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

# Priority arbitration

Embedded System · Chapter 5: Interfacing

- Asked in: 2075 Baisakh, 2+6 marks; 2079 Asoj, 5 marks (asked 2 times)
- 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/priority-arbitration
- Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-5

## Question

What is arbitration? Explain priority arbitration with the help of a block diagram and steps (along with its types).

## Short answer

Priority arbitration uses a central arbiter with per-device request and grant lines. The arbiter grants the bus to the highest-priority requesting device. After the transfer, the device releases the bus; the arbiter serves the next pending request. Priority can be fixed (simple, may starve) or rotating (fairness).

## Answer

**Arbitration** is the method of deciding which one of several peripherals (or processors) requesting the system bus at the same time gets control of it. It is needed because only one master can drive the bus at a time.

#### Priority arbitration
A single **priority arbiter** hardware unit is connected to the bus masters. Each peripheral has its own request line (Req) to the arbiter and its own grant line (Ack) back from the arbiter. The arbiter decides according to a priority order.

```text
        +---------------------+
 Req1 ->|                     |-> Ack1
 Req2 ->|   Priority arbiter  |-> Ack2
 Req3 ->|                     |-> Ack3
 ...    |                     |
        +----------+----------+
              Req  |  Ack
                   v
              Processor / bus
```

#### Steps
1. A peripheral needing the bus asserts its Req line to the arbiter.
2. The arbiter checks all active requests and chooses the one with the highest priority.
3. The arbiter asserts the Ack line of the chosen peripheral (and a bus request to the processor if the CPU owns the bus).
4. The peripheral uses the bus for its transfer, then drops Req.
5. The arbiter removes Ack, and selects the next highest-priority pending request.

#### Types of priority
- **Fixed priority:** each device has a permanent rank, e.g. Req1 > Req2 > Req3. Simple, but low-priority devices may starve.
- **Rotating priority (round robin):** priority changes after each grant, e.g. the device just served becomes lowest. Fair to all devices.
- Related method: **daisy-chain arbitration**, where priority is decided by position along a grant chain.

**Advantages:** fast, fair if rotating, and the priority can be programmed. **Disadvantages:** one request/grant pair per device (more wires) and a more complex arbiter.

## Check yourself

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

1. What is arbitration?
   A. Deciding which of several requesting masters gets the bus
   B. Refreshing memory
   C. Decoding addresses
   D. Converting data formats
   Answer: A. Only one master can drive the bus at a time, so arbitration picks one. (Key point: Arbitration meaning)

2. How does a priority arbiter connect to devices?
   A. Each has its own Req and Ack line
   B. One shared line
   C. A chain
   D. No lines
   Answer: A. Separate lines per device are why wires increase, unlike daisy-chain. (Key point: Priority arbiter lines)

3. In rotating (round robin) priority, what happens after a device is served?
   A. It becomes lowest priority
   B. It stays highest
   C. It is removed
   D. Priority is random
   Answer: A. Rotating priority is fair to all devices. (Key point: Rotating priority)

4. What is the drawback of fixed priority?
   A. Low-priority devices may starve
   B. It is too fair
   C. It is slow
   D. It needs rotation hardware
   Answer: A. A permanent rank lets high-priority devices dominate. (Key point: Fixed priority drawback)

## 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
- [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×
- [PID control operation with block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apid-operation-block-diagram) — Embedded System, chapter 7, asked 4×
- [IC manufacturing steps](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aic-manufacturing-steps) — Embedded System, chapter 8

## Related in other subjects

- [Frequency counter operation with block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Afrequency-counter) — Digital Logic, chapter 11, asked 14×
- [Instrumentation system with block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Ainstrumentation-system) — Basic Electronics Engineering, chapter 7, asked 3×
- [Digital control system block diagram and advantages](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adcs-block-diagram-advantages) — Digital Control System, chapter 1, asked 3×
- [DCS block diagram and S/H tracking/hold modes](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adcs-block-diagram-tracking-hold) — Digital Control System, chapter 1