Q-Channel / Overview

A four-wire low-power handshake

The Q-Channel lets a controller ask a component to become quiescent — so its clock or power can be safely removed — and bring it back. It is the simplest AMBA low-power interface: one request wire, two acknowledge wires, and an activity hint. Start with the Quiescence Handshake to watch the waveform and state machine move together.

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The six handshake states

Q-Channel / Quiescence Handshake

Step the handshake, watch the state

Step through a transaction with ◀ ▶. The waveform and the state machine move together — or click any state node to jump there. Start with Accept, then try the harder paths.

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▶

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                state machine
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Q-Channel / State Machine

The full state machine

Two loops share Q_RUN and Q_REQUEST: the upper accept path gates the clock; the lower deny path keeps the component running. Every transition is one signal edge. Click a state to inspect it.

Node values — Rn=QREQn · An=QACCEPTn · D=QDENY  (1=HIGH, 0=LOW)

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Q-Channel / Signal Reference

Signals & state table

Four wires, six states. CTL→CMP = driven by the controller, CMP→CTL = driven by the component.

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State table

              StateQREQnQACCEPTnQDENYMeaning
            

Only one of the acknowledge signals (QACCEPTn / QDENY) changes per transition — this keeps the handshake safe across asynchronous clock/power domains. QACTIVE is independent of the handshake and can change at any time.

P-Channel / Overview

Negotiated power-state transitions

The P-Channel lets a power controller move a component between power states — on, retention, off — through a four-phase handshake. Where the Q-Channel has a single request wire, the P-Channel adds PSTATE to carry which state is wanted and PACTIVE as a per-mode activity vector. Open Power Transition to step a request through.

Device power states

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The six handshake states

P-Channel / Power Transition

Step a power-state request

Step through a transition with ◀ ▶. The waveform and the state machine move together — or click any state node to jump there. Start with Power-down, then try a denial and a wake-up.

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◀

▶

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                state machine
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P-Channel / State Machine

The four-phase handshake

Two loops share P_STABLE and P_REQUEST: the upper accept path commits the new PSTATE; the lower deny path keeps the current state. Every transition is one signal edge. Click a state to inspect it.

Node values — R=PREQ · A=PACCEPT · D=PDENY  (1=HIGH, 0=LOW)

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P-Channel / Signal Reference

Signals & state table

Five wires, six states. CTL→CMP = driven by the controller, CMP→CTL = driven by the component.

              SignalDriverAssertedMeaning
            

State table

              StatePREQPACCEPTPDENYMeaning
            

PSTATE must be valid before PREQ is asserted and held stable until the handshake completes. Only one of PACCEPT / PDENY is asserted per transition. PACTIVE is independent of the handshake and can change at any time.