Abstract:
As part of the protocol for Common Public Radio Interface/Open Base Station Architecture Initiative (CPRI/OBSAI) systems, multiple data rates are support, which are each supported by one or more reference clock frequencies. Traditionally, timing circuits present used for the physical layer (PHY) paths to determine the data rates for the serial data have been plagued with numerous problems. Here, however, a circuit that performs an automatic rate sense (ARS) of high speed serial signals in a low speed digital domain is provided, which is also relatively easy to implement and robust.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to Common Public Radio Interface/Open Base Station Architecture Initiative (CPRI/OBSAI) systems and, more particularly, to performing Automatic Rate Sense (ARS) detection for CPRI/OBSAI systems. 
       BACKGROUND 
       [0002]    Referring to  FIG. 1  of the drawings, the reference numeral  100  generally designates a portion of a convention communications system. As shown, a base station system  102  operates to provide communications between a network interface  106  and an air interface, which is typically used for wireless communications. The base station system  102  generally comprises radio equipment  108  and a radio equipment controller  110 , which each have a physical layer (PHY)  112  and  114  that communicate with each other over a CPRI/OBSAI link  113 . Typically, though, the CPRI/OBSAI link  113  supports multiple data rates (for example 8), which can be supported by one (or more) of several reference clock frequencies. Setting the reference clock frequency to match the data rate over the CPRI/OBSAI link  113 , however, has been difficult because such detection generally uses high speed circuitry, which is both power and area intensive. Therefore, there is a need for an improved timing circuit with an ARS function. 
         [0003]    Some other examples of conventional circuits are: U.S. Pat. No. 6,158,014; U.S. Pat. No. 7,093,151; U.S. Pat. No. 7,295,554; U.S. Pat. No. 7,359,432; U.S. Pat. No. 7,593,498; U.S. Pre-Grant Publ. Patent No. 2008/0080600. 
       SUMMARY 
       [0004]    A preferred embodiment of the present invention, accordingly, provides a method. the method comprises determining whether the highest clock frequency of a plurality of clock frequencies matches a detected data rate by: starting a timeout counter; enabling a phase locked loop (PLL) following the step of starting the timeout counter; if phase lock is achieved, enabling reception of data; checking whether channel synchronization has been achieved for the data that has been received; and if the timeout counter expires prior to a determination as to whether channel synchronization has been achieved, establishing that the selected clock frequency does not generally match the detected data rate; and repeating the step of determining for each of the remaining clock frequencies, in order from highest to lowest, until a match for the detected data rate is found. 
         [0005]    In accordance with a preferred embodiment of the present invention, the step of checking further comprises: performing comma detection for a plurality of ordered sets, wherein the leftmost bit positions for each ordered set contain a comma for each ordered set; if the comma for each ordered set is determined without an invalid decode error, then performing code-group synchronization; and testing each of a plurality of code-groups. 
         [0006]    In accordance with a preferred embodiment of the present invention, the plurality of ordered sets further comprises three ordered sets. 
         [0007]    In accordance with a preferred embodiment of the present invention, the plurality of code groups further comprises four code-groups. 
         [0008]    In accordance with a preferred embodiment of the present invention, the step of determining further comprises resetting a datapath prior to the step of checking. 
         [0009]    In accordance with a preferred embodiment of the present invention, the step of determining further comprises establishing that the selected clock frequency does not generally match the detected data rate, if the timeout counter has expired prior to achieving phase lock. 
         [0010]    In accordance with a preferred embodiment of the present invention, an apparatus is provided. The apparatus comprises a deserializer having a PLL; a decoder; a channel synchronization circuit that is coupled to the decoder and the deserializer; and an Automatic Rate Sense (ARS) state machine that is coupled to the channel synchronization circuit, wherein the ARS state machine compares a detected data rate to each of a plurality of clock frequencies, in order from the highest frequency of the plurality of clock frequencies to the lowest frequency of the plurality of clock frequencies, to determine which of the plurality of clock frequencies generally matches the detected data rate so as to achieve ARS lock. 
         [0011]    In accordance with a preferred embodiment of the present invention, the ARS state machine: starts a timeout counter; enables the PLL once the timeout counter is started; and enables reception of data, if phase lock is achieved. 
         [0012]    In accordance with a preferred embodiment of the present invention, the channel synchronization circuit further comprises a channel synchronization state machine that checks whether channel synchronization has been achieved for the data that has been received, and wherein the ARS state machine establishes that the selected clock frequency does not generally match the detected data rate, if the timeout counter expires prior to a determination as to whether channel synchronization has been achieved. 
         [0013]    In accordance with a preferred embodiment of the present invention, the channel synchronization state machine, when checking whether channel synchronization has been achieved for the data that has been received: performs comma detection for a plurality of ordered sets, wherein the leftmost bit positions for each ordered set contain a comma for each ordered set; performs code-group synchronization, if the comma for each ordered set is determined without an invalid decode error; and tests each of a plurality of code-groups. 
         [0014]    In accordance with a preferred embodiment of the present invention, the ARS state machine resets a data path prior to checking whether channel synchronization has been achieved for the data that has been received. 
         [0015]    In accordance with a preferred embodiment of the present invention, the ARS state machine establishes that the selected clock frequency does not generally match the detected data rate, if the timeout counter has expired prior to achieving phase lock. 
         [0016]    In accordance with a preferred embodiment of the present invention, an apparatus is provided. The apparatus comprises a deserializer having a PLL; a decoder; and a processor with a storage medium, wherein the processor is coupled to the decoder and the deserializer, and wherein processor has a computer program embodied thereon, and wherein the computer program includes: computer code for determining whether the highest clock frequency of a plurality of clock frequencies matches a detected data rate by: computer code for starting a timeout counter; computer code for enabling the PLL following the starting the timeout counter; computer code for enabling reception of data, if phase lock is achieved; computer code for checking whether channel synchronization has been achieved for the data that has been received; and computer code for establishing that the selected clock frequency does not generally match the detected data rate, if the timeout counter expires prior to a determination as to whether channel synchronization has been achieved; and computer code for repeating the computer code for determining for each of the remaining clock frequencies, in order from highest to lowest, until a match for the detected data rate is found. 
         [0017]    In accordance with a preferred embodiment of the present invention, the computer code for checking further comprises: computer code for performing comma detection for a plurality of ordered sets, wherein the leftmost bit positions for each ordered set contain a comma for each ordered set; computer code for performing code-group synchronization if the comma for each ordered set is determined without an invalid decode error; and computer code for testing each of a plurality of code-groups. 
         [0018]    In accordance with a preferred embodiment of the present invention, the computer code for determining further comprises computer code for establishing that the selected clock frequency does not generally match the detected data rate, if the timeout counter has expired prior to achieving phase lock. 
         [0019]    In accordance with a preferred embodiment of the present invention, the processor with the storage medium having the computer program embodied thereon further comprises an ARS state machine that includes a first processor with a first storage medium having a first computer program embodied thereon and a channel synchronization state machine that includes a second processor with a second storage medium having a second computer program embodied thereon, wherein the second computer program includes the computer code for checking. 
         [0020]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0022]      FIG. 1  is a block diagram of an example of a conventional communications system; 
           [0023]      FIG. 2  is a block diagram of a receive PHY circuit in accordance with a preferred embodiment of the present invention; 
           [0024]      FIGS. 3  is an example flowchart depicting the operation of the ARS state machine of  FIG. 2 ; and 
           [0025]      FIG. 4  is an example flow chart depicting the operation of the channel synchronization state machine of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
         [0027]    Turning to  FIG. 2  of the drawings, an example of a receive PHY circuit  200  in accordance with a preferred embodiment of the present invention can be seen. In operation, this circuit  200  receives serial data (or serial receive data) and outputs parallel data (or parallel receive data), which is generally accomplished through the use of deserializer  202 . The channel synchronization circuit  206  (which includes state machine  208 ), 8b/10b decoder  210 , and ARS state machine  212  can then be used to perform an ARS function, while Management Data Input/Output (MDIO)  214  operates as an interface that can be used to set up the ARS state machine  212 . Additionally, state machines  212  and  208  can be implemented in hardware or can be implemented through software, using one or more processors with one or more storage media (i.e., EEPROM). 
         [0028]    Timing is an important aspect of the functionality of circuit  200 , and circuit  200  typically supports several reference clock frequencies. For example, reference clock frequencies can be 122.88 MHz, 153.6 MHz, 245.76 MHz, and 307.2 MHZ. To be able to determine which reference clock frequency is being used, circuit  300  includes the ARS function, which can be enabled externally through separate pin(s) (i.e., separate pin for each channel). To perform the ARS function, deserializer  202  generally includes a phase locked loop (PLL)  204 , which communicates with ARS state machine  212 . Typically, the channel synchronization circuit  206  monitors the incoming 8b/10b encoded serial receive data (from deserializer  202 ) using both the comma character and 8b/10b disparity errors for a given channel to determine and validate the incoming serial data rate, while decoder  210  examines the data stream for invalid decodes. In other words, channel synchronization state machine  208  (within circuit  206 ) attempts to detect 4 successive commas with no rotation, no running disparity errors, and no invalid decodes. 
         [0029]    Turning now to  FIG. 3 , a flow chart  400  depicting the ARS function, which is performed by state machine  212 , can be seen. Generally, state machine  212  continuously loops through (and overrides previously programmed) deserializer  202  control settings for a given input reference clock frequency until either an incoming serial bit rate is successfully determined, or the ARS function is disabled through a pin or MDIO  214  software control. Initially, the ARS state machine  212  determines if the ARS function is enabled in step  402 , and if so, a timeout counter and the PLL  204  are started or enabled in steps  404  and  406 , respectively. Once the reference clock being checked has achieved a value of 16 or greater (as determined in step  408 ), the ARS state machine  212  waits for phase/frequency lock in steps  410  and  414 . If a timeout condition (step  412 ) is reached while waiting for phase/frequency lock, the timeout counter is restarted (step  404 ). Otherwise, with phase/frequency lock, the receive path is enabled in step  416 , and the data path is automatically reset in step  420 . In step  422 , the channel synchronization state machine  208  determines whether there is channel synchronization (initially for the highest frequency of all of the supported reference clock frequencies) in step  422 . If there is no channel synchronization before a timeout conditions has been reached in steps  424  and  426 , then the processes starts again with step  422  at the next, lower frequency; otherwise, the transmit first-in/first-out (FIFO) memory (not shown) and receive FIFO memory (which is within deserializer  202 ) are reset and a determination is made as to whether the MDIO  214  is gated in step  428 . If the MDIO  214  has been gated, the ARS state machine  212  waits for the transmit FIFO memory (not shown) to reset in step  430  (so as to achieve ARS lock in step  432 ); otherwise, if the MDIO  214  is not gated, then ARS lock has been achieved in step  432 . The channel synchronization state machine  208  can then continually check to see if channel synchronization is maintained in step  434  so that the ARS state machine  212  and channel synchronization state machine  208  can automatically search for the correct reference clock frequency in the event that channel synchronization is no longer maintained. 
         [0030]    Turning to  FIG. 4 , a flow chart  500  for the operation of the channel synchronization state machine  208  can be seen. Generally, the state machine  208  is implemented as specified in Institute of Electrical and Electronics Engineers (IEEE) standard 802.3-2002, clause  36 , which is incorporated by reference herein for all purposes. Typically, an 8b/10b decoder  210  is used in tandem with the state machine  208  to determine if a rate sense is successful at a particular setting; this 8b/10b decoder  218  is also used for the ARS function even if 8b/10 decoding/encoding is disabled for the data path of the selected channel. On the loss of synchronization (i.e., with the assertion of a reset signal RESET) in step  502 , state machine  208  attempts to acquire code-group synchronization by detection of three-ordered sets (for example) containing commas in their leftmost bit positions (for example). Typically, in steps  504 ,  506 , and  508 , state machine  208  attempts to detect commas from these order-sets or code-groups without an intervening invalidation condition (such as an invalid decode ID or no running disparity error NC). If detection of these commas fails, then there is not channel synchronization. If, on the other hand, these commas are detected without an intervening invalidity condition, then the state machine  208  enters an acquisition state in step  510 . In there is an invalid code IC during step  510 , state machine  208  enters a second acquisition state in step  512 . Following step  510 , several other acquisition states can be used in steps  514 ,  516 ,  518 ,  520 , and  522  in the even of an invalid code IC or invalid decode ID. Acquisition of synchronization generally ensures the alignment of multi-code-group ordered sets to even-number code-group boundaries. Typically, steps  514 ,  516 ,  518 ,  520 , and  522  operate to test four (for example) received code-groups using multiple sub-states, effecting hysteresis, to move between a synchronization acquired state and a loss of synchronization state. 
         [0031]    As a result of using the circuit  200 , several advantages can therefore be realized over conventional circuits. For example, implementation of the ARS functionality (as shown in  FIG. 3 through 4 ) is relatively easy and is robust for operations in challenging bit error environments. Additionally, circuit  200  permits highly efficient detection of high speed serial signals in low speed digital domains. 
         [0032]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.