--- title: "Daisy-chain arbitration" subject: "Embedded System" chapter: 5 key: "semester_6/embedded_system:daisy-chain-arbitration" source: https://www.bctengineer.com/q/semester_6/embedded_system/daisy-chain-arbitration site: BCT Engineering --- # Daisy-chain arbitration Embedded System · Chapter 5: Interfacing - Asked in: 2071 Bhadra, 4 marks; 2075 Bhadra, 4 marks; 2076 Baisakh, 6 marks; 2078 Chaitra; 2078 Poush, 5 marks (asked 5 times) - Video: [daisy chain arbitration | Hindi | | Lec-14 | COA | | Niharika panda](https://www.youtube.com/watch?v=w_BrVsOPNgw) — Education 4u - Hindi - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/embedded_system/daisy-chain-arbitration - Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-5 ## Question Explain Daisy-chain arbitration with the help of a block diagram and steps (structure, operation, advantages and disadvantages). ## Short answer Daisy-chain arbitration serially connects a grant line through devices. The arbiter asserts grant to the nearest device; if it requested, it keeps it and uses the bus, else passes it downstream. Priority is fixed by position. Advantages: simple wiring, easy expansion. Disadvantages: starvation, propagation delay, chain breakage if a device fails. ## Answer **Arbitration** decides which of several peripherals (bus masters) requesting the system bus at the same time gets it. In **daisy-chain arbitration** the devices are connected in series along a grant line, so a device's position in the chain gives its priority. #### Structure ```text Bus request (Req, wired-OR, common line) +--------------------------------------------------+ | +----------+----------+----------+ | v | | | | | +-----+ Grant +------+ Gr +------+ Gr +------+ Gr +------+ | CPU |-------->| Dev1 |--->| Dev2 |--->| Dev3 |--->| Dev4 | |/Arbi| +------+ +------+ +------+ +------+ +-----+ (highest) (lowest) ``` All devices share one request line (Req); the grant (Ack/Grant) signal goes from the arbiter to the nearest device and is then passed from device to device. #### Operation (steps) 1. One or more devices assert the common Req line. 2. The arbiter/processor sees Req and issues Grant to the first (nearest) device in the chain. 3. If a device did not request the bus, it passes the Grant on to the next device. 4. A device that did request keeps the Grant (does not forward it) and takes control of the bus. Devices further down the chain cannot get the grant. 5. The device uses the bus, then releases it (drops Req); the arbiter withdraws Grant, and the cycle repeats. (When daisy chaining is used for interrupts, the device that keeps the grant also places its identity/vector on the bus for the CPU.) #### Advantages - Simple, with few wires: only two common lines plus a grant chain. - Easy to add new devices at the end of the chain. - Priority is fixed by position, with no separate priority logic. #### Disadvantages - Fixed priority: devices at the end may starve (no fairness). - The grant takes time to propagate through each device (slow for long chains). - A faulty or removed device breaks the chain. - Priority cannot be changed without rewiring. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In daisy-chain arbitration, what determines device priority? A. Its position along the grant chain B. A separate arbiter priority table C. Its request time D. Its data rate Answer: A. The grant passes from the arbiter through the nearest device first, so closer devices have higher priority. (Key point: Priority by position) 2. What does a device do with the grant if it did not request the bus? A. Passes it to the next device B. Keeps it C. Discards it D. Returns it to the arbiter Answer: A. Non-requesting devices forward the grant; a requesting one keeps it, blocking those behind. (Key point: Grant passing) 3. Which is a disadvantage of daisy-chain arbitration? A. Devices at the end may starve because priority is fixed B. It needs many request lines C. It has rotating priority D. Priority can be changed easily Answer: A. Fixed priority gives no fairness; also a faulty device breaks the chain and grant propagation is slow. (Key point: Disadvantage) 4. How is the request line arranged? A. One common (wired-OR) Req line shared by all devices B. One Req line per device C. No request line D. Req passes in the chain Answer: A. All devices share one Req line, giving few wires; the grant passes through the chain. (Key point: Request line) ## 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 - [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× - [Priority arbitration](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apriority-arbitration) — Embedded System, chapter 5, asked 2× ## 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× - [Data logger block diagram and operation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adl-block) — Instrumentation II, chapter 5, asked 6× - [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×