution, Low-Power Time Synchronization an Oxymoron No More Thomas Schmid, Prabal Dutta, Mani B. Srivastava

In a nutshell… If you need the functionality of a piece of equipment, but it costs too much to run continuously, what can you do? Simple – turn it off when you don’t need it. If you only need a high-frequency (=high- power) clock occasionally, use a slower clock (=lower-power) as your primary clock and only turn on the high-frequency clock when necessary.

Outline Introduction Achieving High Precision – Why we need it – How to do it Achieving Low Power – Temperature controlled clock Virtual High-Resolution Time (VHT) – Two different implementations Results Related work

Introduction Time synchronization is important. Mean synchronization accuracy has not improved much below 1.5 μs. – Low-power synchronization at that level is non- existent. Two problems: – Radio hardware support needed to achieve high- precision message time-stamping. – High-frequency clocks are needed to achieve high- precision time-keeping.

Introduction 2 “Conventional wisdom holds that to achieve a certain level of time synchronization precision, a free-running timebase of comparable frequency is needed.” – Time-keeping costs grow linearly with the required precision. – Power draw from a 8 MHz crystal is 340 times the power draw of 32 kHz crystal. – 8 Mhz->125 ns. 32 kHz->31.3 μs According to the authors, the conventional wisdom is no longer true.

Why the conventional wisdom is wrong: There are modern, low-power radios that support sub-microsecond level message time- stamping. You can use 2 clocks, one fast (8 MHz) and one slow (32 kHz), and only power the fast one when you need the extra speed. – This allows you to scale the power needs with access. – Precision on-demand.

Achieving High Precision Need 2 things: – A high-resolution clock source (frequency f 0 ) A clock can only estimate an event time to a resolution of 1/f 0. – Message time-stamping with accuracy ε < +/- 1/f 0 A node needs to know its frequency error with respect to a reference clock. Without error correction nodes need to re- sequence very frequently. If you have both of those, then you just need to measure clock offsets between network nodes, and apply an existing frequency error estimation technique.

Frequency Error Time offset measurement method: – c A (t) is quantized time count = f 0 *t, c B (t)=(f 0 +f e )*t If both time counts start at 0 then we get: We can use that equation to determine the limit to the resolution with which the frequency error can be estimated.

Frequency Error 2 We can find the minimum resolution of the frequency error estimation by taking the difference between two measurements that are separated by one tick: Example: – f 0 =8MHz, T=10s, error resolution is ppm. – f 0 =32.768kHz, T=10s, error resolution is 3.05 ppm. High frequencies also help improve error estimation. – The greater the clock frequency, the shorter the interval of time needed to synchronize a pair of clocks to a given frequency error resolution.

Accurate Timestamps Need to capture the timestamp in hardware. Most radios have a dedicated interrupt line that fires at a particular point during the reception or transmission of a message. – RBS, Reference Broadcast Synchronization, timestamps the first byte sent or received. One of the first. 10 μs accuracy at ~1 MHz. – FTSP, Flooding Time Synchronization Protocol, timestamps multiple bytes. Accuracy of 1.5 μs. The current standard.

Timestamp Tests Two different radios: – TI CC2420, dedicated interrupt line. – Atmel RF230, interrupt line is multiplexed with other radio state-machine events. 2.4 GHz, max data-rate is 250 kbit/s. Used IEEE , that uses message frames.

TI CC2420 Difference between Start of Frame delimiter rising at the transmitter and at the receiver. Mean of μs, and μs. SD is ns and 40.9 ns. 95% of measurements fall within a 160 ns window. Mean of 3.58 ns, SD of ns.

Atmel RF230 Mean time between transmitter and receiver is 17.4 μs. SD of 290 ns and 370 ns. SD is about 7x higher than TI CC2420.

Results On TI CC2420, need a clock frequency of 12.5 MHz to guarantee with high probability that the message timestamp can be resolved to within +/- 1 clock tick. CC2420 will achieve a better time synchronization accuracy when using high frequency clocks.

Achieving Low Power Need 2 things: – Low-frequency clocks – Infrequent communication Low-frequency: Power draw from a 8 MHz crystal is 340 times the power draw of 32 kHz crystal. – Telos platform at 32 kHz, in sleep mode draws 7.2 μA at 3.0V or 4.5 μA at 2.3V. Infrequent communication: The longer the resynchronization interval, the more likely environmental temperature and clock drift will create error.

Achieving Clock Stability Temperature-Compensated Crystal Oscillators (TCXO). – Hardware solution to regulating frequency with respect to temperature. – Work well, but are too big and expensive. Temperature Compensated Time Sync (TCTS) – Software lookup-table solution using a regular temperature sensor (no new hardware).

TCTS One node has access to an accurate and stable timebase. All other nodes synchronize to the main node, and during resynchronization each node calculates its current frequency error. Temperature and frequency error are cached in a frequency vs. temperature table. At each resychronization, if the current temperature is in the database, then the node will not resychronize because no new time estimate is required. Eventually all operating temperatures will be covered, essentially providing a TCXO timebase.

TCTS Experiment Results Temperature in chamber slowly changed from -10 o C to 60 o C. Quadratic equation: f e (T)=-A*(T-T 0 ) 2 +B, A=temperature coefficient, T 0 =20 o C for room temperature, B=frequency error offset. Can use the equation to find frequency error estimates for previously unobserved temperatures.

Full TCTS Experiment Recalibration set to 30 seconds. TCTS maintains tight synchronization. Power draw: temperature measurement = 66.5 μJ. Sending a message 600 μJ.

Virtual High-Resolution Time Need High-Accuracy – High-frequency clocks – Accurate time-stamping mechanism Need Low-Power – Low-frequency clock – Infrequent synchronization Enter VHT: high-frequency clock only when you need it, otherwise use a low-frequency clock.

VHT Overview During active periods the high-frequency clock is turned on, and a hardware counter counts the number of high-frequency clock ticks that occur during each low-frequency clock interval. There are ρ 0 = f H /f L high-frequency clock ticks during each low-frequency tick. When an event of interest occurs, the system records both counters (high-frequency counter is reset every low-frequency tick). The event time is: t event =C L *ρ 0 + ρ.

Microcontroller-based VHT Need a microcontroller with: – 2 clock inputs, driven by the two different oscillators – 2 timers with capture and compare modes, sourced by the two clocks – Some way to trigger a capture of the high- frequency timer.

Microcontroller-based VHT on CC2420 Both capture units are on the interrupt line. Another capture unit is triggered on the low- frequency rising edge to capture the high- frequency counter. (Sync event) The event captures l 0 and h 1. Event time:

Microcontroller-based VHT on CC2420 Drawbacks: – Uses all available timer resources. No more ADC. – Limited width of the counters. On the CC2420 they are only 16-bits. A 32-bit counter at 8 MHz will overflow every 5 minutes.

FPGA-based Dedicated VHT Smart adder adds 16-bit counter to LTC counting register, or stores LTC register directly.

FPGA-based Dedicated VHT Drawbacks: – Draws more power than if it were directly implemented in a microcontroller. – Timers are off-chip, which means we need a communication interface between the microcontroller and the FPGA.

Microcontroller-based VHT Results Modified timer so incoming or outgoing messages get the 32 kHz clock and the phase of the high-frequency timer. Experiment: – 5 nodes running VHT, synchronizing every 10 seconds – A sixth node sends a beacon every 2 seconds with the other nodes timestamp.

Results The 8 MHz virtual clock provides maximum time resolution of μs. Average accuracy of one tick, with SD of μs.

Power Draw Uses different hardware. Polling interval of 1.6 seconds (radio is turned on to take a sample every 1.6 seconds), duty-cycle of 0.77%. 5 orders of magnitude difference in power draw. Power draw is 3x lower with VHT.

Power model for VHT-equipped Node For a given duty cycle (dc) running Low- Power-Listening (LPL): – Regular node: – VHT node: – P 0 is leakage, P lclk is the slow clock, P hclk is the fast clock, and P radio is the radio power draw.

Conclusions System designers used to have to choose between low-power and high-resolution. Don’t need to make that decision anymore because now both are available. – VHT allows better than 1 μs time-keeping precision and an order-of-magnitude improvement in power draw compared to conventional techniques at 0.1% or lower radio duty cycles.

Related Work Reference Broadcast Synchronization (RBS) was one of the first synchronization protocols. – Not really relevant when low-level software- or hardware-based time-stamping mechanisms are available. Timing-sync Protocol for Sensor Networks (TPSN). – Should have 2x better performance than RBS, but they do not implement clock drift estimation. – IEEE implemented a protocol that is very similar to TPSN that gets better than 100ns accuracy.

More Related Work Flooding Time Synchronization Protocol (FTSP) – Current de facto standard. – Nothing has yet improved on it, but it does not take into account propagation delays. Gradient Time Synchronization Protocol (GTSP) – Corrects the problem where nodes are radio neighbors but are members of different synchronization trees.

Even More Related Work Syntonistor – a device for synchronization that locks onto 60 Hz AC noise found in buildings. – Get accuracy better than 1 ms while only consuming only 58 μW. – Low power is comparable, but accuracy is not. Harmonia time-synchronization system – Closest to VHT. Relies on a TCXO-driven clock that provides a stable 1 Hz signal. – May not be widely applicable, while VHT is drop in.