🪴 TJ's Notes 1.0

Daisy Chaining Dry Run

3 min read

Daisy Chaining method

Assumptions:

  • We have 4 devices: Device 1, Device 2, Device 3, Device 4, and a Bus Arbiter.
  • Device 3 is the one requesting bus access, and we’ll see how the arbitration works.

Initial Setup:

  • All devices are initially in a waiting state for the bus (no device has access yet).
  • The Bus Arbiter manages bus access requests.

Step-by-Step Process:

  1. Device 3 Requests Access (BRQ):

    • Device 3 needs access to the bus. It sends a Bus Request (BRQ) to the Bus Arbiter via the chain.
    • The Bus Request travels through Device 4 → Device 3 → Device 2 → Device 1 → Bus Arbiter.

  2. Bus Arbiter Receives the Request:

    • The Bus Arbiter checks the chain and sees that Device 3 is requesting access. It then sends a Bus Grant (BGT) signal back to Device 3.
    • Device 3 is granted permission to use the bus.

  3. Bus Grant Propagates Back Through the Chain:

    • The Bus Grant (BGT) travels from the Bus Arbiter to Device 1 → Device 2 → Device 3.
    • Now Device 3 is allowed to use the bus. It starts transmitting its data or performing the task it needs on the bus.

  4. Device 3 is Using the Bus:

    • While Device 3 is using the bus, Device 1, Device 2, and Device 4 are idle. They cannot use the bus unless the Bus Arbiter grants them access after Device 3 releases it.

  5. Device 3 Finishes Using the Bus and Releases the Bus (SACK):

    • Once Device 3 is done, it sends a Bus Acknowledgment (SACK) signal to the Bus Arbiter indicating it has completed its task.
    • This releases the bus for other devices to use.

  6. Next Device Requests the Bus:

    • Let’s say Device 2 is the next one to need the bus. It will now send a Bus Request (BRQ) signal.
    • The request is again passed along the chain: Device 4 → Device 3 → Device 2 → Bus Arbiter.
    • The Bus Arbiter sees Device 2’s request and sends a Bus Grant (BGT) signal, allowing Device 2 to use the bus.

  7. Device 2 Uses the Bus:

    • Device 2 now uses the bus, while Device 1 and Device 3 are idle until their turn.

  8. Cycle Repeats:

    • The process repeats itself for each device in the chain. The device requesting access is granted the bus access, and after finishing, the bus is released.

Visualizing the Chain (Flow):

  • Device 3 (Request) → Device 4 → Device 2 → Device 1 → Bus Arbiter (Grant) → Device 1 → Device 2 → Device 3 → Device 4 → Device 1 (Access complete)

This is how the Daisy Chained Bus Arbitration allows multiple devices to take turns accessing the bus without conflict.

Graph View

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