Method and apparatus for accelerating arbitration in a serial bus by detection of acknowledge packets

• US 5,495,481 A
• Filed: 09/30/1994
• Issued: 02/27/1996
• Est. Priority Date: 09/30/1994
• Status: Expired due to Term

• Current Assignee: Apple Inc.
• Original Assignee: Apple Computer Incorporated (Apple Inc.)
• Inventors: Duckwall, William S.
• Primary Examiner(s): MARCELO, MELVIN C
• Application Number:
  US08/316,552
• Time in Patent Office: 515 Days
• Field of Search: 370/82, 370/85.1, 370/85.2, 370/85.6, 370/94.1
• US Class Current: 370/462
• CPC Class Codes: G06F 13/36 for access to common bus or bus system
• CPC Class Codes: G06F 13/368 with decentralised access control
• CPC Class Codes: H04L 12/40084 Bus arbitration

Abstract

Methods and circuitry for arbitrating for control of a serial bus are described. According to one embodiment, one or more nodes of a serial bus are provided with a mechanism for discriminating between data packets and acknowledge packets. If a packet transmitted, repeated, or received by the node is a data packet, the node remains idle for a subaction gap time T_sa to better ensure that the expected acknowledge packet is allowed to successfully propagate throughout the serial bus to the source node. If the packet transmitted by the node is an acknowledge packet, the node is free to begin the arbitration phase of the next subaction if there are no other conditions that prevent further arbitration by that node. To discriminate between data packets and acknowledge packets, a counter is used to determine the length of a transmitted packet, and the length is compared to the expected length of an acknowledge packet. If the length is equal to the expected length of an acknowledge packet, the packet is an acknowledge packet.

Claims

1. In an electronics system, a method for arbitrating for control of a bus by a first node of the bus comprising the steps of:

detecting a packet on the bus by the first node;

determining a length of the packet;

comparing the length of the packet to a first length;

requesting control of the bus by the first node before a predetermined length of time has elapsed if comparing the length of the packet to the first length indicates that the packet is a first type of packet; and

requesting control of the bus by the first node after the predetermined length of time has elapsed if comparing the length of the packet to the first length indicates that the packet is a second type of packet.

2. The method of claim 1, wherein the method further comprises the step of the first node initially receiving the packet from a second node via the bus.

3. The method of claim 1, wherein the method further comprises the steps of:

initially requesting control of the bus by the first node to transmit the packet; and

granting control of the bus to the first node such that the first node may transmit the packet on the bus.

4. The method of claim 1, wherein the packet is an acknowledge packet, the method further comprising the steps of:

initially receiving a data packet addressed to the first node; and

generating the acknowledge packet.

5. The method of claim 1, wherein comparing the length of the packet to the first length indicates that the packet is a first type of packet if the length of the packet is equal to the first length.

6. The method of claim 1, wherein comparing the length of the packet to the first length indicates that the packet is a first type of packet if the length of the packet is less than the first length.

7. In an electronics system including a serial bus, a method for arbitrating for control of the serial bus by a node of the serial bus comprising the steps of:

detecting a packet on the serial bus by the node;

determining a length of the packet by the node;

comparing the length of the packet to a first length by the node;

requesting control of the serial bus by the first node before a subaction gap time has elapsed if comparing the length of the packet to the first length indicates that the packet is an acknowledge packet; and

requesting control of the serial bus by the first node after the subaction gap time has elapsed if comparing the length of the packet to the first length indicates that the packet is a data packet.

8. The method of claim 7, wherein comparing the length of the packet to the first length indicates that the packet is an acknowledge packet if the length of the packet is equal to the first length.

9. The method of claim 7, wherein comparing the length of the packet to the first length indicates that the packet is an acknowledge packet if the length of the packet is less than the first length.

10. In a bus having a plurality of nodes, a node comprising:

at least one port coupled to transmit and receive data on the bus;

a node interconnect coupled to the port, the node interconnect being operative to transfer packets between the port and a local host of the node;

a counter coupled to the node interconnect, the counter being operative to determine a length of a packet transmitted via the node interconnect;

an arbiter coupled to the counter and the node interconnect, the arbiter being operative to compare the length of the packet as determined by the counter to a first length, wherein the arbiter is operative to request control of the bus before a predetermined length of time has elapsed if comparing the length of the packet to the first length indicates that the packet is a first type of packet, and wherein the arbiter is operative to request control of the bus after the predetermined length of time has elapsed if comparing the length of the packet to the first length indicates that the packet is a second type of packet.

11. The node of claim 10, wherein comparing the length of the packet to the first length indicates that the packet is a first type of packet if the length of the packet is equal to the first length.

12. The node of claim 10, wherein comparing the length of the packet to the first length indicates that the packet is a first type of packet if the length of the packet is less than the first length.

13. A serial bus comprising:

a plurality of nodes;

a plurality of point-to-point links, wherein each point-to-point link is coupled between two nodes in a manner such that each node is coupled to every other node via one or more point-to-point links,

wherein at least one node of the plurality of nodes is an accelerated arbitration node comprising;

at least one port coupled to a point-to-point link and operative to transmit and receive data via the point-to-point link;

a node interconnect coupled to the port, the node interconnect being operative to transfer packets between the port and a local host of the accelerated arbitration node;

a counter coupled to the node interconnect, the counter being operative to determine a length of a packet transmitted on the serial bus by the accelerated arbitration node via the node interconnect;

an arbiter coupled to the counter and the node interconnect, the arbiter being operative to compare the length of the packet as determined by the counter to a first length, wherein the arbiter is operative to request control of the bus before a predetermined length of time has elapsed if comparing the length of the packet to the first length indicates that the packet is an acknowledge packet, and wherein the arbiter is operative to request control of the bus after the predetermined length of time has elapsed if comparing the length of the packet to the first length indicates that the packet is a data packet.

14. The serial bus of claim 13, wherein comparing the length of the packet to the first length indicates that the packet is an acknowledge packet if the length of the packet is equal to the first length.

15. The serial bus of claim 13, wherein comparing the length of the packet to the first length indicates that the packet is an acknowledge packet if the length of the packet is less than the first length.