Sequence Body Handshake: start_item / finish_item

Overview

UVM uses a pull model : the driver requests the next item when ready to drive, not when the sequence happens to produce one. The sequence blocks at finish_item until the driver finishes driving and calls item_done(). This back-pressure matches real protocols where the DUT may not accept a new beat every cycle.

This topic breaks the handshake into five focused lessons. Each lesson answers a different why question: why pull beats push, why start_item does not randomize, why missing item_done hangs forever, why objections gate driver startup, and why pipelined reads need item_done(req).

Lessons in this topic

1. Pull Model Overview — timeline diagram of sequence, sequencer, and driver.

2. start_item / finish_item — full apb_wr_seq walkthrough on the sequence side.

3. Driver get_next_item / item_done — run_phase loop and drive_apb.

4. Test start() & Objections — run_phase raise/drop and seq.start(sqr).

5. Pipelining & Read Responses — try_next_item sketch and rdata via item_done(req).

Handshake in the stimulus stack

Legend: [STIM] [SEQ] [DRV] [ITEM] [UVM]

  [STIM] TEST.run_phase
      │ raise_objection
      │ seq.start(sqr)
      ▼
  [SEQ] SEQUENCE.body()
      │ start_item(req)  ──► [SEQ] SEQUENCER queues item
      │ randomize(req)       [ITEM] fields filled
      │ finish_item(req) ◄── blocks until [DRV] item_done
      ▼
  [DRV] DRIVER.run_phase
      get_next_item(req) ◄── pull when ready
      drive_apb(req)     ──► DUT pins
      item_done()

Every sub-lesson expands one arrow in this diagram. Missing item_done is the #1 cause of sequence hangs.

[SEQ] [DRV] handshake contract

  ONE in-flight item per driver–sequencer pair (unless explicitly pipelined)
  PAIRING:  start_item ↔ get_next_item
            finish_item  ↔ item_done
  VIOLATION: get_next_item twice without item_done → sequencer corruption

Key takeaways

• Pull model: driver requests items when ready; sequence blocks at finish_item.

• get_next_item / item_done on the driver must pair with start_item / finish_item on the sequence.

• Missing item_done is the #1 cause of sequence hangs.

Common pitfalls

• Missing item_done() — sequence blocks forever at finish_item.

• Starting sequence before run_phase objections — driver not running.

• Using put() on seq_item_port — wrong API; use get_next_item/item_done.