Abstract:
A method for granting access to a bus is disclosed where a fair arbitration is modified to account for varying conditions. Each bus master (BM) is assigned a Grant Balance Factor value (hereafter GBF) that corresponds to a desired bandwidth from the bus. Arbitration gives priority BMs with a GBF greater than zero in a stratified protocol where requesting BMs with the same highest priority are granted access first. The GBF of a BM is decremented each time an access is granted. Requesting BMs with a GBF equal to zero are fairly arbitrated when there are no requesting BMs with GBFs greater than zero wherein they receive equal access using a frozen arbiter status. The bus access time may be partitioned into bus intervals (BIs) each comprising N clock cycles. BIs and GBFs may be modified to guarantee balanced access over multiple BIs in response to error conditions or interrupts.

Description:
BACKGROUND OF INVENTION 
   The present invention relates in general to methods and apparatus for arbitrating access to a communication bus on an integrated circuit and, in particular, to the processor local bus (PLB) used on system-on-a-chip (SOC) implementations. 
   On-chip bus systems that are used to communicate between a number of functional units use various methods to arbitrate among bus masters that manage bus access for the function or device. Relative to access to the bus, a bus master is the transfer initiator and a slave is the transfer receptor. A bus master actually controls the bus paths on which the address and control signals flow. Once these are set up, the flow of data bits goes between the transfer initiator and the receptor. There are various types of arbitration schemes: round robin, daisy chain, centralized, distributed, etc. Each of the arbitration schemes attempts to provide the various bus master devices access that is fair or access that is based on a predetermined fixed priority sequence. The round robin arbitration scheme is a “fair” method that continuously repeats a sequence, such as the polling of a series of bus masters, one after the other, over and over again searching for an access request. For the round robin arbitration method, a bus master (BM) that requires a certain amount of guaranteed bandwidth may be starved from gaining sufficient access to the bus if one or more other BMs are also continuously requesting a bus access. At best, the BM will only be guaranteed 100/N percent of the bus bandwidth where N represents the number of continuously requesting BMs coupled to the bus. In a fixed arbitration method, the potential for starvation will also occur if the BM that is requesting the bandwidth is relegated to a secondary priority when another BM which has a higher priority continuously requests access to the bus. 
   A fair arbitration scheme is desirable, but it would be advantageous to have an arbitration scheme that was also able to guarantee selected BMs a certain bandwidth while not wasting bus resources if these selected BMs were not requesting a bus access. It also would be useful if a bandwidth selected for a device could be changed dynamically if system utilization indicated that the bandwidth requirements had changed. It would also be desirable to have an arbitration scheme that allowed BMs not assigned a certain bandwidth to be guaranteed a chance to access the bus if it was determined that they were being “locked” out by the arbitration scheme. 
   The efficient utilization of a bus in a system, where the data traffic environment may be continuously changing, is key to realizing the maximum performance capability of the system. There is, therefore, a need for a method and apparatus for better managing access to a bus so that selected BMs may be assigned a certain amount of bus access and thus a certain bandwidth while preventing other bus masters without assigned bandwidth from being locked out. 
   SUMMARY OF INVENTION 
   Each bus master (BM) that manages the access of a device or functional unit to a shared bus has a Grant Balance Factor value (hereafter GBF) assigned that defines its guaranteed access to the bus in relation to the other BMs. The GBF for each BM is stored in a Grant Balance Register (GBR) which is decremented by one each time the BM is granted access to the bus. In one embodiment, BMs with a GBF greater than zero are given priority to the bus in a “stratified” protocol. In this stratified protocol, requesting BMs with the same highest GBF are arbitrated first. Each time one of these BMs is granted an access, its GBF is decremented and its priority essentially drops to the next level. Fair arbitration continues among the requesting BMs with the highest GBF. If there are no requesting BMs with the highest GBF, requesting BMs with the “next” highest level GBF are arbitrated. The BMs that have an initial GBF equal to zero have no guaranteed priority but may still get access. If a requesting BM&#39;s GBR value decrements to zero, it may be treated with the same or a different priority than a requesting BM with an initial GBR equal to zero. However, if its GBR value has decremented to zero and there is no other requesting BM with a GBR value greater than zero or requesting BM with an initial GBF equal to zero, then its request may be serviced. Additionally, the time during which bus accesses are granted is partitioned into bus intervals (BIs) which may be programmed via an Arbitration Bus Interval (ABI) register. Bus activity is checked during each BI. If bus request activity drops below a predetermined level during a BI, the GBFs may be reset to a predetermined programmed value. If bus request activity drops below a predetermined level during a BI, an interrupt may be issued to stop the BI and the GBFs may be reset to a predetermined programmed value. If the BI has expired, the GBRs are reset to a predetermined programmed value. 
   The BI is set as a programmed number of k clock cycles. In this manner, a bus (e.g., a processor local bus (PLB)) access time may be partitioned into n BIs where each BI comprises k clock cycles. In this embodiment, a user should program the total number of access times for all BMs to be less than the total time allocated per BI. This should insure that all of the BMs have an access granted within a given BI. Each time a BM is granted an access, its GBF is decremented and its GBF is reset to a programmed value at the end of the BI. However, if all the GBFs have decremented to zero and the BI has not been reached, the arbiter will revert to a fair arbitration (e.g., round robin) to handle any requests from BMs that had initial GBFs equal to zero. BMs with a GBF equal to zero compete for bandwidth left over after all the BMs with a GBF greater than zero have been serviced to their level determined by their GBF. When there are multiple requesting BMs with a GBF equal to zero, it is possible that only some of them will get an access granted during a given BI. To insure that over multiple BIs these particular BMs will get an equal opportunity for an access request to be granted, the state of the arbiter&#39;s polling sequence is frozen at the end of a BI. During the next BI, the arbiter will continue from this frozen state when it is granting accesses to the requesting BMs with initial GBFs equal to zero. This guarantees that any one of these requesting BMs that was “locked out” from getting an access request granted during a BI will get a virtual higher priority in its next fair arbitration cycle. 
   In another embodiment, the BI is dynamically modified to vary bus accesses depending on system access requirements. Two error reporting mechanisms are employed by the arbiter when it is in the dynamic BI mode:
         (a) an error is flagged if a user sets the BI too small such that requesting BMs that have GBFs greater than zero are locked out of receiving accesses to the bus. If all the GBFs greater than zero have not decremented to zero at the end of the BI, an error is flagged in a status register indicating the master identification (ID) of the requesting BM(s) that did not get an access request serviced during the particular BI.   (b) an error is flagged if requesting BMs that have a GBF equal to zero never or very seldom get the opportunity to have an access request serviced because the user has set the BI so small that only requesting BMs with a GBF greater than zero ever get access requests serviced. Likewise, if BMs with a GBF equal to zero never get an access request serviced over a number m of successive BIs, an error is flagged in the status register. The number m may be programmed as a variable value. Over m successive BIs, if it appears that the BI has been set too short, the arbiter may decide to increase the BI or modify the GBFs of selected BMs in the system. In either case, the arbiter ensures that a requesting BM that is being denied access to the bus is allowed greater opportunity to have an access granted in a subsequent BI.       

   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. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     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: 
       FIG. 1  is a flow diagram of method steps in one embodiment of the present invention with Grant Balance Factor values assigned to the bus masters (BMs); 
       FIG. 2  is a flow diagram of method steps in another embodiment of the present invention where the status of the arbiter is frozen after a bus interval (BI) has expired; 
       FIG. 3  is a flow diagram of method steps in an embodiment of the present invention where the BI may be dynamically modified; 
       FIG. 4A  illustrates the bus access time is partitioned into BIs; 
       FIG. 4B  illustrates exemplary BIs and which BMs have been granted bus access requests; 
       FIG. 4C  illustrates how the frozen status of the arbiter guarantees equal access to requesting BMs with an initial GBF equal to zero; and 
       FIG. 5  is a block diagram of a data processing system with a central processing system that may contain a processor with a processor local bus (PLB) or an I/O bus that employs bus arbitration according to embodiments of the present invention; and 
       FIG. 6  is a block diagram illustrating a communication network according to embodiments of the present invention connecting bus master interface units and slave interface units. 
   

   DETAILED DESCRIPTION 
   In the following description, numerous specific details may be set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits and sub-systems may have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details such as specific clock frequency, cycle time, etc. have not been included. 
   Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. In the following explanation, a bus master (BM) that is requesting a bus access may be referred to as a “requesting BM” for simplification. An arbitration bus interval may be referred to as a bus interval (B) for simplification. Grant Balance Factors (hereafter GBFs) are values that are stored in Grant Balance Registers (GBRs) that may be selectively decremented. In the following, instead of referring to decrementing the GBR, the GBF is decremented indicating that the GBF value stored in the GBR is reduced by one. 
     FIG. 5  is a block diagram of a data processing system  500  configured to use embodiments of the present invention. Central Processing Unit (CPU)  534 , Read Only Memory (ROM)  516 , Random Access Memory (RAM)  514 , I/O adapter  518 , Communications Adapter  535 , User Interface Adapter  522 , and Display Adapter  536  are examples of functional units with interface units that are communicating on a bus  512 . These functional units have either BM (request initiators) interface units (e.g. BM  01 , BM  04  and BM  05 –BM  07 ) or bus slave (BS) (request receivers) interface units (e.g., BS  02  and BS  03 ). Bus  512  may be managed to give requesting BMs access using a fair arbiter (not shown). Using embodiments of the present invention, selected BMs may be guaranteed a particular bus bandwidth and other requesting BMs may not be starved off the bus. I/O Adapter  518  may be coupled to tape drive  540  or disk drive  520  for storing data. Communications adapter  535  enables CPU  534  to communicate with remote functional units via network  541 . Display Adapter  536  is coupled to display  538  for display data from CPU  534 . User Interface Adapter  522  may be used to couple user input/output functional units like keyboard  524 , mouse  526 , track ball device  532 , and speaker  528 . 
   System  550 , in data processing system  500 , comprising CPU  534 , ROM  516 , RAM  514 , I/O Adapter  518 , Communications Adapter  535 , User Interface Adapter  522  and Display Adapter  536  may all reside on a system-on-a-chip integrated circuit (IC). Bus  512  would then be considered a processor local bus (PLB). In this system-on-a-chip (system  550 ), access to PLB (Bus  512 ) for functional units CPU  534 , ROM  516 , RAM  514 , I/O Adapter  518 , Communications Adapter  535 , User Interface Adapter  522  and Display Adapter  536  is managed by corresponding bus masters BM  01  and BM  04 –BM  07 . Embodiments of the present invention are used to give each of selected requesting BMs within BM  01  and BM  04 –BM  07  a guaranteed bus bandwidth while insuring that BMs without a guaranteed bandwidth are not locked out from access to Bus  512 . 
     FIG. 6  is a block diagram of a system  600  according to embodiments of the present invention. Units  601 – 603  incorporate BM interfaces for controlling access to the communication link  607  (bus or switch) between these and other units. Units  601 – 6   03  have BM  1 –BM N and are units that are considered “request initiators.” For example, BM  1 –BM N make an access request to arbitrator  608  and an access is granted as the result of arbitration between the requesting master and other masters with the same priority. A central processing unit (CPU) is a device that would have a master interface since it may need to request information from random access memory (RAM) (shown in unit  604 ) therefore it would be a “request initiator.” Unit  604  with RAM is a BS unit and is a “request receiver.” For example, RAM would not typically request an access to the bus, but rather, it would receive a signal from a BM interface (request initiator) requesting that information stored in the RAM be read and forwarded over the communication link to the requesting unit (e.g., CPU in unit  601 ). Since access to the RAM in unit  604  may have only one port, a requesting master would have to be granted access by the arbitrator  608  based on some protocol. Units  601 – 603  with BM  1 –BM N have direct access to the communication link  607  via connections  612 – 614  and unit  604  with Slave  1  has direct connection  615 . Units may also have a shared bus connection to communication link  607 . Units  605 – 606  are connected to communication link  607  via connection  611  and shared bus  610 . GBRs  609  contain the GBFs for each master and are accessed for use by arbitrator  608  or during an update process to establish new GBF values. 
     FIG. 1  is a flow diagram  100  detailing method steps in one embodiment of the present invention. In step  101 , each BM managing access to a bus is assigned a GBF. The number of clock cycles (n cycles) for each access time determining a bus interval (BI) is also set. In step  102 , a test is done to determine if there are any “first” BMs. Hereafter, a first BM is defined as a BM that is requesting bus access and also had an initial GBF greater than zero. If the result of the test is YES, then in step  103  bus request access is granted to a selected first BM using a fair arbitration starting with the first BMs with the highest GBF. The GBF of the selected first BM is decremented by one if the GBF is greater than zero. Step  104 , is an optional step where a first BM whose GBF has been decremented to zero is removed from the arbitration queue. A branch is then taken to step  107  where a test is done to determine if a present BI has expired. If the result of the test in step  107  is YES, then in step  108  all the GBFs are reset to their initial programmed values. A branch is then taken back to step  102 . If the result of the test in step  107  is NO, then in step  109  a test is done to determine if an activity period with no access request has expired. This is an optional step where a decision to reset the GBFs is made prior to expiration of a present B. If the result of the test in step  109  is NO, then a branch is taken directly back to step  102  where arbitration continues amongst first BMs which have not had their GBF decremented to zero. If the result of the test in step  109  is YES, then an interrupt may be issued to stop the BI. A branch is then taken to step  108  where the GBFs are reset to their initial programmed value. 
   If the result of the test in step  102  is NO, then there are no first BMs with a GBF greater than zero. A test is then done in step  105  to determine whether there are any second BMs requesting access. Hereafter, a “second” BM is defined as a BM requesting access with an initial GBF equal to zero. A first BM with a GBF that has decremented to zero may be treated differently than a second BM that had an initial GBF equal to zero. If the result of the test in step  105  is YES, then in step  106  a selected second BM is granted access using fair arbitration. Step  107  is then executed as described previously. If the result of the test in step  105  is NO, then a test is done in step  110  to determine if there are any first BMs whose GBF are now zero that are requesting an access. If the result of the test in step  110  is YES, then step  103  is again executed and first BMs with GBFs equal to zero are given an opportunity to have additional access requests granted. If the result of the test in step  110  is NO, then step  107  is again executed awaiting time out of the present BI. 
     FIG. 2  is a flow diagram  200  detailing method steps in another embodiment of the present invention. In step  201 , each BM managing access to a bus is assigned a GBF. The number of clock cycles (n cycles) for each access time determining a bus interval (BI) is also set. In step  202 , a test is done to determine if there are any first BMs requesting bus access with GBFs greater than zero. If the result of the test in step  202  is YES, then in step  203  an access is granted to a selected first BM using a fair arbitration starting with the first BMs with the highest GBF. The GBF of the selected first BM is decremented by one if the GBF is greater than zero. A branch is then taken to step  206  where a test is done to determine if a present BI has expired. If the result of the test in step  206  is YES, then in step  207  all the GBFs are reset to their initial programmed values and an arbiter sequence status determined during arbitration of second BMs is frozen. A branch is then taken back to step  202  where arbitration continues among any first BMs with a GBF greater than zero. If the result of the test in step  206  is NO, then a branch is taken directly back to step  202 . 
   If the result of the test in step  202  is NO, then there are no requesting first BMs with a GBF greater than zero. A test is then done in step  204  to determine whether there are any second BMs requesting access. If the result of the test in step  204  is YES, then in step  205  a selected second BM is granted access using fair arbitration starting at a “frozen” arbitration sequence status. If this is the first time second BMs have been arbitrated for access, then the frozen sequence status is the initial round robin location, otherwise it is the arbitration sequence status stored from the previous arbitration of second BMs preceding expiration of the corresponding BI. 
   After an access is granted to second BMs from step  205 , step  206  is then executed. If the result of the test in step  204  is NO, then a test is done in step  209  to determine if there are any first BMs (BMs whose initial GBF was greater than zero) whose GBF is now zero that are requesting an access. If the result of the test in step  209  is YES, then step  203  is again executed and first BMs with GBFs equal to zero are given an opportunity to have additional access requests granted. If the result of the test in step  209  is NO, then step  206  is again executed awaiting time out of the present BI. 
     FIG. 3  is a flow diagram  300  detailing method steps in another embodiment of the present invention. In step  301 , each BM managing access to a bus is assigned a GBF. The number of clock cycles (n cycles) for each access time determining a bus interval (BI) is also set. In step  302 , a test is done to determine if there are any first BMs requesting bus access. If the result of the test in step  302  is YES, then in step  303  a bus access is granted to a selected first BM using a fair arbitration starting with the first BMs with the highest GBF. The GBF of the selected first BM is decremented by one if the GBF is greater than zero. A branch is then taken to step  306  where a test is done to determine if a present BI has expired. The branches taken after the test in step  306  will be explained following the explanation of steps leading to step  305  which also leads to step  306 . 
   If the result of the test in step  302  is NO, then there are no first BMs with a GBF greater than zero. A test is done in step  304  to determine whether there are any second BMs requesting access. If the result of the test in step  304  is YES, then in step  305  a selected second BM is granted access using fair arbitration starting at a “frozen” sequence status of the arbiter. If this is the first time second BMs have been arbitrated for access, then the frozen sequence status is the initial round robin location, otherwise it is the arbitration sequence status stored from the previous arbitration of second BMs preceding expiration of the corresponding BI. 
   In step  306 , a test is done to determine if a present BI has expired. If the result of the test in step  306  is YES, then in step  307  an error is flagged if requesting BMs did not get their requests serviced. The requesting BMs could be first BMs that did not get a request serviced or a second BM that never got a request serviced. In step  310 , a test is done to determine if errors have been flagged over successive m BIs. If the result of the test in step  310  is YES, then the BI is extended or selected GBFs are modified in step  311  and step  312  is then executed. If the result of test in step  310  is NO, step  312  is executed next. In step  312 , the arbiter sequence status resulting from arbitrating access for the second BMs is frozen and all the GBFs are reset to their initial programmed value. A branch is then taken back to step  302 . If the result of the test in step  306  is NO, then a branch is taken directly back to step  302 . 
   If the result of the test in step  304  is NO, then a test is done in step  313  to determine if there are any first BMs whose GBF has been decremented to zero that are requesting an access. If the result of the test in step  313  is YES, then a branch is taken back to step  303  and first BMs with GBFs equal to zero (there no requesting first BMs with GBFs greater than zero) are given an opportunity to have additional access requests granted. If the result of the test in step  313  is NO, then step  306  is again executed awaiting time-out of the present BI. 
     FIG. 4A  is a block diagram of the bus access time partitioned into BIs  401 – 406 . In this particular example, each BI is partitioned into an arbitrary 100 clock cycles. 
     FIG. 4B  is a block diagram of BIs  407 – 409  where the access time granted to each BM corresponds to the value of the index M (short for Master) where M 0  corresponds to BM 0 , M 1  corresponds to BM 1 , M 2  corresponds to BM 2 , M 3  corresponds to BM 3  and Mn corresponds to one or more BMs with a zero guaranteed access time. For example, let M 0 =3, M 1 =1, M 2 =1, M 3 =1 and Mn=0. To insure these relative guaranteed access times (M 0 −Mn), the BM would be assigned corresponding GBFs wherein the highest GBF would go to the BM with the highest guaranteed access time. In this example, the BI (e.g., each of BI 0 –BIn) has 8 access periods. In this example, all BMs are assumed to be continuously requesting a bus access). M 0  has the highest GBF and was granted the first access. The GBF of M 0  will then be decremented. In the second period, M 0  still has the highest GBF but is not requesting an access so the arbiter chooses from among M 1 –Mn. Of the remaining BMs, M 1 –M 3  have the same priority. Since they all are requesting, the arbiter would pick the first one polled by a fair arbitration. M 1 , M 2 , and M 3  are sequentially granted a request and their GBFs are decremented by one to zero. In the fifth period M 0  is again requesting; its GBF is 2 so it is the highest priority regardless of other requests so it is granted the next access and its GBF is decremented to 1. In the sixth period M 0  is still the highest priority and is again granted an access and it GBF is decremented to zero. In the seventh and eighth periods all the BMs (M 0 –Mn) have the same GBF of zero. In this case, the arbiter considers all the remaining BMs as equals, including Mn which had no guaranteed access time. In the seventh and eighth period, Mn is the only requesting BM that had an initial GBF equal to zero and it is granted access in both these time periods. BI 0 –BIn are shown to have the same granted accesses to each BM. This would only occur if, during each B, the BMs had exactly the same bus requests during each of the illustrated BI time periods. 
     FIG. 4C  is a diagram of three BIs, BI 1 –BI 3 , illustrating an embodiment of the present invention using a frozen arbiter sequence status. For this example, all of the BMs are assumed to be continuously requesting a bus access to simplify the explanation. BM 0 –BM 5 , have initial GBFs corresponding to GBF 0 =3, GBF 1 =2, GBF 2 =1, GBF 3 =0, GBF 4 =0, and GBF 5 =0. The diagram row headings correspond to initial GBFs and the column headings correspond to which BM is granted an access during a particular access time. The individual cell values indicate the value of the GBFs during an access time. Also, a cell that has a “G” indicates that this particular BM was granted the access during the access time. For example, in BI 1  access time one, BM 0  is granted an access and its GBF is equal to three before being decremented. The GBFs of BM 1 –BM 5  are 2, 1, 0, 0, and 0 respectively. In access time two, the GBF 0  has been decremented to two and BMI which now has the same priority as BM 0  is granted the access by fair arbitration. Fair arbitration continues among the BMs with the highest GBFs until all GBFs are equal to zero. In BI 1 , this occurs in access time  7 . At access time  7 , all the BMs have equal GBF as the BMs which had initial GBFs greater than zero have had there guaranteed access times fulfilled. 
   In access time  7 , the arbiter switches to the BMs which did not have guaranteed access times (BM 3 –BM 5  had GBFs equal to zero). The arbiter starts polling starting with a frozen previous sequence status. Since this is the first BI and there is no previous sequence status and thus the frozen status is the present initial status. Since it was assumed that all BMs were continuously requesting access, BM 3  is granted an access during access time  7  and then BM 4  is granted an access in access time  8 . After access time  8 , a new BI 2  starts (BI 1  ended) and all GBFs are reset to their initial programmed values and the sequence status of granting accesses for the BMs with an initial GBF equal to zero is “frozen” becoming the frozen sequence status. Now in BI 2  period one, BM 0  is again the highest priority and accesses are granted in accordance with the sequence described for BI 1 . This is true because of the assumed condition that all BMs are continuously requesting a bus access. As with BI 1 , all of the guaranteed bus access times are fulfilled at access time six. In BI 1 , BM 5  did not get its request serviced. Because the sequence status of the arbiter relative to the BMs with an initial GBF equal to zero was frozen from BI 1 , BM 5  is given priority during BI 2  and its request is serviced first after the guaranteed access times for BM 0 –BM 2  have been fulfilled. At the end of BI 2 , BM 3  was the last bus request granted of the BMs with initial GBFs equal to zero. Freezing the arbiter status after BI 2  guarantees that BM 4  will have priority in BI 3  after the guaranteed access times for BM 0 –BM 2  have been fulfilled.  FIG. 4C  illustrates how the BMs without guaranteed access times all have been granted two accesses over the three bus intervals BI 1 –BI 3 . 
   In  FIG. 4C , it took six access time periods for the guaranteed access times for BM 0 –BM 2  to be fulfilled. If each of the bus intervals BI 1 –BI 3  were only six time periods (not shown), none of the requesting BMs with a GBF equal to zero would have ever had their pending access requests granted. In one embodiment of the present invention, an error would be generated and stored when this happened. A programmed number m of BIs would determine if any action is taken based on the error condition. For example, with m equal to one, the BI may be extended to enable more access times in a BI when during a single BI a requesting BM with a GBF equal to zero did not get its bus access request serviced. Also, the GBFs of selected BMs may be modified to give better balance among requesting BMs. An m greater than one would require more than one successive BIs with an error condition before an BI extension or GBF modification was implemented. In another embodiment, an error condition may also be flagged if a BM with a GBF greater than zero was not getting serviced to its guaranteed access time during m successive BIs. Again in this case, the BI may be dynamically extended or selected GBFs may be modified to insure bandwidth guarantees are met. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.