JAJSQ97J may 2009 – january 2021 XIO2001
PRODUCTION DATA
The bridge supports both software-directed power management and active state power management through standard PCI configuration space. Software-directed registers are located in the power management capabilities structure located at offset 48h (see Section 8.4.32). Active state power management control registers are located in the PCI Express capabilities structure located at offset 70h (see Section 8.4.42).
During software-directed power management state changes, the bridge initiates link state transitions to L1 or L2/L3 after a configuration write transaction places the device in a low power state. The power management state machine is also responsible for gating internal clocks based on the power state. Table 8-10 identifies the relationship between the D-states and bridge clock operation.
CLOCK SOURCE | D0/L0 | D1/L1 | D2/L1 | D3/L2/L3 |
---|---|---|---|---|
PCI express reference clock input (REFCLK) | On | On | On | On/Off |
Internal PCI bus clock to bridge function | On | Off | Off | Off |
The link power management (LPM) state machine manages active state power by monitoring the PCI Express transaction activity. If no transactions are pending and the transmitter has been idle for at least the minimum time required by the PCI Express Specification, then the LPM state machine transitions the link to either the L0s or L1 state. By reading the bridge’s L0s and L1 exit latency in the link capabilities register, the system software may make an informed decision relating to system performance versus power savings. The ASLPMC field in the link control register provides an L0s only option, L1 only option, or both L0s and L1 option.