Abstract:
A synchronization apparatus, comprising: a USB device having a USB microcontroller, circuitry for observing USB traffic, and circuitry for decoding from a USB data stream a periodic data structure (such as a clock carrier signal) containing information about a distributed clock frequency and phase and outputting a decoded carrier signal; and circuitry for receiving the decoded carrier signal, for generating a software interrupt upon receipt of a predefined data packet (such as a SOF packet) and for passing the software interrupt to the USB microcontroller; wherein the USB microcontroller is configured to respond to the software interrupt (such as with an interrupt service routine provided therein) by generating an output signal adapted to be used as a synchronization reference signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS APPLICATION 
     This application is the U.S. National Stage of International Application No. PCT/AU2008/000663, filed on May 12, 2008, which is based on and claims the benefit of the filing date of U.S. Patent Application Ser. No. 60/938,148, filed May 15, 2007, the content of which as filed is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for providing a USB based synchronization and timing system, of particular but by no means exclusive use in providing clocks, data acquisition and automation and control of test and measurement equipment, instrumentation interfaces and process control equipment, synchronized to an essentially arbitrary degree in either a local environment or in a distributed scheme. 
     BACKGROUND OF THE INVENTION 
     The USB specification is intended to facilitate the interoperation of devices from different vendors in an open architecture. USB data is encoded using differential signalling (viz. two wires transfer the information) in the form of the difference between the signal levels of those two wires. The USB specification is intended as an enhancement to the PC architecture, spanning portable, desktop and home environments. 
     The USB specification assumes that devices differ. This is true for the intended environments in which devices from a multiplicity of manufacturers are connected, but there exist other environments (such as certain common industrial or laboratory environments) that require a specification for operating multiple devices of a similar nature in a synchronized manner. The specification does not sufficiently address this issue. Such environments are typically those where testing, measuring or monitoring is performed, and which require the devices to be synchronized to a more accurate degree than is specified. The USB specification allows limited inter-device synchronization by providing a 1 kHz clock signal to all devices. However, many laboratory and industrial environments require synchronization at megahertz frequencies and higher. 
     USB employs a tiered star topology, where hubs provide attachment points for USB devices. The USB host controller which is located on the user&#39;s personal computer (PC), laptop or personal digital assistant (PDA) contains the root hub, which is the origin of all USB ports in the system. The root hub provides a number of USB ports to which USB functional devices or additional hubs may be attached. 
     In turn, one can attach more hubs (such as USB composite devices) to any of these ports, which then provide additional attachment points via ports for further USB devices. In this way, USB allows a maximum of 127 devices (including hubs) to be connected, with the restriction that any device may be at most five levels deep. 
     The root hub in the host transmits a Start of Frame (SOF) signal packet every 1 ms to every device, the time between two SOF packets being termed a frame. Each module receives this SOF packet at a different time, owing to electrical delays inherent in the USB topology, which means that there may be a significant time delay (specified as ≦380 ns) between the receipt of a signal at a device connected directly to the host controller and at a device that is five levels down. This is a severe restriction when it is desired to synchronize devices at megahertz levels and above. Furthermore the USB specification allows the host controller to fail to transmit up to five consecutive SOF tokens. 
     Current synchronization between a USB host and a USB device is possible by two types of USB transfers, Interrupt and Isochronous. Interrupt transfers allow guaranteed polling frequencies of devices with minimum periods of 125 μs, whereas isochronous transfers guarantee a constant transfer rate. Both methods require there to be traffic between the device and host for synchronization to take place and therefore reserve more bandwidth for higher degrees of synchronization. This unfortunately means that the available USB bandwidth can be used up before the maximum number of devices has been connected. This approach also places on the host the great computational burden of keeping 127 devices synchronized to the host by means of software, yet still fails to address maintaining synchrony between the devices as to the host the individual devices represent separate processes. 
     Devices that contain a physical transducer of some kind, such as a laser diode or a photodetector, may require clock and trigger information. A device such as a laser diode with a modulated light output at 1 MHz may use a clock signal to perform transducer functions at regular intervals or at a constant frequency. A trigger signal is usually used to start or end an operation at a set time. In the laser diode example, a trigger signal could be used to turn the modulated light output on or off. 
     These clock and trigger signals can be used to synchronize a multiplicity of devices to each other (and hence constitute what is referred to below as “synchronization information”), provided that the signals are common and simultaneous to all devices. ‘Common’ and ‘simultaneous’ here mean that the variation in time of these signals between the devices is less than a specified quantity, δt. In the laser diode example, this would enable a multiplicity of laser diodes to modulate their light output at one frequency. The modulation frequency of all devices would be the same, and their waveforms would be in-phase. The current USB specification (viz. 2.0) allows for a δt of up to 0.35 μs. For a signal with a frequency of 1 MHz and a period of 1.0 μs, this delay represents almost half of the period. It is thus unusable as synchronization information for routine use. 
     Devices such as hubs and USB controller chips commonly use some amount of phase locking in order to decode the USB protocol. It is the purpose of the SYNC pattern in the USB protocol to provide a synchronization pattern for another electronic circuit to lock to. However, this is intended to synchronize the device to the USB bit streams to an accuracy sufficient to interpret MHz bit streams. It is not intended to synchronize two separate devices to each other to the level required by many test and measurement instruments. The USB specification—to the extent that it deals with inter-device synchronization—is mainly concerned with synchronizing a USB-CD audio stream sufficiently for output on a USB-speaker pair. The requirements of such an arrangement are in the kHz range and, for this application, the USB specification is satisfactory. However, the specification does not address the potential problems of synchronizing, for example, 100 USB-speaker pairs. 
     As discussed above, USB communication transfers data during regular 1 ms frames (or, in the case of the High-Speed USB specification, in eight micro-frames per 1 ms frame). A Start of Frame (SOF) packet is transmitted to all but Low-Speed devices at the beginning of each frame and to all High-Speed devices at the beginning of each micro-frame. The SOF packet therefore represents a periodic low resolution signal broadcast to all but Low-Speed devices connected to a given Host Controller. 
     This SOF packet broadcast occurs at a nominal frequency of 1 kHz (in the case of the High-Speed USB specification, 8 kHz). However the USB specification allows a very large frequency tolerance (by instrumentation standards) of some 500 parts per million. The background art utilises this low resolution frequency signal that is broadcast to each of the devices to provide clock synchronization, but only to the somewhat ambiguous frequency provided by the USB Host Controller. 
     U.S. Pat. No. 6,343,364 (Leydier et al.) discloses an example of frequency locking to USB traffic, which is directed toward a smart card reader. This document teaches a local, free-running clock that is compared to USB SYNC and packet ID streams; its period is updated to match this frequency, resulting in a local clock with a nominal frequency of 1.5 MHz. This provides a degree of synchronization sufficient to read smart card information into a host PC but, as this approach is directed to a smart card reader, inter-device synchronization is not addressed. 
     U.S. Pat. No. 6,012,115 and subsequent continuation U.S. Pat. No. 6,226,701 (Chambers et al.) address the USB SOF periodicity and numbering for timing. These documents disclose a computer system that can perform an accurate determination of the moment in time a predetermined event occurred within a real-time peripheral device by using the start of frame pulse transmitted from a USB host controller to peripheral devices connected to it. 
     However the methods taught by these documents do not involve the measurement of the frequency of a periodic data structure contained within the USB data traffic for determination of the absolute frequency of the master clock in the USB Host Controller, and in some cases rely on the provision of an additional counter in the host. 
     U.S. Pat. No. 6,092,210 (Larky et al.) discloses a method for connecting two USB hosts for the purpose of data transfer, by employing a USB-to-USB connecting device for synchronizing local device clocks to the data streams of both USB hosts. Phase locked loops are used to synchronize local clocks and over-sampling is used to ensure that data loss does not occur. This document, however, relates to the synchronization of the data streams of two USB hosts with each other (and with limited accuracy) such that transfer of information is then possible between said Hosts. This document does not teach any method for synchronizing a plurality of USB devices to a single USB Host or to a plurality of USB hosts. 
     The USB specification was written with audio applications in mind, and U.S. Pat. No. 5,761,537 (Sturges et al.) describes how to synchronize two or more pairs of speakers with individual clocks, where one pair operates off a stereo audio circuit in the PC and the other pair is controlled by the USB. Both speaker pairs use their own clocks, so they need to be synchronized so this document teaches one technique for maintaining synchronization of the audio signals despite possible clock skew between the asynchronous clocks. 
     U.S. patent application Ser. No. 10/620,769 (Foster et al.) discloses a synchronized version of the USB, in which the local clock of each device is synchronized on a given USB to an arbitrary degree. This document also discloses a method and apparatus for providing a trigger signal to each device within the USB such that an event may be synchronously initiated on multiple devices by the trigger signal. 
       FIG. 1  is a schematic diagram of an exemplary background art synchronized USB device  10 , connected to a digital USB  12 , via a USB bus connector  14 . USB device  10  contains an integrated USB interface and microcontroller  16 , USB device function circuitry (such as a digitally controlled transducer)  18 , bus sampling circuitry  20 , digital I/O bus decoding circuitry  22 , synchronization phase comparator  24  and synchronised clock  26 . 
     USB device  10  is connected by bus connector  14  to digital USB  12 . Digital USB  12  contains USB data and control signals for the USB device  10 ; bus sampling circuitry  20  observes the internal USB traffic  30  and generates a replica  32  thereof. Digital I/O bus decoding circuitry  22  decodes clock carrier signal  34  from signal  32 . Synchronization phase comparator  24  compares decoded clock carrier  34  with divided clock signal  36  from synchronized clock  26  such that the output local clock signal  28  (at a substantially higher frequency than carrier signal  34 ) is locked to carrier signal  34  in frequency and phase. 
     In this arrangement, synchronization is achieved by detecting and extracting information from USB  12  as USB signal traffic enters USB device  10  and by generating local clock signal  28 . 
     This architecture for synchronization of the local clock on each of a plurality of USB devices employs periodic data structures present in the USB traffic. An embodiment disclosed in U.S. application Ser. No. 10/620,769 essentially locks the local clock in frequency and phase to the detection of a SOF packet token at the USB device. Circuitry is employed to observe traffic through the USB and decode a clock carrier signal from bus traffic (in one embodiment, SOF packets), which results in a nominal carrier signal frequency of 1 kHz (or 8 kHz for USB High Speed). The local clock signal from a controlled oscillator clock is locked to the reception of the USB SOF packet in both phase and frequency. This ensures that all devices attached to the root hub are locked in frequency to the point at which they receive the SOF packet token. However, the approach described in U.S. application Ser. No. 10/620,769 is limited in its ability to provide a precisely known clock frequency to each device. 
     Further, although this disclosure teaches the highly accurate clock synchronization of devices attached to a USB, the disclosed approach employs a precision controlled oscillator, typically in the form of a voltage controlled voltage oscillator, and particular care must be taken to provide stable supply voltages. A closed loop control circuit is then applied to the precision oscillator. This adds both cost and complexity to the design of a synchronized USB device. 
     Another synchronized USB device, disclosed in International Patent Application No. PCT/AU2007/000155 filed 15 Feb. 2007 (Foster et al.), is shown schematically in  FIG. 2 . The technique of this disclosure allows the generation of accurate clock frequencies on board the USB device regardless of the accuracy of the clock in the Host PC. Referring to  FIG. 2 , USB device  40  includes a bus connector  44 , bus interface circuitry  46 , a microcontroller  48 , USB device function circuitry (such as a digitally controlled transducer)  50  and synchronization circuitry in the form of synchronizer  52  (comparable to digital I/O bus decoding circuitry  22 , synchronization phase comparator  24  and synchronised clock  26  of  FIG. 1 ). Bus interface circuitry  46  acts as a transceiver for USB data detected at bus connector  44 , passing the USB data to microcontroller  48 . USB device  40  also includes circuitry  54  that observes the internal bus link and passes a replica  56  of USB traffic  58  to synchronizer  52 . Microcontroller  48  provides information  60  to synchronizer  52  for accurate frequency synthesis of clock signal  62 . Microcontroller  48  communicates with device function circuitry  50  through communication bus  64 . 
     The carrier signal, once decoded from the USB traffic, is combined with a scaling factor to generate synchronization information and hence to synthesize a local clock signal with precise control of the clock frequency. In this way, the frequency of the local clock signal can be more accurate than the somewhat ambiguous frequency of the carrier signal by the use of additional information signal  60  from microcontroller  48 . 
     This arrangement is said to be able to produce a local clock signal to arbitrarily high frequencies, such as a clock frequency of tens of megahertz, and thus to ensure that the local clock of each device connected to a given USB is synchronized in frequency. U.S. application Ser. No. 10/620,769 also teaches a method and apparatus to further synchronize multiple local clocks in phase by measurement of signal propagation time from the host to each device and provision of clock phase compensation on each of the USB devices. 
     While such synchronous USB systems can perform accurate clock synchronisation between USB devices with accurate clock frequency generation, they require special hardware components to decode data present on the USB and precision determination of the moment in time of carrier signal reception. These components are required in addition to the normal USB bus interface circuitry and microcontroller ( 46  and  48  respectively of  FIG. 2 ) so these approaches are not compatible with a generic implementation of USB using off the shelf USB interface microcontrollers. 
     Additionally, the USB specification constraints the level of capacitance that the USB device can present to the bus. The effective capacitance of USB each data line to ground in the presence of the parallel effective resistance to ground is very tightly controlled. There is generally only a small capacitance margin with compliant USB devices. Addition of a parallel data pathway circuit (comparable to that of bus sampling circuitry  20  of  FIG. 1 ) to a conventional USB device would typically exceed the capacitance limits. 
     SUMMARY OF THE INVENTION 
     In a first broad aspect, the invention provides a synchronization apparatus, comprising:
         a USB device having a USB microcontroller, circuitry for observing USB traffic, and circuitry for decoding from a USB data stream a periodic data structure (such as a clock carrier signal) containing information about a distributed clock frequency and phase and outputting a decoded carrier signal; and   circuitry for receiving the decoded carrier signal, for generating a software interrupt upon receipt of a predefined data packet (such as a SOF packet) and for passing the software interrupt to the USB microcontroller;   wherein the USB microcontroller is configured to respond to the software interrupt (such as with an interrupt service routine provided therein) by generating an output signal adapted to be used as a synchronization reference signal.       

     The USB microcontroller may comprise the circuitry for observing the USB data stream. 
     The circuitry for receiving the decoded carrier signal and generating a software interrupt may be provided within the USB microcontroller, though it may alternatively be provided as circuitry external to the USB microcontroller or, indeed, external to the USB device. 
     The apparatus may comprise a hybrid USB interface and microcontroller that comprises the circuitry for decoding the periodic data structure and the circuitry for receiving the decoded carrier signal and generating the software interrupt. 
     The apparatus may comprise separate decoding hardware that generates a decoded SOF signal and passes it to the USB microcontroller, wherein the USB microcontroller generates the software interrupt in response thereto. 
     The periodic data structure may comprise, for example, any of the USB packet signal structures defined in the USB specification, command sequences sent to the USB device, data sequences sent to the USB device, OUT tokens, IN tokens, ACK tokens, NAK tokens, STALL tokens, PRE tokens, SOF tokens, SETUP tokens, DATA0 tokens, DATA1 tokens, or programmable bit pattern sequences in the USB data packets. 
     There is also provided a method for synchronizing a plurality of USB devices, employing a plurality of synchronization apparatuses as described above. 
     According to the first broad aspect there is also provided a synchronized USB comprising the apparatus described above. 
     According to a second broad aspect the present invention provides a method for assigning a validity weighting to a plurality of decoded synchronization reference signals, comprising;
         observing a decoded carrier signal;   prompting a software interrupt based on a predefined data packet (such as a SOF packet) in the decoded carrier signal;   executing in response to the software interrupt an interrupt service routine that generates an interrupt output signal;   determining a USB microcontroller instruction that was executing when said software interrupt was received;   determining a number of instruction cycles required by the USB microcontroller instruction; and   assigning a timing validity weighting factor to the timing reference signal based on the uncertainty in reception time of software interrupt.       

     The second broad aspect can employ one or more statistical methods to filter, average or otherwise process the repetitive carrier signal pulses for greater accuracy and stability. 
     In a third broad aspect the present invention provides a method for synchronizing the local clock of a USB device, comprising;
         observing a USB data stream;   decoding a periodic data structure from the USB data stream;   prompting a software interrupt based on the periodic data structure;   executing in response to the software interrupt an interrupt service routine;   generating an interrupt output signal with the interrupt service routine;   passing the interrupt output signal as a reference timing signal to synchronized clock circuitry having a phase locked loop architecture; and   locking a local clock (such as a high frequency local crystal oscillator clock) in phase and frequency with the synchronized clock circuitry based on the reference timing signal.       

     The periodic data structure may comprise, for example, any of the USB packet signal structures defined in the USB specification, command sequences sent to the USB device, data sequences sent to the USB device, OUT tokens, IN tokens, ACK tokens, NAK tokens, STALL tokens, PRE tokens, SOF tokens, SETUP tokens, DATA0 tokens, DATA1 tokens, or programmable bit pattern sequences in the USB data packets. 
     The third broad aspect can also be used to generate a synchronized USB using a plurality of synchronized USB devices according to this method. 
     In a fourth broad aspect the present invention provides a method for synchronizing the local clock of a USB device, comprising;
         observing a USB data stream;   decoding period data structure SOF packets in the USB data stream;   using a free running oscillator clock with multiple outputs, each with a respective phase (which will typically differ from one another);   upon receipt of the decoded periodic data structure selecting the output that is most in phase with the decoded periodic data structure; and   phase adjusting the free running clock based on the selected phase to compensate for the frequency of the free running clock being incorrect or the free running clock being unsynchronized.       

     The method will generally comprise repeatedly selecting from the phases of the outputs a phase that is most in phase with the decoded periodic data structure and phase adjusting the free running clock based on the selected phase. 
     The periodic data structure may comprise, for example, any of the USB packet signal structures defined in the USB specification, command sequences sent to the USB device, data sequences sent to the USB device, OUT tokens, IN tokens, ACK tokens, NAK tokens, STALL tokens, PRE tokens, SOF tokens, SETUP tokens, DATA0 tokens, DATA1 tokens, or predefined bit pattern sequences in the USB data packets. 
     Thus, the most appropriate phase of the clock (which is not frequency controlled) can be chosen at each periodic data structure (e.g. SOF) event. This may be used with software (ISR) or hardware type periodic data structure detection. 
     The method may include using a free running oscillator with multiple phase delayed outputs. 
     The method may include using a free running oscillator with multi-tap delay generator to generate the multiple phases. 
     According to the fourth aspect there is also provided a synchronized USB using a plurality of synchronized USB devices according to the above-described method. 
     According to one broad aspect, the invention provides a method for distributing a timing reference signal, comprising:
         connecting a device having a processor to a data bus;   monitoring the data bus with the processor for data signals local to the device, wherein the data contains a periodic data structure;   the processor responding to detection of the periodic data structure by generating a software interrupt;   the processor responding to the software interrupt by executing an interrupt service routine;   the interrupt service routine generating a timing reference signal.       

     The periodic data structure may be one of a plurality of periodic data structures, and the method include the processor responding to detection of each of the periodic data structures by generating respective software interrupts, the processor responding to the software interrupts by executing respective interrupt service routines, and the interrupt service routine generating respective timing reference signals. 
     The method may further comprise:
         identifying a software instruction being processed by the processor during reception of the respective software interrupt;   determining a number of instruction cycles required by the processor to execute the software instruction; and   assigning a weighting factor to each of the respective timing reference signals, wherein the weighting factor is a measure of the uncertainty in the period of time between generation of the respective software interrupt and the respective timing reference signal.       

     The method may further comprise:
         generating a plurality of weighted timing reference signals by combining the plurality of weighting factors with the plurality of timing reference signals, such that the contribution of a given one of the timing reference signals is weighted by its corresponding weighting factor.       

     The method may further comprise:
         locking a local clock of the device to the plurality of weighted timing reference signals in both frequency and phase.       

     The method may further comprise:
         statistically processing the weighted timing reference signals and thereby locking the frequency and phase of the local clock more precisely.       

     The data bus may be a USB and the device is a USB device. 
     The processor may be a microcontroller with an embedded USB physical interface, or a field programmable gate array or other form of programmable logic containing an embedded USB physical interface. 
     In one embodiment, the periodic data structure comprises any of the USB packet signal structures defined in the USB specification, command sequences sent to the USB device, data sequences sent to the USB device, OUT tokens, IN tokens, ACK tokens, NAK tokens, STALL tokens, PRE tokens, SOF tokens, SETUP tokens, DATA0 tokens, DATA1 tokens, or predefined bit pattern sequences in the USB data packets. 
     In another broad aspect, the invention provides a method for weighting a plurality of clock carrier signals comprising:
         monitoring a data bus for a periodic data structure with a processor;   the processor responding to the periodic data structure by generating a software interrupt;   the processor responding to the software interrupt by executing an interrupt service routine;   the interrupt service routine generating a timing reference signal;   determining the software instruction being processed by the processor during reception of the software interrupt;   determining a number of instruction cycles required by the processor to execute the software instruction, wherein the number of instruction cycles represents a measure of the uncertainty in determination of the time the processor issued the software interrupt; and   weighting the plurality of clock carrier signals according to an uncertainty in the time that the processor issued the software interrupt.       

     The periodic data structure may be one of a plurality of periodic data structures, and the method comprise:
         the processor responding to each of the periodic data structures by generating respective software interrupts;   the processor responding to the software interrupts by executing respective interrupt service routines;   the interrupt service routine generating respective timing reference signals;   assigning a weighting factor to each of the timing reference signals, wherein the weighting factor is a measure of the uncertainty in the period of time between generation of the software interrupt and the respective timing reference signal;   generating a plurality of weighted timing reference signals by combining the plurality of weighting factors with the plurality of timing reference signals, such that the contribution of a given one of the timing reference signals is weighted by its corresponding weighting factor.       

     The method may include determining the uncertainty in the period of time between generation of the software interrupt and the timing reference signal with a computing device. 
     The method may include determining the uncertainty in the period of time between generation of the software interrupt and the timing reference signal with a lookup table. 
     According to a broad aspect, the invention provides an apparatus for weighting a plurality of timing reference signals comprising:
         a processor or microcontroller that supports interrupt service routines;   circuitry for observing data traffic locally to the processor;   circuitry for decoding a periodic data structure from the data traffic;   circuitry for generating an acknowledgement signal corresponding to decoding of the periodic data structure;   circuitry for generating a software interrupt of the processor configured to issue an interrupt service routine upon receipt of the acknowledgement signal; and   circuitry for generating a timing reference signal in response to commands from an interrupt service routine.       

     The apparatus may further comprise:
         an output for reporting which instruction was being executed when the software interrupt occurred;   a mechanism for determining the uncertainty in the period of time between generation of the software interrupt and the timing reference signal;   a mechanism for calculating the weighting factor for each of the plurality of timing reference signals.       

     The mechanism for determining the uncertainty in the period of time between generation of the software interrupt and the timing reference signal may be, for example, a computing device or a lookup table. 
     The processor or microcontroller may have an embedded USB physical interface. The processor or microcontroller may be a field programmable gate array or other form of programmable logic, which may optionally have an embedded USB physical interface. 
     The invention also provides an apparatus for synchronising a USB device, comprising:
         a USB;   a processor or microcontroller having architecture supporting software interrupts and interrupt service routines;   circuitry for observing USB traffic local to the USB device when the USB device is in data communication with the USB;   circuitry for decoding a periodic data structure from traffic on the USB;   circuitry for generating an acknowledgement signal corresponding to decoding of the periodic data structure;   circuitry for generating a software interrupt of the processor upon receipt of the acknowledgement signal, wherein the software interrupt initiates an interrupt service routine configured so as to generate an output signal adapted to be used as a local synchronization reference signal; and   circuitry for generating a timing reference signal in response to commands from the interrupt service routine.       

     The apparatus may further comprise:
         a local clock; and   circuitry for controlling the frequency and phase of the local clock, wherein the timing reference signal is usable to control the local clock frequency and/or phase.       

     The processor or microcontroller may comprise the circuitry for observing USB traffic local to the USB device. 
     The processor or microcontroller may comprise the circuitry for decoding a periodic data structure from traffic on the USB. 
     The processor or microcontroller may comprise the circuitry for generating an acknowledgement signal corresponding to decoding of the periodic data structure; 
     The processor or microcontroller may comprise the circuitry for generating a software interrupt of the processor upon receipt of the acknowledgement signal. 
     The processor or microcontroller may comprise the circuitry for generating a timing reference signal in response to commands from the interrupt service routine. 
     The processor or microcontroller may comprise the local clock. 
     The circuitry for observing USB traffic local to the USB device may not be contained within the processor. 
     The circuitry for decoding a periodic data structure from traffic on the USB may not be contained within the processor or microcontroller. 
     The circuitry for generating an acknowledgement signal corresponding to decoding of the periodic data structure may not be contained within the processor. 
     The invention also provides a method for synchronizing the local clock of a USB device, comprising;
         observing a USB data stream;   decoding periodic signal structure packets in the USB data stream;   using a free running oscillator clock with multiple outputs, each with a respective phase;   upon receipt of the decoded periodic signal structure packets, selecting from the phases of the outputs a phase that is most in phase with the decoded periodic signal structure packets; and   phase adjusting the free running clock based on the selected phase to compensate for the frequency of the free running clock being incorrect or the free running clock being unsynchronized.       

     The method may comprise repeatedly selecting from the phases of the outputs a phase that is most in phase with the decoded periodic signal structure and phase adjusting the free running clock based on the selected phase. 
     The periodic data structure packets may comprise, for example, any of the USB packet signal structures defined in the USB specification, command sequences sent to the USB device, data sequences sent to the USB device, OUT tokens, IN tokens, ACK tokens, NAK tokens, STALL tokens, PRE tokens, SOF tokens, SETUP tokens, DATA0 tokens, DATA1 tokens, or predefined bit pattern sequences in the USB data packets. 
     The method may comprise using a free running oscillator with multiple phase delayed outputs. 
     The method may comprise using a free running oscillator with multi-tap delay. 
     The invention also comprises a method for synchronising the free running local clock of a device with a plurality of selectable phase shifted outputs, the method comprising:
         (i) the device receiving a periodic reference signal;   (ii) on receipt of each of the periodic reference signal, the device detecting which of the plurality of selectable phase shifted outputs is most in phase with the periodic reference signal; and   (ii) employing the most in phase of the phase shifted outputs as the local clock signal.       

     The method may include periodically adjusting the local clock signal in phase by repeating steps (i) to (iii). 
     The method may include selecting the phase shifted output that is most in phase with the periodic reference signal when assessed over a plurality of clock cycles. 
     The method may further comprise:
         determining the cumulative phase error of the free running oscillator outputs experienced during the period between successive cycles of the periodic reference signal; and   periodically selecting the next most appropriate of the plurality of phase shifted outputs, within the period between successive cycles of the periodic reference signals, in order to continually minimise the absolute phase error of the output clock signal.       

     It should be noted that the various features of each of the above aspects of the invention can be combined as desired. 
     In addition, apparatuses according to the invention can be embodied in various ways. For example, such devices could be constructed in the form of multiple components on a printed circuit or printed wiring board, on a ceramic substrate or at the semiconductor level, that is, as a single silicon (or other semiconductor material) chip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In order that the present invention may be more clearly ascertained, embodiments will now be described, by way of example, with reference to the accompanying drawing, in which: 
         FIG. 1  is a schematic diagram of a background art synchronized USB device; 
         FIG. 2  is a schematic diagram of another background art synchronized USB device; 
         FIG. 3  is a schematic diagram of a synchronized USB device according to a first embodiment of the present invention; 
         FIG. 4  is a schematic diagram of the command execution stack of two USB interface/microcontrollers according to the embodiment of  FIG. 3 , showing software interrupt latency and uncertainty; 
         FIG. 5  is a timing diagram of the command execution stack of  FIG. 4 ; 
         FIG. 6  is a schematic diagram of a synchronized USB device according to a second embodiment of the present invention; 
         FIG. 7  is a schematic diagram of the synchronizer circuitry of the USB device of  FIG. 6 ; and 
         FIG. 8  is a schematic timing diagram of the internal circuitry of the synchronizer circuitry of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A USB device according to a first embodiment of the present invention is shown schematically at  80  in  FIG. 3 , with a digital USB  82 . USB device  80  includes a bus connector  84 , for connection to USB  82 , USB interface/microcontroller  86 , digital input/output circuitry  88  (in the form, for example, of a digital transducer such as an analog to digital converter, pressure transducer or strain gauge) and synchronized clock circuitry  90 . 
     Clock synchronization information in the form of a repetitive carrier signal is extracted from USB  82  by USB interface/microcontroller  86 . Conventional USB microcontrollers (such as the Cypress EZUSB-FX brand series of USB microcontrollers) can be configured to execute a software interrupt on reception of the Start of Frame (SOF) packet in the USB data stream, and this is so in this embodiment; the interrupt service routine that executes in response to reception of the SOF packet is programmed to generate a reference timing signal  92  (at either 1 kHz for USB Full Speed or 8 kHz for USB High Speed), which is passed to synchronized clock circuitry  90 . Synchronized clock circuitry  90  contains a phase locked loop architecture to lock a high frequency local crystal oscillator clock in phase and frequency using low frequency reference signal  92 . 
     Synchronized output clock signal  94  is configured to synchronously control the operation of digital input/output circuitry  88  according to data and commands  96  provided from USB interface/microcontroller  86 . This architecture allows decoding of carrier signals for use by synchronizing circuitry without using dedicated additional hardware components to observe the USB data stream in parallel to the USB interface/microcontroller  86 . 
       FIG. 4  is a schematic diagram of, respectively, first and second command execution stacks  110 ,  112  of two examples of USB interface/microcontrollers  86  according to the embodiment of  FIG. 3 . Command stacks  110 ,  112  execute from top to bottom; execution time  114  increases from top to bottom in this view. Microcontroller instructions are integer multiples of a single instruction cycle time and are represented as blocks of time. 
     At some point in first command stack  110 , microcontroller  86  receives a physical interrupt stimulus in the form of a SOF interrupt signal  116  corresponding to an internal hardware register receiving a valid SOF packet in the data stream. This occurs at some point during, in this example, instruction  118  (shown arbitrarily as a three cycle instruction in this example). After completion of instruction  118 , microcontroller  86  pushes instructions onto the stack and executes an Interrupt Service Routine (ISR)  120 . ISR  120  is generally a series of instructions, but is represented as one instruction here for simplicity. ISR  120  responds to interrupt signal  116  at the next available time and is programmed to generate a software interrupt output signal  122  (which is comparable to reference timing signal  92  of  FIG. 3 ). There is a time delay t 1  between reception of physical interrupt signal  116  and software interrupt output signal  122 . 
     Time delay t 1  is variable and depends on both the length of an instruction cycle and the position within an instruction cycle that physical interrupt signal  116  is received. This is demonstrated by reference to second exemplary command stack  112 . In this example, the microcontroller receives a physical SOF interrupt signal  126  at some point during instruction  128  (which is shown in this figure as a single cycle instruction). After completion of instruction  128 , microcontroller  86  executes ISR  130 . ISR  130  is generally a series of instructions but has been represented as one instruction here for simplicity. ISR  130  responds to interrupt signal  126  and generates software interrupt output signal  132 . There is a time delay t 2  between reception of the physical interrupt signal  126  and the outputting of software interrupt output signal  132 . 
     Hence, there is uncertainty—corresponding in magnitude to these time delays t 1  and delay t 2 —in determining the exact reception time of a SOF packet. This uncertainty depends on the duration of the currently executing instruction and the position within that instruction at which a physical SOF interrupt signal is received by the hardware interrupt generator. This is typically given by the period of the longest instruction on a given microcontroller. The best case (i.e. minimum) uncertainty is therefore given by the single cycle instruction cycle, of length Δt. 
       FIG. 5  is a schematic timing diagram  150  for the exemplary command execution stacks  110 ,  112  of  FIG. 4 .  FIG. 5  includes the reception  152  of a SOF packet and timing diagrams  154  and  156  corresponding to command stacks  110  and  112  of  FIG. 4  respectively. Reception  152  of the SOF packet results in a hardware interrupt signal  158  occurring during instruction  160 . At the completion of instruction  160 , interrupt service routine  162  generates local reference signal  164  which is used as the repetitive synchronization reference signal. Similarly hardware interrupt signal  158  occurs during instruction  166  (corresponding to instruction  128  in command stack  112  of  FIG. 4 ). Interrupt service routine  168  then generates local reference signal  170 . 
     Some microcontrollers have instruction cycle times that range from single instruction cycles to ten or more instruction cycles for complex operations. There is thus significant variability in the uncertainty of any given determination of SOF reception time. If the structure of the software is known, the limits can be determined by the longest instruction execution time and structure of the code. Given the asynchronous nature of the SOF rate and microcontroller code execution there is a random distribution of accurate and inaccurate determinations of SOF reception time. 
     However it is possible to determine the accuracy of any given determination of SOF reception time, as it is possible to determine which instruction was executing during reception of the SOF and therefore the length of the instruction and of the resulting uncertainty of measurement. This allows any measurements of SOF reception time that are highly uncertain to be identified and hence discarded. Alternatively some form of statistical weighting can be applied to determinations of SOF reception time based on the length of instruction that was executing during its reception, to reduce reliance on measurements that rely on lengthy instructions. The statistical weighting can determine how each measurement is used in controlling the phase locked loop synthesized clock of a synchronized USB device, and only the most reliable determinations of SOF reception time used in controlling the synchronized clock circuitry  90 . Furthermore the use of filtering, averaging or other statistical means can be used to improve the accuracy and stability of the synchronized clock. 
     There is a fixed latency  172  associated with each measurement of SOF reception time. This is the time taken to execute interrupt service routine  162  or  168  respectively. This is constant for each class of microcontroller (assuming identical constant clock frequency and zero batch to batch inconsistencies). 
     In this way, the present invention provides a means of synchronizing the local clock of a USB device in both frequency and phase, to a repetitive reference carrier signal contained in the USB data stream without using special hardware components that might, for example, add additional parallel capacitive loading to the bus. Additionally, the present invention provides a means of decoding data from the bus and generating a software interrupt based synchronizing reference signal. Such interrupt based reference signals do not require the highly accurate temporal resolution of prior art hardware based synchronization disclosures. 
     Furthermore the present invention provides a method for estimating the uncertainty of timing accuracy of any given interrupt based synchronizing reference signal and utilizes filtering or statistical means to improve the accuracy and stability of said synchronized clock. 
     A USB device according to a second embodiment of the present invention is shown schematically at  200  in  FIG. 6 , with a USB  202 . USB device  200  includes a bus connector  204 , with which USB device  200  is attached to USB  202 . USB device  200  has a bus interface/microcontroller  206 , USB device function circuitry (such as a digitally controlled transducer)  208 , bus sampling circuitry  210  (comparable to bus sampling circuitry  20  of  FIG. 1 ) and synchronization circuitry  212 . 
     Bus interface circuitry  206  observes the USB data stream present at bus connector  204  and passes a replica  214  of the USB traffic  216  to synchronization circuitry  212 . Synchronization circuitry  212  generates synchronous local clock signal  218  which is passed to USB device function circuitry  208 , which uses it to synchronously execute commands  220  from bus interface/microcontroller  206 . Synchronization circuitry  212  decodes periodic clock carrier signal from USB traffic  216 . 
       FIG. 7  is a schematic diagram of synchronization circuitry  212  of the USB device  200  of  FIG. 6 . Synchronization circuitry  212  has an input port  232  for receiving replica  214  of the USB data traffic, a synchronous clock output port  234 , a matched filter  236 , a multi-tap free running oscillator clock  238  and a clock selector  240 . 
     Matched filter  236  receives a USB data stream  242  from input port  232 . Matched filter  236  decodes the periodic carrier signals (in the form, in this embodiment, of SOF packet tokens) from USB data stream  242  and generates clock synchronizing signal  244 . Multi-tap free running oscillator  238  (which may be in the form of free running oscillator coupled to a multi-tap phase delay generator) generates a plurality of phase delayed clock signals  246 . Clock selector  240  uses clock synchronizing signal  244  to select output clock signal  248 , being a selected one of plurality of phase delayed clock signals  246 . It will be appreciated by those skilled in the art that there are many ways of utilizing an inexpensive free running oscillator to generate a plurality of phase delayed cock signals. 
     In this way synchronization circuitry  230  uses an inexpensive free running oscillator and clock selector circuitry to control the output clock signal phase in a digital control loop to maintain course synchronization. Such a system will generally be less stable and less accurate than a conventional analog phase locked loop architecture (which provides precision phase and frequency control), but this method provides a relatively inexpensive alternative synchronization system because it employs a series of steps (embodied as software or firmware operating in a field programmable gate array) that determines the most appropriate of a plurality of free running clocks to be used at any given time. 
     In order to more fully explain the operation of synchronization circuitry  230 ,  FIG. 8  provides a schematic diagram  260  of the clock and control signals within synchronization circuitry  212  of  FIG. 7 . 
     Decoded Start of Frame signal  262  (comparable to  244  of  FIG. 7 ) provides the reference signal to which a local synchronized clock must be synchronized. A perfectly synchronized ideal clock  264  is shown for reference along with two phases, first clock phase  266  and second clock phase  268  of local free running oscillator (comparable to two of the plurality of clock phases  246  of  FIG. 7 ). Output clock  270  (comparable to output clock signal  248  of  FIG. 7 ) is the synchronized clock signal (comparable to  218  of  FIG. 6 ) that is used to control the synchronized USB device. 
     Start of Frame decoded pulse  272  constitutes the beginning and decoded pulse  274  constitutes the end of a synchronization window. The ideal clock  264  is synchronized in frequency and phase such that the leading edge of clock pulse  276  is aligned with decoded pulse  272  and the leading edge of clock pulse  278  is aligned with decoded pulse  274 . (The leading edge of the clock pulse may in fact be any repeatable point of the waveform, but the leading edge is chosen for simplicity in this discussion). 
     The free running oscillator is not operated at a precisely controlled frequency and phase, but rather is allowed to vary in frequency across its entire range. 
     First clock phase  266  represents one phase of multiphase clock  237  of  FIG. 7  and is synchronised  280  with decoded pulse  272 . It will be apparent from a comparison of first clock phase  266  and ideal clock  264  that the free running clock is running slower than the ideal clock frequency. This is indicated by phase lag  282 , which first clock phase  266  exhibits with respect to ideal clock  264  after several clock cycles (exaggerated in this view for clarity). Although first clock phase  266  is synchronized with decoded pulse  272 , by the time the next decoded pulse  274  is received, first clock phase  266  is out of phase as shown by clock pulse  284 . 
     Second clock phase  268  is not synchronized  286  (or in phase) with decoded SOF  272  but, owing to the unsynchronized frequency of local free running oscillator clock  238  of  FIG. 7 , second clock phase  268  is in phase  288  with decoded SOF  274 . 
     Clock selector  240  of  FIG. 7  acts as a digital switch to route one of the plurality of phase delayed clock signals  246  to output port  234  of  FIG. 7 . Clock selector  240  of  FIG. 7  switches synchronously with reception of decoded SOF pulse, selecting the most appropriate one (i.e. most accurately aligned) of the plurality of phase delayed clock signals  246 . 
     In the example of  FIG. 8 , clock selector circuitry  240  of  FIG. 7  has selected first clock phase  266  at SOF pulse  272  and first clock phase  266  continues to be transmitted as output clock  270  until reception of next decoded SOF pulse  274 . Second clock phase  268  is most accurately in phase (viz. clock pulse  288 ) of the plurality of phase delayed clock signals  246  at the moment clock selector circuitry  240  of  FIG. 7  receives decoded SOF signal  274 . Therefore second clock phase  268  continues to be transmitted as output clock  270  until reception of next decoded SOF pulse. The final clock cycle  294  of output clock  270  prior to reception of decoded SOF  274  is of a different duration to the rest of the output clock cycles. Given the rate of reception of decoded SOF packets and a reasonably selected frequency tolerance for the free running oscillator clock, final clock cycle  294  is only in error by a small fraction of a clock cycle. In this way, the free running oscillator clock is phase adjusted in a digital fashion every time a new decoded SOF is received. 
     The typical frequency tolerance of free running oscillators is of the order of 50 parts per million to 100 parts per million. A 50 parts per million tolerance for a typical 10 MHz data acquisition oscillator corresponds to a worst possible frequency error of 500 cycles per second. In a USB High Speed system, SOF packets are received at a rate of 8 kHz (or with a period of 125 μs). This means that a maximum of 500 cycles of clock error occur in each 125 μs SOF period, or 1/16 of a cycle. One sixteenth of a cycle at 10 MHz corresponds to a worst case phase error of 6.25 ns. Cumulative phase error of this magnitude in successive SOF periods is unacceptable for data acquisition applications. However according to the second embodiment of the present invention digital correction of this magnitude of phase error in each SOF period provides an acceptable data acquisition clock. 
     In this way, the simple digital phase only control loop is used with an inexpensive free running oscillator clock to maintain an output clock frequency that is synchronized with repetitive incoming clock carrier signal from a USB within some acceptable bounds. Furthermore a plurality of USB devices each disposed with similar circuitry forms a synchronized USB. 
     Modifications within the scope of the invention may be readily effected by those skilled in the art. It is to be understood, therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove and that combinations of the various embodiments described herein are readily apparent to those skilled in the art. 
     In the preceding description of the invention, except where the context requires otherwise owing to express language or necessary implication, the words “Host Controller” are used to refer to a standard USB Host controller, a USB-on-the-go Host Controller, a wireless USB Host Controller or any other form of USB Host Controller. 
     In the preceding description of the invention, except where the context requires otherwise owing to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, that is, to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 
     Further, any reference herein to prior art is not intended to imply that such prior art forms or formed a part of the common general knowledge.