Which method has the fastest response time in centralized bus arbitration?

In the centralized bus arbitration, the independent request mode has the fastest response time; the control logic of the centralized bus arbitration is basically concentrated in one place and requires a central arbiter. The independent request mode is because each device sharing the bus has a For the bus request line BRi and the bus grant line BGi, the response time is fast, and it takes less time to determine the device that responds first, and there is no need to query one device after another.

The operating environment of this tutorial: Windows 10 system, DELL G3 computer.

Which method has the fastest response time in centralized bus arbitration

Among the three centralized bus arbitration methods, the independent request method has the fastest response time, because it is a peripheral Independent request to the CPU

Bus arbitration. Multiple devices or modules in the system may apply for the right to use the bus at the same time. In order to avoid bus conflicts, the bus arbitration mechanism needs to be reasonably Applicants who need to occupy the bus in the local control and management system. When multiple applicants make bus requests at the same time, a certain priority algorithm is used to arbitrate which one should obtain the right to use the bus.

According to the location of the bus arbitration circuit, the arbitration method is divided into two categories: centralized arbitration and distributed arbitration:

Centralized bus arbitration

The control logic of centralized bus arbitration is basically concentrated in one place and requires a central arbiter. It is divided into chain query mode and counter timing query mode. , Independent request method;

(1) Chain query method

The main features of the chain query method: The bus authorization signal BG is serially transmitted from an I/O interface is passed to the next I/O interface. If the interface where the BG arrives has no bus request, the query continues; if the interface where the BG arrives has a bus request, the BG signal will no longer query downward, and the I/O interface obtains bus control. The device closest to the central arbiter has the highest priority, achieved through the interface's priority queuing circuitry.

Advantages of the chain query method: Only a few lines can be used to achieve bus arbitration according to a certain priority, and it is easy to expand the equipment.

Disadvantages of the chain query method: It is very sensitive to circuit faults in the query chain. If there is a fault in the circuit of the chain in the interface of the i-th device, then the devices after the i-th device will not be able to work. The priority of the query chain is fixed. If a device with a high priority receives frequent requests, a device with a lower priority may not be able to use the bus for a long time.

(2) Counter timing query method

When any device on the bus requests to use the bus, it sends a bus request through the BR line. After the central arbiter receives the request signal, the counter starts counting when the BS line is "0", and the count value is sent to each device through a set of address lines. Each device interface has a device address discrimination circuit. When the count value on the address line is consistent with the device address requesting the bus, the device sets the BS line "1" and obtains the right to use the bus. At this time, the count query is terminated.

Each count can start from "0" or from the stop point. If it starts from "0", the priority of each device is the same as the chain query method, and the priority order is fixed. If starting from the stopping point, each device uses the bus with equal priority.

The initial value of the counter can also be set by program, which can easily change the priority order, but this flexibility comes at the expense of increasing the number of lines.

(3) Independent request mode

Each device sharing the bus has a pair of bus request lines BRi and bus authorization lines BGi. When a device requires the use of the bus, the device's request signal is sent. The queuing circuit in the central arbiter decides which device's request should be responded to first and gives the device an authorization signal BGi.

Advantages of the independent request method: The response time is fast, and it takes less time to determine the device that responds first. There is no need to query one device after another. Secondly, the control of priority is quite flexible. It can be fixed in advance or changed through program; you can also block (forbid) a certain request and not respond to requests from invalid devices.

Extended knowledge

Distributed arbitration

Distributed arbitration There is no need for a central arbiter, each potential master function module has its own arbitration number and arbiter. When they have a bus request, they send their unique arbitration numbers to the shared arbitration bus, and each arbiter compares the number obtained on the arbitration bus with its own number. If the number on the arbitration bus is large, its bus request will not be responded to and its arbitration number will be revoked. Finally, the winner's arbitration number remains on the arbitration bus. Obviously, distributed arbitration is based on the priority arbitration strategy

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