Abstract:
The present invention provides a method of arbitration for resources which allows requestors from multiple frequency domains. Most requestors generate requests at full speed. A small number of low-speed requesters generate requests every two full-speed cycles, and hold their requests for two full-speed cycles. The arbitration method gives priority to the requests from the low-priority requesters and guarantees that two requests made by the half-speed requestors at the beginning of a low-speed cycle will be granted over the course of the low-speed cycle. The requests generated by the low-speed requestors are issued in phases. Issuance of later phases of a request is blocked when the request has been granted in an earlier phase.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to the allocation of resources in a processor and, more particularly, to a method of arbitration which allows requesters from multiple frequency domains and allows simultaneous grants for the lower-frequency requesters.  
       BACKGROUND  
       [0002]     When several devices make conflicting requests for shared resources, the requests must be arbitrated to determine allocation of the resources. For example, when a bus is connected to a cache, cache allocate and snoop requests may be given priority for some applications and designs. For some of these designs, these requests can arrive simultaneously. This arbitration priority is application specific and other designs may choose to have a different priority.  
         [0003]     Self-arbitration can be used to select among high-priority requests. For example, when there is both a reload and snoop request on the bus, one can be chosen. This method delays the other request until the first request is completed, which may be a matter of several cycles. Further, a self arbitration would add an additional arbitration point and make the requestors aware of the arbitration.  
         [0004]     Therefore, there is a need for a method of arbitration for giving priority to high-priority requests, but with a lessened delay to the high-priority requests that are not selected.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a method of arbitration for resources which allows requesters from multiple frequency domains. Most requesters generate requests at full speed. A small number of low-speed requestors generate requests every two full-speed cycles, and hold their requests for two full-speed cycles. The arbitration method gives priority to the requests from the low-priority requesters and guarantees that two requests made by the half-speed requestors at the beginning of a low-speed cycle will be granted over the course of the low-speed cycle. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     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:  
         [0007]      FIG. 1  shows a flow diagram illustrating the processing of requests for arbitration from full-speed requestors and slow speed requesters;  
         [0008]      FIG. 2A  shows a flow diagram illustrating the timing of the processing of requests for arbitration when there are two slow speed requests during a slow speed cycle; and  
         [0009]      FIG. 2B  shows a flow diagram illustrating the timing of the processing of requests for arbitration when there is one slow speed request during the first low-speed cycle. 
     
    
     DETAILED DESCRIPTION  
       [0010]     In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail.  
         [0011]     It is further noted that, unless indicated otherwise, all functions described herein may be performed in either hardware or software, or some combination thereof. In a preferred embodiment, however, the functions are performed by a processor such as a computer or an electronic data processor in accordance with code such as computer program code, software, and/or integrated circuits that are coded to perform such functions, unless indicated otherwise.  
         [0012]      FIG. 1  shows a flow diagram illustrating the processing of requests for arbitration from full-speed requesters and slow speed requestors. The half-speed requesters generate requests every two full-speed cycles. Requests A  102 , B  104 , and C  106  through N  108  arrive for arbitration. Requests A  102  and B  104  are from half-speed requesters. The requests are active through two full speed cycles, and consist of two phases, one per cycle. The first phase, during the first full-speed cycle, is a low-priority request. The second phase, during the second full-speed cycle, is a high-priority request. It is blocked if the low-priority request was granted during the previous cycle.  
         [0013]     Requests A  102  and B  104  pass to the request blocking logics  110  and  112 . The request blocking logics  110  and  112  transmit low priority requests to the arbiter  114  during the first full-speed cycle of the request. Request A  102  is granted during the low-priority phase. During the second full-speed cycle of the request, the blocking logic  110  does not issue a high-priority request for request  102 . Request B  104  was not granted during the low-priority phase. During the second full-speed cycle of the request, the blocking logic  112  issues the high-priority request phase for request  102 , and the arbiter  114  grants it.  
         [0014]     The arbiter  114  always prefers the low-speed requesters. If there are two low-priority requests during the first phase of the two-phase low-speed request cycle, the arbiter  114  grants one of them. If there is one request from a low-speed requester, either a low-priority request during the first phase of the two-phase low-speed request cycle or a high-priority request during the second phase of the two-phase low-speed request cycle, then the arbiter  114  grants it. If there are requests from the high-speed requestors but not from the low-speed requesters, then the arbiter  114  grants one of the requests from the high-speed requesters.  
         [0015]     This method of arbitration guarantees that the two requests A  102  and B  104  from the low-speed requestors win arbitration, even if both are issued at the same time. One of the requests A  102  and B  104  will be granted during the first-phase of the two-phase low-speed request cycle, as a low-priority request. The other request will issued as a high-speed request during the second phase of the cycle, and will be granted. The request granted during the first phase of the two-phase cycle will not be issued as a high-speed request during the second cycle.  
         [0016]     It further guarantees that all of the requests are granted within one low-speed cycle. Further, there is only one arbitration point, and this method allows the high-priority requestors to be totally unaware of any arbitration point at all. Provided that a resource is available whenever a requestor wins arbitration, the requests A  102  and B  104  do not require a grant, because they are always guaranteed to win arbitration at their half-speed domain, and have a resource available. Since they are guaranteed to win arbitration, these high-priority requesters can be designed to send their requests up without waiting for a grant from arbitration.  
         [0017]      FIG. 2A  shows a flow diagram illustrating the timing of the processing of requests for arbitration when there are two slow speed requests during a slow speed cycle. During the first full-speed cycle, low-speed requesters issue requests A and B and high-speed requesters issue requests C and D. The requests A and B go to the blocking logics  110  and  112  and generate low-priority requests. The arbiter  114  grants one of them, in this case, request A. During full-speed cycle 2, the request B goes to the blocking logic  112 , is generated as a high-priority request, and is granted by the arbiter  114 . The blocking logic  110  blocks the high-priority request phase from request A. During cycle 3, the arbiter grants request C, from a high-speed requester. During cycle 4, the arbiter grants request d, from a high-speed requester. Under this method, requests from the low-speed requesters are given priority over the requests from the high-speed requesters. The requests A and B from low-speed requestors are granted before the requests C and D from the high-speed requestors. Further, both requests, A and B, from the low speed requesters are granted during a single low-speed cycle.  
         [0018]      FIG. 2B  shows a flow diagram illustrating the timing of the processing of requests for arbitration when there is one slow speed request during the first low-speed cycle. During the first full-speed cycle, a low-speed requesters issue requests A and high-speed requesters issue requests C and D. The request A goes to a blocking logic, which generates a low-priority request, granted by the arbiter  114 . During the second full-speed cycle, the second half of the half-speed cycle, the blocking logic blocks the high-priority phase of request A. The arbiter grants request C, from a high-speed requester C. During cycle 3, request B from a low-speed requester is issued as a low-priority request by a blocking logic and granted by the arbiter  114 . During cycle 4, the arbiter grants request D from a high-speed requestor. The blocking logic blocks the high-priority phase of request B, because the low-priority request issued during the first phase of the low-speed cycle was granted. Thus, requests A, C, B, and D were granted in sequence. The requests from the low-speed requestors were given priority over the requests from the high-speed requesters, when there are requests from both speeds of requesters during the same cycle.  
         [0019]     In an alternative embodiment, the low-speed requesters operate at one-third speed. Low-speed requests are issued every three high-speed cycles and last for three high-speed cycles. The low-speed requests consist of three phases. The blocking logics block requests during the latter phases when they had been granted during earlier phases. Again, the requests from low-speed requestors are given priority over the requests from high-speed requesters. Three requests made during a low-speed cycle are guaranteed to be granted.  
         [0020]     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. Many such variations and modifications may be considered desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.