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

# Priority arbitration vs daisy chain

Embedded System · Chapter 5: Interfacing

- Asked in: 2074 Bhadra, 5 marks
- Video: [Daisy Chain, Polling & Independent Request bus contention methods | Bus Contention & Bus Arbitration](https://www.youtube.com/watch?v=YSt_47u6rvc) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/embedded_system/priority-arbitration-vs-daisy
- Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-5

## Question

Describe priority arbitration method and compare it with daisy-chain arbitration.

## Short answer

Priority arbitration employs a central arbiter with separae Req/Ack lines per device, granting the bus to the highest-priority requester (fixed or roating). Compared to dasy-chain, it offers factory, programmable priority and better fault tolerane but needs more wires. Dasy-chain uses fewer wires and fixed position-based priority but is slower and less fault-tolerant.

## Answer

#### Priority arbitration
In priority arbitration, a single arbiter circuit has one request line (Req) and one grant line (Ack) per peripheral. When several devices request the bus, the arbiter selects the one with the highest priority, according to a fixed order or a rotating (round-robin) order, and asserts its Ack. The device uses the bus and then drops Req, and the arbiter selects the next device.

```text
 Req1 ->|          |-> Ack1
 Req2 ->| Arbiter  |-> Ack2
 Req3 ->|          |-> Ack3
```

#### Comparison with daisy-chain arbitration
| Point | Priority arbitration | Daisy-chain arbitration |
|---|---|---|
| Lines | Separate Req and Ack for each device | One common Req; Grant passes through devices in series |
| Priority decided by | Arbiter logic (fixed or rotating) | Physical position in the chain |
| Wiring | More (2 per device) | Less (2 plus chain) |
| Flexibility | Priority can be changed or rotated | Fixed, changes only by moving devices |
| Fairness | Possible with rotating priority | Poor; far devices may starve |
| Speed | Fast, grants decided in parallel | Slower; grant ripples through devices |
| Expansion | Needs more arbiter inputs | Easy to add devices at the end |
| Fault tolerance | One device failure does not block others | A broken link blocks all devices after it |

## Check yourself

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

1. In priority arbitration, how does the arbiter decide who gets the bus?
   A. Grants the bus to the slowest device first
   B. Grants the device physically closest in a chain
   C. Lets all devices transmit and detects collisions
   D. Grants Ack to the highest-priority pending Req (fixed or rotating order)
   Answer: D. The arbiter picks the highest-priority requester using a fixed or rotating order and asserts its Ack. (Key point: Priority arbiter operation)

2. Which statement about lines is correct?
   A. Priority arbitration uses separate Req/Ack per device; daisy chain uses one common Req with grant passed in series
   B. Both use only a single line
   C. Priority arbitration uses one common Req; daisy chain uses separate pairs
   D. Daisy chain uses more wires than priority arbitration
   Answer: A. Priority arbitration needs more wiring (2 per device); daisy chain needs fewer lines. (Key point: Wiring comparison)

3. Why is priority arbitration more flexible than daisy chain?
   A. It needs fewer wires
   B. Its grant ripples through each device
   C. It has a lower hardware cost
   D. Priority can be changed or rotated in arbiter logic, while in a daisy chain it depends on physical position
   Answer: D. Daisy-chain priority changes only by moving devices; rotating priority also gives fairness. (Key point: Fairness and flexibility)

4. Which is a disadvantage of daisy-chain arbitration compared with priority arbitration?
   A. It cannot add devices
   B. Grants are decided in parallel
   C. A broken link blocks all devices after it, and the grant ripples slowly
   D. It needs a complex arbiter
   Answer: C. Grant ripples serially (slow) and a broken link blocks the following devices. (Key point: Speed and fault tolerance)

## Related in Embedded System

- [Priority and daisy-chain arbitration briefly](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aarbitration-priority-daisy-brief) — 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×