COE 758 Lecture Notes - Lecture 8: Bus Mastering, Memory Bus

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Bus system is used to connect i/O device to processor
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Only one device at a time is allowed to be bus master
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Hardware
Protocol scheme to use
We must use arbitration
Each device must request the bus (bus request)
1.
Arbitor based on a priority select, select one device that has the highest priority and generates a grant
for this device
2.
When device is finished using the bus, it must release bus request
3.
Steps to arbitrate for the bus:
Must give the lowest priority device a chance to use the bus
Processor is always given grant to use the bus, always by a bus Master
Processor memory bus:
What should you do if multiple devices require the bus at the same time?
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IO communication with processor
Daisy Chain (first type of arbiration)
1.
Step 1. Device requests the bus by placing request signal active
Step 2. If arbiter receives request, it generates one grant
Step 3. If device1 is not requesting in b us, it parses grant to next device, until it reaches the requesting
device and will not be forwarded to next
Step 4. When it finishes it will release the request, and generate release signal
Advantages: simple, which device with highest priority will be the first device
Last device may be prevented from having a chance to use the bus
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Problem: slow as it needs to ripple through each device
Centrilized Arbiter (2nd type of arbitration)2.
Lecture 8
Monday, 12 November 2018 09:08 AM
Lectures Page 1
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