Abstract:
A digital communications link, protocol and related circuits are provided which achieve a scaleable performance rate through a combination of clock scaling and/or variable frame sizing. The system is used within a personal computer, thus allowing the latter to be interoperable with any number of different communications protocols, including xDSL based transmission standards, and to set up communications links of varying capacity and performance.

Description:
This application is a continuation of U.S. application Ser. No. 09/255,235, filed Feb. 22, 1999, now U.S. Pat. No. 6,345,072. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to a digital subscriber line (DSL)-link interface and protocol for a DSL modem. The present invention has particular applicability to personal computer (PC) systems where the digital control and analog front end portions of such a DSL modem are not implemented in a single integrated circuit (IC) for various technical and cost-effective reasons. 
     BACKGROUND OF THE INVENTION 
     It has been consistent in the electronics industry that complex logic functions are integrated into smaller and smaller form factors. For instance, a popular growing trend is to combine audio and telephone capabilities in one device, including graphic and video functions in a computer chipset, and to integrate all computer peripheral functions into a single IO chip. A related general trend in the PC industry is the migration of more and more devices, components, functionality, etc. to the motherboard. This makes sense for a variety of engineering and cost reasons, including the fact that this approach conserves available conventional expansion slots on the system bus, is easier to configure, reduces the total mother board form factor and BOM cost, and preserves compatibility with legacy systems. 
     At the same time, there are certain components that are not optimally combined or tightly integrated together for a variety of reasons, some of which are engineering related, and some of which are regulatory related. As noted above there is considerable interest in placing a full scale modem down (directly on) the motherboard, to obtain the aforementioned benefits. From an engineering perspective, however, it is difficult to place the analog I/O portions of a modem directly on a motherboard because of various noise sources present on the latter. Therefore, the signal to noise ratio (SNR) performance of the modem system will decrease. Similarly, from a regulatory perspective, these analog portions of a modem must undergo a certification process in accordance with local applicable telecom rules and regulations before they can be implemented in a commercial product. If it were necessary to subject every motherboard to a new certification process (simply because it now incorporated some modem front end circuitry), this would impede and slow down the development of new products considerably. 
     Thus, there is a general perception in the industry at this time that, at least for the PC environment, it makes more sense to separate the analog and digital portions of a high speed modem. This has led to a proposal, authored by Intel Corporation, known as the Audio/Modem Riser Specification, in which it is suggested that the analog I/O portions of a modem be moved to a card that is physically separated but easily connected to a motherboard, Version 1.1 of this document, revised on Sep. 10, 1998, is available for downloading via the Internet at Intel&#39;s corporate web site, and is hereby incorporated by reference. In this specification, a portion of which is reproduced in FIG. 1A, it is proposed that the portions of the modem which are susceptible to noise, and which implicate certification issues, be moved to what is known as a Homologated Riser Card. This card has I/O connections to the outside world (i.e., such as to a conventional telephone line) but at the same time fits in a riser connector directly to the motherboard. As the specification indicates, this riser interface is preferably standardized, so the motherboard makers can implement audio/modem capability with lower BOM costs, and with fewer concerns about vendor proprietary variations (i.e., because there is a common footprint that can be stuffed with competing pin compatible implementations). The specification provides a detailed and comprehensive explanation of the electrical and physical requirements required to comply with the proposed standard. 
     As depicted in FIG. 1A, a modem  100  includes an analog I/O portion  110  located on a riser card  111 . This analog I/O portion (sometimes referred to as a codec) is then interfaced through a digital link  120  (known as AC-97) to the digital controller  130  of the modem. The latter digital portion is located directly on a motherboard  131  and generally includes appropriate modem control circuitry, data pumps, and other functional circuits typically associated with those aspects of a modem. These functions can also be performed, for example, by a so-called host signal processing (HSP) embodiment, in which case the AC 97 digital controller (at least for the modem portions anyway) can consist of as little as a very simple bus interface. The preferred implementation for this digital portion is described in another specification published by Intel known as Audio Codec 97. Version 2.1 of this document, published on May 22, 1998, is likewise available for downloading at Intel&#39;s corporate web site, and is also hereby incorporated by reference. As noted therein, the AC97 digital controller can include a wide range of capabilities, including multiple digital audio channels, and suitable interfaces to a PCI bus, a digital I 2 S bus, etc. 
     A description of the various I/O control, and power/ground signals in the AC-97 link passing through the riser interface is provided at Tables 1-5 of the aforementioned A/M Riser specification. As seen generally in FIG. 1B, in a voice band modem implementation, the AC 97 link between the digital controller  130  and the analog CODEC  110  consists of a five wire, bi-directional, serial TDM format interface. The signals supported, include the following: RESET (a master hardware reset), BITCLK (12.288 MHz serial data clock), SYNC (48 KHz fixed rate sample sync), SDATA_OUT (serial, time division multiplexed input data stream to the PC), and SDATA_IN (serial, time division multiplexed output data stream from the PC). The format and nature of these signals is well-known, and is described in the aforementioned specifications. Digital controller  130  can also including suitable interfaces for other contemporary expansion busses, such as PCI bus  141 , USB  142 , etc. 
     While the A/M Riser specification is a step in the right direction, it does not provide for functionality that would permit implementation of an xDSL type modem at this time. For example, xDSL requires a much higher bit rate than that provided by the AC97 link (at a 2.2 MHz sampling and 16 bits wide for an ADSL downstream signal, the total bit rate is 35.2 Mb/s, which is much higher than the 12.288 MHz serial clock). Furthermore, there are various power and control signaling requirements in xDSL that were not foreseen by AC97. Therefore, the AC97 and A/M Riser specification are limited to supporting at most simple analog type (V.90 and the like) modems. This is a significant drawback, since xDSL technologies (and especially ADSL) are expected to be in widespread use in a very short time. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to overcome the aforementioned problems. 
     Another object of the present invention, is to provide a DSL modem which is configurable as a motherboard device within a personal computing system, in a manner similar to that only currently possible for voice band type modems; 
     A related object of the present invention is to provide a DSL modem that has analog codec circuitry and digital controller circuitry both tightly coupled to a PC motherboard, in a manner similar to that only current provided for voice band type modems; 
     Another object of the present invention is to provide a DSL interface that can properly handle DSL data rates, DSL control signals, and other xDSL related signal requirements so that it is suitable for linking the analog and digital portions of a DSL modem of the aforementioned type; 
     A related object of the present invention is to provide a DSL interface that is extremely flexible, and can be used in any one of a number of different xDSL technologies, including HDSL, SDSL, ADSL, reduced rate ADSL, etc.; 
     Another object of the present invention is to provide an improved signal protocol for a DSL link, having data/control word formatting and timing specifications especially suited for a DSL data stream; 
     A further object of the present invention is to provide a DSL interface that minimizes the required signal and power pins associated with a standardized specification for analog codec components in a personal computing system, and is therefore cost effective, and can be easily integrated by system vendors; 
     Yet a further object of the present invention, is to provide a DSL digital controller and a DSL codes which can utilize the above novel DSL interface; 
     An additional object of the present invention, is to provide a personal computer motherboard which is adapted to use the aforementioned DSL modem and DSL interface. 
     Another object of the present invention is to minimize the required logic complexity in order to support all above functions and features. 
     These and other objects are effectuated by the following aspects of the present invention. 
     An improved digital communications link of the present invention connects a digital controller section of an xDSL modem—which is preferably located on a system motherboard of a computing system—to a separate analog section of the xDSL modem—which is located at a position substantially free of electronic noise from other electronic components on said motherboard, which could materially affect the operation of such analog section. The data path/link is generally configured in the following manner: (a) a plurality of receive signal lines are set up for receiving data from a remote site; (b) a plurality of transmit signal lines are designated for transmitting data to remote site; (c) a bit clock signal line is set up for carrying a clock signal, which clock signal is used in connection with communicating the data to and from the remote site. The bit clock signal line can carry any desired clock signal needed according to data transmission requirements of said digital communications link, thus providing a scalable interface that is easily adaptable for use in any number of different motherboard environments. 
     An additional word clock signal, derived from the bit clock signal, marks the boundary for each sample word received or transmitted on said plurality of receive signal lines and/or plurality of transmit signal lines respectively. 
     Another aspect of the present invention includes the use of an improved protocol for exchanging operational and/or control information between separated digital and analog sections of an xDSL modem. This operational and/or control information preferably includes information relating to real time control settings for circuits located within the analog codec, power management, and the like. 
     During normal operation, the aforementioned receive and/or transmit signal lines can also be used for carrying control words for use by the xDSL modem, which control words are embedded as part of the normal data stream. As a compromise between flexibility and speed, these control words preferably have either a first or second length. The control word has a start bit, followed by a length indicator bit, and then the set of command bits corresponding to the actual command/data value. Idle bits are transmitted continuously between control words. Unlike prior art schemes, the control words are transmitted asynchronously, therefore, in an embedded operations channel (EOC) that has an effective rate equal to the word clock signal. 
     According to another useful feature of the present invention, the aforementioned link also includes provision for setting up a multi-channel data frame. In this way, two or more data channels can be set up between the digital controller and analog section, with data transferred through a first channel during a first time period of the multi-channel data frame, and through a second channel during a second time period of the multi-channel data frame. To accomplish this, the word clock signal can be adapted to have a different duration for different number of data channels. In one variation, the data channels can have different transmit and/or receive data rates, so as to accommodate multiple codecs, or unequal receive/transmit data stream bit rates (such as ADSL). Accordingly, the single channel limitation of the prior art can be also be overcome by this capability of the present invention. 
     Yet another useful aspect of the present invention provides that, due to the unique selection of receive/transmit/clock lines and signal protocols, the same receive and/or transmit signal lines can also be used to support a compatible data interface between the digital controller and an ATM based hardware xDSL modem. In other words, the data path can handle ATM packaged data and clocking information, making it extremely versatile and useful for incorporation within a motherboard application. 
     These aspects of the present invention thus permit an xDSL motherboard, and an xDSL modem to be configured in a manner similar to a voice-band modem. 
     A computer motherboard, DSL digital controller, DSL codec, and DSL modem of the present invention include otherwise conventional hardware (and/or software in an HSP application) and are all implemented with appropriate interface circuitry and control logic necessary to effectuate the above architecture, characteristics and functionality of the aforementioned DSL link. 
     Although the inventions are described below in a preferred embodiment associated with a personal computing system, it will be apparent to those skilled in the art the present invention would be beneficially used in any environment where it is desirable to link separated analog/digital portions of an xDSL modem. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a diagram illustrating how an analog portion of a prior art voice band grade modem is mechanically placed in a riser card and electrically connected to a digital portion of such modem integrated into a computer motherboard in a personal computing system 
     FIG. 1B is an electrical block diagram depicting the various signals carried by a prior art digital link for a voice band grade modem integrated as part of a motherboard in a personal computer system; 
     FIG. 2A is a functional block diagram of a preferred embodiment of a DSL digital link, DSL modem, and motherboard of the present invention; 
     FIG. 2B is a more detailed functional block diagram of the portions of the circuitry illustrated in FIG. 2A; 
     FIGS. 3A-3C are preferred embodiments of timing specifications, data formatting, and control signal protocols used in the DSL link of the present invention; 
     FIG. 4 is a preferred embodiment of a signal and timing protocol that can be used by the DSL link of the present invention to support an ATM interface. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     DSL Modem System Overview 
     A preferred embodiment of the present invention is depicted generally in FIGS. 2A and 2B. This embodiment is similar in some respects to the prior art voice band modem implementation shown in FIG. 1, and, for that reason, like components/circuits are designated with like numerals. In FIG. 2A, an xDSL modem  200  as shown consists of two physically separated (but electrically coupled) circuitry sections, a DSL Modem Analog Circuit  205 , and a DSL Modem Digital Circuit  230 . Except for those portions necessary for implementing the xDSL digital link of the present invention, the specifics of these two circuitry sections are well known in the art, and any suitable hardware/logic/firmware combination could be used for effectuating the typical functions of an xDSL modem. For example, DSL Modem Analog Circuit  205  is typically a combination of a codec and various analog components necessary to support an xDSL link. It will be apparent to those skilled in the art that portions of DSL Modem Analog Circuit  205 , as well portions (or even all) of DSL Modem Digital Circuit  230  may be implemented in single chip (IC) form either alone or in combination with a larger functional device. With reference to FIG. 2A again, DSL Modem Digital Circuit  230  includes circuitry for performing some of the necessary signal processing associated with an xDSL link, data buffering, and host interface functions. Additional control logic sets up and maintains the xDSL compatible data link by responding to appropriate commands from the host. It also receives hardware status such as power loss detection from the DSL Analog Circuit  205  via the DSL link and passes this information forward to the host for necessary actions. 
     Various software drivers associated with the operating system can interact with DSL Modem Digital Circuit  230  through various available system busses such as a PCI Bus to ensure proper data flow between such circuit and applications running on a host system. In a typical PC system, the functions performed by DSL Modem Digital Circuit  230  is embodied as part of a PCI Bus Controller chipset, such as those known in the art as either South Bridge or North Bridge. The purpose of this integration is to increase system integration flexibility and to reduce BOM costs. Thus, for purposes of the present discussion, DSL Modem Digital Circuit  230  is also generally referred to herein as a DSL Digital IC, and this IC can be integrated alone as a standalone device, or, as noted immediately above, as part of a PCI North/South Bridge IC. In additional to DSL link, this overall chip may contain additional various interfaces such as a PCI bus  241 , USB bus  242 , and AC-Link  243 . Furthermore, it may generate additional control signals such Wake Up to the host for various modem operation needs. Other system controller functions (CPU, memory and PCI agent related) normally performed by such PCI Bus Controller chipset can be implemented in any conventional, well-known fashion. 
     In general, the hardware, firmware, software, and logic for performing the above functions in DSL Modem Digital Circuit  230  is dependent on the functions performed in DSL Modem Analogy Circuit  205 . For example, when oversampling is used in A/D and D/A , the digital filters for interpolation and decimation filters can be done on either side of the DSL link. Therefore, for purposes of the present invention, the actual implementation of DSL Modem Digital Circuit  230  and Digital Modem Analog Circuit  205  can be accomplished through a number of variations that are compatible with the teachings herein. In many cases, as just described in connection with the filtering function, various signal processing or logic operations can be effectuated on either side of the link with no loss of performance or flexibility. The present invention, in fact, specifies a universal DSL link interface that allows a very flexible implementation for the two blocks and for future DSL technologies. 
     DSL Analog Section Overview 
     FIG. 2B provides a more detailed electrical schematic illustration of the components shown generally in FIG.  2 A. As shown, a DSL Analog Modem Circuit  205  is connected to a phone line through a hybrid circuit  207  and processes the received signal through a receive amplifier  209 , filter  211 , A/D  213 , and, in embodiments where filtering is implemented on this side of the modem, a post digital filter  214 . A ring detect circuit  212  is used to detect a remote wake up signal, which is some pre-designated xDSL signal. This wake up signal is passed on where it used, for example, to re-activate portions of the DSL modem and a host PC incorporating the same. A series of transmission counterpart stages are also shown in FIG. 2B, including a digital filter  214 ′ (optional as noted above), D/A circuit  213 ′, and a transmit amplifier and driver stage  211 ′, for generating the xDSL analog transmit signal to the phone line through the hybrid. Also associated with DSL Analog Modem Circuit  205  is a DSL link interface block DSL-A  216 , which transmits signals in the DSL link to DSL Digital Modem Circuit  230 , and converts received signals in the DSL link to various data and control signals for the internal circuits within DSL Analog Modem Circuit  205 , including control registers  215 . Also inside DSL-A  216  is a clock circuit (not shown) which generates the necessary clocks for internal blocks and external DSL link based on an input from a System Master Clock as shown. Again, some or all of the functions of DSL Analog Modem Circuit  205  may be grouped and implemented in single chip form. For example, DSL-A codec  218 , incorporating control registers  215 , DSL-A Interface  216 , digital filters  214 ,  214 ′, and A/D  213  and D/A  213 ′ is preferably embodied in a single integrated circuit (IC), and a separate IC is preferably used to embody analog front end sections (i.e. receive/transmit drivers  209 ,  209 ′ and receive/transmit filters  211  and  211 ′). 
     It should be emphasized that the above description is only a typical embodiment of DSL Analog Modem Circuit  205  suitable for use with the present invention, and that other variations of an analog codec circuit will be apparent to those skilled in the art. Other voice modem related functions can also be incorporated in this stage, as noted in the aforementioned Intel specifications noted above. 
     To improve transmission performance against various noise sources from the PC motherboard and coupled to the components in the analog front end (AFE) section, DSL Modem Analog Circuit  205  can be physically placed in a separate printed circuit board (PCB) or riser card  211 . This provides maximum noise isolation from DSL Modem Digital Circuit  230 . This approach is similar to that done in the prior art for analog modems. Note that it is not necessary in all cases that DSL Modem Analog Circuit  205  be placed on a separate riser card  211 , but it is important that such circuitry be isolated in some fashion so that it is substantially free from electronic noise that could impair its performance. Accordingly, DSL Modem Analog Circuit  205  could conceivably still be mounted on the same motherboard  231  as DSL Modem Digital Circuit  230 , so long as the former is sufficiently isolated from other motherboard noise sources. As an example, a small isolating enclosure, shielding cage, or even insulative layer coatings might be used to protect and shield portions of DSL Modem Analog Circuit  205  from EMI and other noise, and this could result in smaller displacements between the analog and digital sections of an xDSL modem. Other appropriate manifestations will be apparent to those skilled in the art. 
     DSL Digital Section Overview 
     On the other side of DSL Link  220  is DSL Modem Digital Circuit  230 , which includes a counterpart DSL-D Interface  233 . As noted earlier, DSL Modem Digital Circuit  230  is preferably implemented as a single DSL Digital IC integrated within a South/North bridge controller within a PC system. The functions performed by this circuit include I/O transfers between a PC host and DSL Analog Circuit  205 . Depending on the specific DSL implementation, part of the data pump functions can be done inside Receiver Buffer and Processing  234  and Transmitter Buffer and Processing  234 ′. For example, DSL functions such as IFFT and FFT can be performed by  234 ′ and  234 , respectively. DSL-D Interface  233  interacts with and is responsible for coordinating data/control signals on the digital side of DSL Link  220 . As with its counterpart on the analog side, DSL-A Interface  216 , this circuit performs data conversion and control signaling via DSL Link  220 . To interface with the PC host and various peripheral devices, DSL Digital IC  230  may also include various bus connections and interfaces. These include PCI Bus  241  and associated PCI Bus Interface  238  to the PC host, USB  242  and USB Interface  238 ′ to external USB devices, and AC-Link  243  and DC97 Interface  238 ″ to external audio and analog modem codecs. 
     Again, it should be mentioned that the above description for the Digital IC  230  is but one typical embodiment for use with the present invention in a PC system, and that other variations of the same will be apparent to those skilled in the art. As noted earlier, some of the functions performed in DSL Digital IC  230  can instead be implemented on the analog side, and vice-versa. 
     As noted above, functions performed by Transmitter Buffer and Processing  234 ′ and Receiver Buffer and Processing  234  depend on the specific xDSL implementation. In the case of host signal processing, where the present invention can be used for great benefits, only minimal functions need be performed inside  234  and  234 ′. Instead, most xDSL functions are performed by the host CPU. The degree of integration of such functions by a CPU are, of course, a function of the available MIPs for performing the same. While it is not practical or economical at this time to implement a full rate ADSL data pump with contemporary CPU technologies, it is nevertheless quite possible to achieve reduced rate ADSL functionality (i.e., such as that specified by the proposed ITU-T G.992.2 standard) in a PC in most cases by utilizing the teachings of applicants&#39; prior disclosures, including in Ser. No. 08/884,895 and 08/884,957 which are hereby incorporated by reference. Thus, the hardware needed to implement DSL Digital IC  230  can be extremely inexpensive (simple interface circuitry) and cost-effective for a motherboard manufacturer. 
     DSL Link Specification/Protocols 
     A DSL link  220  couples DSL Modem Analog Circuit  205  to DSL Digital IC  230  through DSL-A Interface  216  and DSL-D Interface  233  respectively. This link includes a number of data and control signal lines. Specifically, there are four (4) receive data lines RX 1 -RX 4 ; four (4) transmit data lines TX 1 -TX 4 ; a variable rate bit clock signal (CLOCK); a word clock signal (WORD CLOCK) to show the word boundary for each sample; and a reset signal (RESET) from the DSL Digital IC  230  to reset the DSL Modem Analog Circuit  205 . These signals are explained in detail below. As can be seen, the applicants have carefully evaluated and calculated an optimal set of data/control lines and signal formatting to minimize the total number of pin assignments and maximize the usability for various xDSL requirements. 
     Receive data lines RX 1 -RX 4  carry digital samples generated by A/D  213  and assembled and transmitted across the link by DSL-A Interface  216 ; DSL-D interface  233 , conversely dis-assembles and passes these samples on for further signal processing. Unlike prior art audio modem codecs, which utilize a single receive data line, the present invention makes use of four parallel lines. This comes from a compromise between the single line prior art approach (which minimizes the number of pin assignments but requires a high clocking rate) and a fully parallel approach (which minimizes the clock rates but requires more driving power and a higher pin count). This compromise approach has the following advantages: (1) since xDSL signals are broadband signals, it minimizes clock frequencies and consequently device power consumption, (2) it reduces the total pin count to an acceptable level, (3) it provides an embedded channel for control signaling (explained shortly), and (4) enables various features such as using the same DSL link  220  as a logically equivalent Utopia interface (explained shortly). 
     For the same reasons, four (4) transmit data lines TX 1 -TX 4  carry digital samples from DSL Modem Digital Circuit  220  assembled by DSL-D Interface  233  into suitable form where they are then received and dis-assembled by DSL-A Interface  216  and processed by D/A converter  213 ′ in DSL Modem Analog Circuit  205  to generate the analog transmit xDSL signal. Signal line RESET is activated when the PC host tries to reset DSL Modem Analog Circuit  205 ; this can occur, for example, when the xDSL modem must be initialized or re-initialized. 
     Signal line CLOCK carries a scalable rate clock signal, f clock  generated by the clock generator inside DSL-A  216 , which can be any suitable and well-known conventional clocking circuit. For a given xDSL link of signal frequency bandwidth B, oversampling factor P, up to 15 bits per sample, f clock  is given by: 
     
       
           f   clock =2 *B*P *16/4 
       
     
     In a preferred embodiment, a sample word consists of 16 bits: 1 control bit and up to 15 sample bits. Therefore, the period of the signal line WORD CLOCK consists of four (4) serial clock periods. That is, 
     
       
           f   WC   =f   clock /4 
       
     
     Unlike the prior art fixed clock rate signaling, the clock signal rates for CLOCK and WORD CLOCK will vary depending on the particular DSL link in place. The present DSL Digital Communication Link  220  does not impose any restriction on the clock rate, since it is expected that such should be scalable in a manner that supports a wide range of potential options for an xDSL modem in a personal computing system. In other words, this variability for the clock rate provides system motherboard manufacturers with the ability to mix and match compatible combinations of digital and analog xDSL modem components without concern about the available clocking rate across the connecting interface. This optimizes options for both motherboard suppliers, and. xDSL modem component vendors, since they only need to ensure that they are matching combinations of hardware that are otherwise compatible. For example, in an ADSL modem application, the T1.413 issue 2 standard calls for a full downstream (Nyquist) sampling rate of 2.2 MHz, where the signal bandwidth B is 256 tones*4.3125 kHz/tone=1.1 MHz. With P being 4× oversampling, CLOCK and WORD CLOCK are 35.328 and 8.832 MHz, respectively. The only important consideration, therefore, is to make certain that the particular DSL Digital IC  230  can support this clock rate in conjunction with the selected DSL Modem Analog Circuit  205 . In this respect, therefore, DSL Digital Communications Link  220  supports a scalable clock rate that is based purely on the desired price and performance objectives of a system vendor. To flexibly adjust the clock rates, the external Master clock can be divided by a programmable value specified in a register inside DSL-A Interface  216 , which can be set up by the host through the control channel embedded in the DSL Link  220  (explained shortly). 
     To reduce the total pin count in DSL Link  220 , the same CLOCK and WORD CLOCK are used by the corresponding interfaces (DSL-A  216  and DSL-D  233 ) for both transmit and receive data. 
     The word clock rate, f wc , provides timing for transmit and receive data samples. As illustrated generally in FIG. 3A, each pulse in the WORD CLOCK represents the beginning of a sample word. As shown, four sampling CLOCKs are used in a word period. This allows up to 16 bits (4 lines*4 cycles) per sample word. In a typical DSL application, a high speed codec will not require more than 15 bits per sample. Accordingly, the present scheme accommodates even the expected worst case for an xDSL modem by setting 16 bits per WORD CLOCK period, in which at least one of these bits can be designated for control signaling. Of course, while the preferred embodiment herein utilizes the specified clocking format discussed above, and an accompanying set of four data transmission lines, other combinations of these parameters will be apparent to those skilled in the art from the present teachings. Accordingly, the present invention is not limited to any one particular combination of such parameters. 
     As mentioned above, the present invention also provides capability for communicating operational and/or control information easily and flexibly between the analog and digital sections of modem  200 . This can be seen in FIG. 3B which illustrates how at least one operational control bit (D 15 ) is reserved or allocated for an embedded operation channel (EOC). The EOC can be used, for example, for passing the following types of information across the link. 
     (1) transmitter/receiver gains,; 
     (2) interrupts to the Digital IC  230 ; 
     (3) information to control VCXO in DSL Modem Analog Circuit  205  for timing recovery; 
     (4) power management information, such as an ADSL wake-up signal, or a power loss signal received by DSL Digital IC  230  from the motherboard. The power loss signal from the motherboard, in the case of ADSL, is used by DSL Modem Digital Controller  230  for generating a dying gasp message to be transmitted to a remote transceiver through DSL Modem Analog Circuit  205 . Other types of control information associated with a conventional xDSL link can also be accommodated. 
     It can be observed from FIG. 3B that EOC has an effective bit rate equal to the word clock. The present invention, therefore, provides a method for providing control signaling information that does not require a separate time division multiplexed control word sent over the link as is done in the aforementioned AC97 prior art data link. This operational information is encoded by DSL-D Interface  233  into the data stream using any suitable logic, and is then decoded and acted upon by corresponding logic in DSL-A Interface  216  so that, for example, suitable adjustments are made to digital filters  214 ,  214 ′, A/D  213 , D/A  213 ′, and amplifier and filter stages  209 ,  209 ′ and  211 ,  211 ′ respectively. This operational information can be passed in both directions across the link so that control registers  215  within DSL Analog Circuit  205  can be read or configured in any desired manner by DSL Digital IC  230 . Conversely, if desired, information concerning data and settings of control registers  235  within DSL Digital IC can be monitored and changed by control logic in DSL Analog Circuit  205 . 
     Preferably, control information (i.e., control data or control data addressing) in the EOC is presented in asynchronous form in a manner analogous to that used in a conventional PC serial port or RS 232  to simplify logic design. This means that a start bit is set to logical 0 when there is control data, and logical 1 otherwise. After the start bit, a fixed number (1+N c  or 1+2 N c ) of control bits fills out the remainder of the control word signal. To optimize the balance between short control words (to minimize delays) and long control words (improved addressing range), two different control word lengths are provided (i.e., the control word length is either N c  or 2N c ). To signal a short control word (say N c =16 bits), the bit immediately following the start bit is set to a logical 0; to signal a long control word (say 32 bits) the bit immediately following the start bit is set to a logical 1. Following the set of bits for the actual control word, the EOC is then stuffed with all logical 1 values, corresponding to idle bits. Because the above architecture is relatively flexible, it is of course possible (and may be desirable in some environments) to increase the number of embedded operation channels by using an additional bit (for example D 14) of the transmit/receive data words across the link. 
     Additional Features in DSL Link 
     Variable Number of DSL Channels 
     Another useful aspect of the present invention is the provision in the present xDSL digital link for a variable number of channels. The aforementioned AC97 prior art can only handle a fixed number of channels (12) in the transmit/receive directions, and this is not practical for an xDSL link for a number of reasons. First, the receiving and transmitting data streams may not be operating at the same sampling clock. This will occur, for example, when there are different oversampling ratios, or where there are different signal bandwidths (e.g., ADSL) for upstream and downstream transmissions. Second, there may be multiple channels for transmission and receive directions. For instance, an echo cancellation data path might be used in some environments for echo suppression. Finally, it may be useful to support additional DSL Modem Analog Circuits  205 ′,  205 ″, (not shown) connected in parallel with DSL Modem Analog Circuit  205  using the same set of data signal, control and clock lines described above. 
     To achieve this variable number of channels capability, a frame structure for the word clock cycles is illustrated in FIG.  3 C. As illustrated, a certain number M (programmable by software and set to a value of 2 as shown) of word clock cycles forms a frame clock to support M separate DSL channels (or separate analog circuits) in each direction. It is unnecessary, of course, for all M channels to be active at one time. To mark the beginning of a frame, the word clock is held high for a duration of two clock cycles. At startup, M is initially set to 1. To operate with multiple channels, DSL Digital IC  230  passes a control signal via the EOC noted earlier, to set M to some higher integer value in the DSL Modem Analog Circuit  205 , such as M=2. A multi-channel frame is then indicated by WORD CLOCK being set high for two clock cycles. As depicted in FIG. 3C, the data for channel  2  (or the second DSL Modem analog circuit) is presented at the beginning of the frame cycle (i.e., when WORD CLOCK is high for two cycles) and the data for channel  1  (or the first DSL Modem analog circuit) is presented during the second half of the frame cycle (during the time WORD CLOCK is high for a single cycle). Thus, in the multi-channel data frame of FIG. 3C, there are two data channels, and data is transferred through a first channel during a first time period of the multi-channel data frame, and through a second channel during a second time period of the multi-channel data frame. 
     In this manner, multiple codecs can be included in DSL Modem Analog Circuits  205 . As is apparent, the value for M can be programmed as necessary by DSL Digital IC  230  based on the particular DSL link requirements, the specific implementation of DSL Modem Analog Circuits  205 , etc. 
     Furthermore, when the bit rates at the output of Digital Filters  214  and  214 ′ have an integer relationship, the provision of multiple channels is a necessary feature. For example, in the case of ADSL where the sampling rates equal the Nyquist rates, the upstream and downstream rates are 275 kHz and 2.2 MHz, respectively (a ratio of 1:8). Therefore, the present invention allows a DSL modem to be configured with a value of M=8, thus setting up only one active channel in the upstream direction and 8 active channels in the downstream direction. 
     The various clocks, buffers, latches, control logic, etc., for implementing this aspect of the present invention, as well as for performing the formatting, assembling, and dis-assembling operations on the Rx and Tx data samples by the various DSL-A and DSL-D interfaces are well-known in the art, and therefore any suitable combination of the same which effectuates the above functionality can be used for the present invention. 
     Reuse of DSL Link for External Hardware DSL Implementation 
     As mentioned earlier, the use of DSL Link  220  is most attractive to a host based DSL modem implementation requiring minimal logic inside Digital IC  230 . When the CPU in the motherboard is not fast enough, it is desirable to use the DSL Link to connect Digital IC  230  to an external hardware DSL implementation. In this case, another useful aspect of the present invention is illustrated in FIG.  4 . As shown, when an external hardware solution for a DSL modem implementation exists, a reasonable interface to use with such implementation is one based on the ATM Utopia I or Utopia II interface. This is because ADSL technology has already been defined to interface with ATM in both T 1 . 413  Issue 2 and ITU-T G.992 standards. In this configuration, DSL Digital IC  230  would be linked through DSL Digital Link  220  to a hardware based xDSL modem in FIGS. 2A and 2B, instead of interfacing directly to DSL Analog Modem Circuit  205 . In such instance, of course, since most of the signal processing and control functions would be located within the hardware xDSL modem, DSL Digital Controller  230  could be simplified accordingly. The reason this is possible is because the same 10 signal lines described above (RX 1 -RX 4 , TX 1 -TX 4 , CLOCK and WORD CLOCK) can serve a dual purpose and act as an ATM interface as well. As above, for the same four sampling cycles per word clock, the following data can be transported over DSL digital link  220 : 
     1. First clock cycle period: RX 1 -RX 4  are used for Control, 0, RxClav, TxClav; TX 1 -TX 4  are used for Control, 0, RxEnb and TxEnb. 
     2. Second clock cycle period: RX 1 -RX 4  are used for RxSoc, RxAddr [2:0], while TX 1 -TX 4  are used for TxSoc, TxAddr [2:0]. 
     3. Third clock cycle period: RX 1 -RX 4  are used for RxData [7:4], while TX 1 -TX 4  are used for TxData [7:4]. 
     4. Fourth clock cycle period: RX 1 -RX 4  are used for RxData [3:0], while TX 1 -TX 4  are used for TxData [3:0]. 
     As defined above, the signals Control, RxClav, TxClav, RxEnb, TxEnb, RxSoc, TxSoc, RxAddr, and TxAddr are defined in the UTOPIA Specification, Level 1, Version 2.01, published Mar. 21, 1994 by the ATM Forum, and incorporated by reference herein. It can be seen from the above that an ATM type interface can also be implemented by the DSL-A and DSL-D interfaces to exploit the embedded control channel characteristics of the present digital link with great advantage. 
     One significant advantage of the present invention, therefore, is the fact that the DSL Digital Link  220  can be incorporated within a motherboard of a personal computing system to enable great flexibility in configuring options for an xDSL communications capability. For example, a motherboard manufacturer (or ultimate system vendor) can configure any particular base level enabled system to include any desired level of xDSL modem performance and functionality, including either a host based soft modem solution, or a pure hardware type modem solution. This is accomplished by simply stuffing the board with various combinations; for example, a simplified version of DSL Digital Controller  230  can be combined with an ATM based hardware DSP xDSL modem, or, alternatively, a configuration such as shown in FIG. 2B can be implemented for a software modem implementation. The flexibility of the data lines of the communications link, and the use of an embedded control channel, permits easy adoption of optimized solutions for any particular application. 
     Thus, the configuration and selection of the data signal lines, the use of an embedded control channel, the data clocking mechanism, the ability to provide multiple data channels, and the format of control words used in the embedded control channel, all contribute to making the present invention easily adaptable and usable with a number of different combinations of digital and analog circuits associated with xDSL modems. 
     Although the present invention has been described in terms of a preferred embodiment, it will be apparent to those skilled in the art that many alterations and modifications may be made to such embodiments without departing from the teachings of the present invention. Accordingly, it is intended that the all such alterations and modifications be included within the scope and spirit of the invention as defined by the following claims.