---
title: "Priority and daisy-chain arbitration briefly"
subject: "Embedded System"
chapter: 5
key: "semester_6/embedded_system:arbitration-priority-daisy-brief"
source: https://www.bctengineer.com/q/semester_6/embedded_system/arbitration-priority-daisy-brief
site: BCT Engineering
---

# Priority and daisy-chain arbitration briefly

Embedded System · Chapter 5: Interfacing

- Asked in: 2073 Bhadra, 8 marks
- Video: [BUS and BUS Arbitration in  Hindi | COA | Computer Organization and Architecture Lectures](https://www.youtube.com/watch?v=7pUhkzVI6mA) — Last moment tuitions
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/embedded_system/arbitration-priority-daisy-brief
- Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-5

## Question

What is arbitration? Explain Priority Arbitration and Daisy Chain Arbitration in brief.

## Short answer

Arbitration decides which requester gets the bus. Priority arbtration uses a central arbiter with individual Req/Ack lines, granting the highest-priority request (fixed or roating). Daisy-chain shares one Req line; the Grant passes serially, and the first requesting device in the chain captures it, with priority set by position.

## Answer

**Arbitration** is the process of deciding which of several devices requesting the shared system bus at the same time gets it. Only one master can use the bus at a time, so an arbiter grants access based on a rule.

#### Priority arbitration
- A central **priority arbiter** has a separate Req/Ack pair for each device.
- Devices assert Req; the arbiter grants Ack to the device with the highest priority among pending requests.
- Priority can be **fixed** (permanent rank) or **rotating** (round-robin, for fairness).
```text
 Req1,Req2,Req3 --> [Priority arbiter] --> Ack1,Ack2,Ack3
```
- Merits: fast, programmable priority. Demerits: more wires, complex arbiter, starvation with fixed priority.

#### Daisy-chain arbitration
- All devices share one Req line; the grant (Ack) signal passes serially through the devices.
- A device that did not request passes the grant on; a requesting device keeps it and uses the bus. Priority is set by position (nearest to the arbiter is highest).
```text
 Arbiter --Grant--> Dev1 --> Dev2 --> Dev3
    ^--------------- Req (common) ----------
```
- Merits: simple, few wires, easy to extend. Demerits: fixed priority, slow grant propagation, chain breaks if a device fails.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is arbitration in a bus system?
   A. Allocating addresses to memory chips
   B. Detecting errors in transferred data
   C. Converting between serial and parallel data
   D. Deciding which of several simultaneous requesters gets the shared bus
   Answer: D. Only one master can use the bus at a time, so the arbiter decides who gets it by a rule. (Key point: Meaning of arbitration)

2. How is a central priority arbiter connected to the devices?
   A. A single bidirectional line
   B. One shared Req line and a serial grant
   C. A separate Req/Ack pair for each device
   D. Only an Ack line per device
   Answer: C. Each device has its own Req and Ack with the arbiter, which grants the highest-priority pending request. (Key point: Priority arbiter wiring)

3. In daisy-chain arbitration, what does a device do with the grant signal if it did not request the bus?
   A. Returns it to the arbiter and cancels it
   B. Keeps it for later
   C. Blocks it
   D. Passes it on to the next device
   Answer: D. Non-requesting devices pass the grant on; the first requesting device in the chain keeps it. (Key point: Daisy-chain grant passing)

4. In a daisy chain, what determines priority and what is a demerit?
   A. Position nearest the arbiter is highest; chain breaks if a device fails
   B. Random choice; slow clock
   C. Round-robin order; very complex arbiter
   D. Device ID; needs many wires
   Answer: A. Priority is by position, which is fixed, and a failed device breaks the chain; grant propagation is also slow. (Key point: Daisy-chain priority and demerits)

## Related in Embedded System

- [Priority arbitration vs daisy chain](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apriority-arbitration-vs-daisy) — Embedded System, chapter 5
- [Daisy-chain arbitration](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Adaisy-chain-arbitration) — Embedded System, chapter 5, asked 5×
- [Priority arbitration](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apriority-arbitration) — Embedded System, chapter 5, asked 2×