CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Stage of International Application No. PCT/GB2018/053593, filed Dec. 11, 2018, which was published in English under PCT Article 21(2), which in turn claims the benefit of Great Britain Application No. 1720596.4, filed Dec. 11, 2017.

TECHNICAL FIELD

The present invention relates to transferring system timing signals over asynchronous interfaces, particularly in electronic devices arranged to carry out radio communications, for example Long Term Evolution (LTE) communications, in particular for Internet of Things applications.

BACKGROUND

Throughout the course of the past few decades, the extent and technical capabilities of cellular-based radio communication systems have expanded dramatically. A number of different cellular-based networks have been developed over the years, including the Global System for Mobile Communications (GSM), General Packet Radio Services (GPRS), Enhanced Data rates for GSM Evolution (EDGE), and Universal Mobile Telecommunications System (UMTS), where GSM, GPRS, and EDGE are often referred to as second generation (or “2G”) networks and UMTS is referred to as a third generation (or “3G”) network.

More recently, the Long Term Evolution (LTE) network, a fourth generation (or “4G”) network standard specified by the 3rdGeneration Partnership Project (3GPP), has gained popularity due to its relatively high uplink and downlink speeds and larger network capacity compared to earlier 2G and 3G networks. More accurately, LTE is the access part of the Evolved Packet System (EPS), a purely Internet Protocol (IP) based communication technology in which both real-time services (e.g. voice) and data services are carried by the IP protocol. The air interface of LTE is often referred to as Evolved UMTS Terrestrial Radio Access (or “E-UTRA”).

However, while “classic” LTE connections are becoming increasingly prevalent in the telecommunications industry, further developments to the communication standard are being made in order to facilitate the so-called “Internet of Things” (IoT), a common name for the inter-networking of physical devices, sometimes called “smart devices”, providing physical objects that may not have been connected to any network in the past with the ability to communicate with other physical and/or virtual objects. Such smart devices include: vehicles; buildings; household appliances, lighting, and heating (e.g. for home automation); and medical devices. Such smart devices are typically real-world objects with embedded electronics, software, sensors, actuators, and network connectivity, thus allowing them to collect, share, and act upon data. These devices may communicate with user devices (e.g. interfacing with a user's smartphone) and/or with other smart devices, thus providing “machine-to-machine” (or “machine type”) communication. However, the development of the LTE standards makes it more practical for them to connect directly to the cellular network.

3GPP have specified two versions of LTE for such purposes in Release 13 of the LTE standard. The first of these is called “NarrowBand IoT” (NB-IoT), sometimes referred to as “LTE Cat NB1”, and the second is called “enhanced Machine Type Communication” (eMTC), sometimes referred to as “LTE Cat M1”. It is envisaged that the number of devices that utilise at least one of these standards for IoT purposes will grow dramatically in the near future.

From a communications perspective, LTE standards (including NB-IoT and eMTC) use orthogonal frequency division multiple access (OFDMA) as the basis for allocating network resources. This allows the available bandwidth between to be shared between user equipment (UE) that accesses the network in a given cell, provided by a base station, referred to in LTE as an “enhanced node B”, “eNodeB”, or simply “eNB”. OFDMA is a multi-user variant of orthogonal division multiplexing (OFDM), a multiplexing scheme in which the total bandwidth is divided into a number of non-overlapping sub-bands, each having its own sub-carrier frequency. In OFDM, unlike other frequency division multiplexing (FDM) schemes, each of these sub-carriers are orthogonal to one another such that cross-talk between sub-bands is ideally eliminated and removing the need for inter-carrier guard bands.

Within an electronic device, there are typically several entities (e.g. subsystems of the device) that are not necessarily synchronised to one another. In other words, each entity has its own internal timer, which is usually implemented using a counter. Each of these counter-based timers is arranged to try and keep track of the system time. In LTE, this system time is a clock that runs at 30.72 MHz. The different entities are separated by an asynchronous interface, for example a power domain crossing and/or a clock domain crossing.

In order to attempt to ensure that both entities have the same internal reference to the system time, typically synchronisation signals are passed from one entity to the other. For example, in a radio receiver, one entity might be a radio frequency (RF) front-end circuit portion and another entity might be a baseband (BB) circuit portion, where both the RF and BB circuit portions have their own internal clocks. The RF front-end is typically arranged to send a timing event signal to the BB circuit portion via a synchroniser such that the BB circuit portion can capture the timing event and set its own counter-based timer to the same value as the timer of the RF circuit portion.

However, the Applicant has appreciated that passing signals over an asynchronous interface introduces uncertainty which is not desirable. The degree of uncertainty depends on, inter alia, the clock rates of the entities that transmit and receive the timing event signal and on the specific synchroniser being used.

SUMMARY OF THE INVENTION

When viewed from a first aspect, the present invention provides an electronic device comprising:a first circuit portion comprising one or more components, including a first counter, which are clocked by a first clock signal, wherein the first clock signal has a first frequency, said first circuit portion being arranged to receive a data stream comprising a plurality of data signals; anda second circuit portion comprising one or more components clocked by a second clock signal having a second frequency and a second counter not clocked by the second clock signal, wherein the first clock signal is not synchronised to the second clock signal;wherein the second circuit portion is arranged to:receive samples of the data stream from the first circuit portion at a sample rate;time-stamp each received sample with a count value of the second counter; and increment the count value of the second counter by a predetermined increment value for each received sample from the data stream.

This first aspect of the invention extends to a method of operating an electronic device which comprises:a first circuit portion comprising one or more first circuit components, said one or more first circuit components including a first counter; anda second circuit portion comprising one or more second circuit components and a second counter;
the method comprising:clocking the first circuit components by a first clock signal, wherein the first clock signal has a first frequency, clocking the second circuit components but not the second counter by a second clock signal having a second frequency, wherein the first clock signal is not synchronised to the second clock signal;said first circuit portion being receiving a data stream comprising a plurality of data signals;said second circuit portion receiving samples of the data stream from the first circuit portion at a sample rate;time-stamping each received sample with a count value of the second counter; andincrementing the count value of the second counter by a predetermined increment value for each received sample from the data stream.

Thus it will be appreciated by those skilled in the art that embodiments of the present invention provide an electronic device with two asynchronous circuit portions, representing two different clock domains, where the second circuit portion can maintain certainty over its timing without the need for dedicated synchronisation signals from the first circuit portion. Instead the second circuit portion uses the arrival time of the samples to keep track of the system time and to time-stamp the received samples.

In accordance with this elegant approach, the second counter may therefore maintain its previous value until the second circuit portion receives and time-stamps a sample from the first circuit portion, at which time it may increment its count value by the increment value. This leads to the second counter having ‘coarser’ timing than the first counter, however this is not of detriment where, as is preferred, the second counter's count value is only used by the second circuit portion for time-stamping the samples it receives, and thus the timings correspond accurately to those in the first circuit portion.

The acquisition and maintenance of the timing within the second circuit portion is agnostic to any variations in transfer latency that are introduced by the asynchronous interface between the first and second circuit portions. Any operations, for example receiver operations including sampling buffer switches, or starting and stopping the sampling process, may then be timed based on the count values of the second counter.

The predetermined increment value by which the second counter increments its count value could be any suitable count value, and those skilled in the art will appreciate that the choice of predetermined increment value will simply determine the conversion factor(s) that should be applied in order to ‘translate’ the time-stamp for a given sample into the time base of the first circuit portion, second circuit portion, and/or system time.

In some embodiments, however, the predetermined increment value is a ratio between the second clock frequency and the sample rate. In other words, dividing the second clock frequency by the sample rate yields the number of ticks of the second clock that occur during a single sample period and thus causes the second counter to ‘jump’ to the count value that it would have reached had it been running normally, clocked by a clock signal synchronised to the first clock signal. Thus, in accordance with such embodiments, the second circuit portion increments its own counter based on incoming samples by making use of the known second clock frequency and the known sampling rate, which may also provide the advantage that there is no need for the second clock frequency to be the same as the first clock frequency.

Typically, the second clock frequency is not equal to the sample rate and thus, in at least some such embodiments, the ratio is not 1:1. For example, in at least some embodiments, the second clock frequency is greater than the sample rate such that the ratio is greater than 1:1.

The time-stamps applied by the second counter may still give accurate values measured in the second clock but synchronised to the first clock. The time-stamps produced by the second circuit portion are, in such embodiments, in the time base of the second clock domain, however these can be translated into the time base of the first circuit portion by multiplying the time-stamps by a conversion factor equal to a ratio between the first clock frequency and the second clock frequency.

In an alternative set of embodiments, the predetermined increment value is a ratio between the first clock frequency and the sample rate. In other words, dividing the first clock frequency by the sample rate yields the number of ticks of the first clock that occur during a single sample period. Incrementing the second counter by this value may advantageously mean that the time-stamps provided by the second circuit portion (using the count value of the second counter) are in the time base of the first circuit portion, without needing to transfer any information regarding the first clock signal across the asynchronous interface between the respective clock domains of the first and second circuit portions. These time-stamps can be translated from the time base of the first circuit portion into the time base of the second circuit portion by multiplying the time-stamps by a conversion factor equal to a ratio between the second clock frequency and the first clock frequency.

Typically, the first clock frequency is not equal to the sample rate and thus, in at least some such embodiments, the ratio is not 1:1. For example, in at least some embodiments, the first clock frequency is greater than the sample rate such that the ratio is greater than 1:1.

In some embodiments, the electronic device comprises memory having a plurality of memory locations, wherein the second circuit portion is arranged to store each incoming sample from the data stream into the memory such that each sample is stored at the next consecutive memory location. Those skilled in the art will appreciate that the term ‘next consecutive memory location’ as used herein means the next memory location in the order in which memory locations are accessed from the beginning to the end of the available memory locations; while this is typically the next physical memory location, in some memory arrangements there may be a logical mapping of memory locations that is not necessarily commensurate with the physical order of the memory locations. By storing the samples in this way, the storage address used for any given sample are directly related to a specific count value of the second counter.

While the memory may be of any type suitable for the storage of the samples, in a set of embodiments the device further comprises a processing circuit portion that comprises the memory and a processor arranged to access said memory. Those skilled in the art will appreciate that, this may be implemented using memory based on the same chip as the processor, often referred to as ‘tightly coupled memory’ (TCM).

It will be appreciated that the principle of the present invention may readily be applied to any type of electronic device having an asynchronous interface between different circuit portions. However, the Applicant has appreciated that the principles of the present invention are particularly applicable to radio receiver devices and in some embodiments, the first circuit portion comprises a radio frequency circuit portion; and the second circuit portion comprises a baseband circuit portion; wherein the data stream is carried by one or more radio frequency signals that are received by the radio frequency circuit portion; and wherein the radio frequency circuit portion is arranged to convert the one or more radio frequency signals to baseband signals comprising the data stream and to pass samples of said baseband signals to the baseband circuit portion.

Such a radio receiver device may be beneficial for the reception of radio signals complying to various different protocols known in the art per se. However, in a preferred set of such embodiments, the radio frequency signals are LTE signals. When operating in accordance with the LTE specification, the system frequency is dependent on the bandwidth in use, where LTE provides bandwidth options of 1.4, 3, 5, 10, and 20 MHz, having sampling rates and thus system frequencies of 1.92, 3.84, 7.68, 15.36, 23.04, and 30.72 MHz respectively.

In some embodiments, the electronic device is arranged such that, on initialisation, the second counter is set to an initialisation value. This initialisation value may be a default, i.e. predetermined, value. For example, in some embodiments, the initialisation value is zero. In such embodiments, all of the time-stamps will be relative to the time at which the second counter was initialised and thus there will be an offset between the stored time-stamps and the system time.

However, in some alternative embodiments, the initialisation value is set to a system count value on initialisation. This may, by way of example, be achieved using a handshaking procedure. Thus, in accordance with such embodiments, there may be no offset between the stored time-stamps and the system time. Once reception of the samples begins, the second counter is incremented by the increment value for every received sample.

In some embodiments, the second circuit portion is arranged to receive the data stream from the first circuit portion via a synchroniser. This synchroniser acts as a bridge over which data can be passed between the two asynchronous clock domains, i.e. over the asynchronous interface, without the clock domains themselves needing to be synchronised.

While it will be appreciated that there are a number of different types of synchroniser known in the art per se, for example handshaking-based synchronisers or synchronisers that use two or more flip-flops in series. However, in some embodiments, the synchroniser is arranged to receive the samples from the first circuit portion, wherein an input of the synchroniser is clocked by the first clock signal and an output of the synchroniser is clocked by the second clock signal. In some such embodiments the synchroniser comprises a first-in-first-out synchroniser. Those skilled in the art will appreciate that a first-in-first-out (FIFO) synchroniser acts as a buffer where the incoming samples are added to the back of the queue by the first circuit portion at the first clock frequency and are taken from the front of the queue by the second circuit portion at the second clock frequency.

In a set of embodiments, the first and second clock frequencies are equal. In such embodiments, incrementing the count value of the second counter by the increment value, provides a direct correspondence between the first and second counters because they have the same time base. In other words, the time-stamps according to the count value of the second counter directly match the time of the first counter. However, the applicant has appreciated that the two counters could have different time bases and a conversion factor may be used to relate timings in one clock domain to timings in the other clock domain as described hereinabove. Thus, in an alternative set of embodiments, the first clock frequency is different to the second clock frequency.

