DesignWare Technical Bulletin Article

USB 2.0 IP with Link Power Management Extension

Michael Liu, Corporate Applications Engineer

What is it?

USB 2.0 Link Power Management Addendum is officially released as an Engineering Change Notice (ECN) to apply to USB specification Revision 2.0. In this ECN, it defines a power management feature for USB called Link Power Management (LPM). This feature is similar to the existing USB 2.0 suspend/resume, however, it has transitional latencies of tens of microseconds between defined power states. This is a much finer granularity as compared to the USB 2.0.s suspend/resume, whose transitional latencies are in tens of milliseconds.

In the ECN, it introduces a formal terminology for bus line-states in terms of power states. The fast transition of a root port from an enabled state (called L0 state) to a new Sleep state (called L1 state) is also defined. The ECN has also defined the corresponding L1 state to L0 state transition. Further, the ECN defines the method for extending the existing USB 2.0 protocol to accommodate the newly introduced explicit L1 entry.

Using a newly defined USB transaction, a USB host would actively initiate a L0 to L1 transition. This would allow a device to detect the host.s intent and respond immediately. This feature is favorably compared to the existing suspend method in USB 2.0, which requires 3 orders of magnitude more transitional time on USB wires. Similarly, the L1 to L0 transition is compared to USB 2.0 resume. Both the host and device can initiate the resume. However, the duration of resume signaling has been redefined to be 3 orders of magnitude faster in LPM.

Why is there a LPM?

The ECN intends to make USB 2.0 a relevant technology for today.s mobile platforms, upon which many hand-held devices have been designed and marketed. The focus of LPM is to provide a more favorable power management method for mobile product development. The existing USB 2.0 power management feature is simply not suitable for many of the mobile products being developed nowadays. Preliminary lab testing has shown that the LPM feature allows for power consumption optimization across both the USB 2.0 host and devices. A direct impact on the hand-held device application in terms of its power consumption reduction is that the effective battery life cycles will be extended by 15-20%.

LPM Power States

In ECN, it formally defines 4 power management states for USB as follows:

  • L0 (On)
  • L1 (Sleep): New & finer granularity
  • L2 (Suspend)
  • L3 (Off)
Please note that the L0, L2 and L3 states are formal names for conditions already defined in USB 2.0 specification. The link in this context is intended to encapsulate the downstream facing port of a host or hub, the upstream facing port of a device and the data lines connecting the ports.

The L1 Sleep state is a newly introduced link power state. It differs from the L2 Suspend in 2 ways: the transitional latencies are much shorter and there are no explicit power draw requirements. The comparison of the 2 states is provided in the following table:

 L1 (Sleep)L2 (Suspend)
EntryExplicitly entered via LPM extended transactionImplicitly entered via 3ms of link inactivity
Exit Device or host-initiated via resume signaling;

Remote-wake can be (optionally) enabled/disabled via the LPM transaction.

Device- or host-initiated via resume signaling;

Device-initiated resumes can be (optionally) enabled/disabled by software

LatenciesEntry: ~10us

Exit: ~70 us to 1ms (host-specific)

Entry: ~3ms

Exit: >0ms (OS-dependent)

Device Power ConsumptionDevice power consumption level is application and implementation specificDevice power consumption is limited to: ≤500 uA or ≤2.5mA

The explicit port commands are used to control a particular USB link.s L0 and L1 state transitions.

LPM Protocol Extension

The LPM ECN uses the existing USB 2.0 reserved PID value 0000B and defines this PID value the EXT PID. Further, the ECN defines an extended token phase used to extend the USB 2.0 transaction protocol. Now the token phase has 2 token packets as illustrated below:

Please note that the first token packet is a standard token packet with its newly defined EXT PID. The second token is the extended token packet with a SubPID that is specific to the LPM extension. When its value is LPM, then the 11-bit content would contain definitions that are related to the LPM. In that content definition, it would further describe link states, resume capability and timing duration.

LPM Transactions

A host would use LPM transactions to transition a host to device link to a specific link power state. An LPM transaction is a two phase extended USB transaction as shown below, consisting of an extended token phase and a handshake phase.

When a host is ready to transition a port from L0 to L1 state, it issues a port command that will result in an LPM transaction being initiated on the affected port. The attached device of the port would send ACK if it is ready to make such a transition or a NYET handshake if it is currently not able to do so. The device must send a STALL if it does not support the requested link state. If the device detects errors or it does not understand the protocol extension transaction, then it sends no handshake.

LPM Impacts

LPM is backward compatible with USB 2.0 systems. A USB 2.0 host would continue to work with properly with a device that supports LPM. Similarly, a USB 2.0 device would work properly with a host that supports LPM. The only time when LPM is used is when both host and device support such a feature.

LPM enhancement requires changes on both host and device platforms to really take advantage of this feature. There will be implications on hardware, software and compliance testing,

For hardware implications, there are no changes to the physical analog layer. The L1 state is identical to USB 2.0 suspend in terms of line-state. There are no changes to resume signaling levels, end of resume signaling event and subsequent line states. However, new state machines are required at both host and device to accommodate the shorter event times. Further, a device must be able to decode the new extended protocol to recognize an LPM transaction.

For software implications, the host controller design would need to allow software to exert control on LPM feature management. This is host controller implementation specific. Minimally, a host controller needs to provide the software a means of control to expose the compliance testing of L1 and new resume timings.

For compliance testing, it is easy to test the LPM feature since L1 is an explicit transaction, it is easily captured via the bus analyzer. Similarly, the resume signaling is also easily captured and measured using existing procedures.

When is it available?

General availability for the DesignWare USB 2.0 IP with LPM is scheduled for Q4 2008. Please contact Synopsys for more details about the DesignWare USB 2.0 IP solutions.