IDT 89HPES16T4AG2, User Manual

Page: 75 / 165

IDT Power Management
PES16T4AG2 User Manual 6 - 3 May 23, 2013
Notes The PME_Turn_Off / PME_TO_Ack protocol may be initiated by the root when the switch is in any
power management state. When the PES16T4AG2 receives a PME_Turn_Off message, it broadcasts the
PME_Turn_Off message on all active downstream ports. The PES16T4AG2 transmits a PME_TO_Ack
message on its upstream port and transitions its link state to L2/L3 Ready after it has received a
PME_TO_Ack message on each of its active downstream ports. This process is called PME_TO_Ack
aggregation.
The aggregation of PME_TO_Ack messages on downstream ports is abandoned by the PES16T4AG2
when it receives a TLP on its upstream port after it has received a PME_Turn_Off message on that port but
before it has responded with a PME_TO_Ack message. Once a PME_TO_Ack message has been sched-
uled for transmission on the upstream port and the PME_TO_Ack aggregation process has completed,
received TLPs at that point may be discarded.
If the TLP that causes PME_TO_Ack aggregation to be abandoned targets the PES16T4AG2, the
PES16T4AG2 responds to the TLP normally. If the TLP that causes aggregation to be abandoned targets a
downstream port and the port is in L0, the TLP is transmitted on the downstream port. If the downstream
port is not in L0 (i.e., it is in L2/L3 Ready), the switch transitions the link to Detect and then to L0. Once the
link is reaches L0, the TLP is transmitted on the downstream port.
When PME_TO_Ack aggregation is abandoned, the PES16T4AG2 makes no attempt to abandon the
PME_Turn_Off and PME_TO_Ack protocol on downstream ports. Devices downstream from the
PES16T4AG2 are allowed to respond with a PME_TO_Ack and transition to L2/L3 Ready. When a TLP is
received that targets the downstream port, the switch transitions the link to Detect and then to L0. Once the
link reaches L0, the TLP is transmitted on the downstream port.
In order to avoid deadlock, a downstream port that does not receive a PME_TO_Ack message in the
time-out period specified in the PME_TO_Ack Time-Out (PMETOATO) field in its corresponding
PME_TO_Ack Timer (PMETOATIMER) register declares a time-out, transitions its link to L2/L3 Ready, and
signals to the upstream port that a PME_TO_Ack message has been received. If instead of being transi-
tioned to the D3
cold
state the PES16T4AG2 is transitioned to the D0
uninitialized
state, the PES16T4AG2
resumes generation of PM_PME messages.
Power Budgeting Capability
The PES16T4AG2 contains the mechanisms necessary to implement the PCI-Express power budgeting
enhanced capability. However, by default, these mechanisms are not enabled. To enable the power
budgeting capability, registers in this capability should be initialized and the Next Pointer (NXTPTR) field in
one of the other enhanced capabilities should be initialized to point to the power budgeting capability. The
Next Pointer (NXTPTR) of the power budgeting capability should be adjusted if necessary.
The power budgeting capability consists of the four power budgeting capability registers defined in the
PCIe 2.0 base specification and eight general purpose read-write registers. See section Power Budgeting
Enhanced Capability on page 8-55 for a description of these registers.
The Power Budgeting Capabilities (PWRBCAP) register contains the PCI-Express enhanced capability
header for the power budgeting capability. By default, this register has an initial read-only value of zero. To
enable the power budgeting capability, this register should be initialized via the serial EEPROM. The Power
Budgeting Data Value [0..7] (PWRBDV[0..7) registers are used to hold the power budgeting information for
that port in a particular operating condition.
The PWRBDV registers may be read and written when the Power Budgeting Data Value Unlock
(PWRBDVUL) bit is set in the Switch Control (SWCTL) register. When the PWRBDVUL bit is cleared, these
registers are read-only and writes to these registers are ignored. To enable the power budgeting capability,
the PWRBDV registers should be initialized with power budgeting information via the serial EEPROM.