Kontron COMe Starterkit Eval T2 - Manual (Page 157)

Power and Reset
3.2. ATX and AT Style Power Control
3.2.1. ATX vs AT Supplies
ATX power supplies are in common use in contemporary PCs. ATX supplies have two sets of
power rails: a set for normal operation (12V, 5V, 3.3V and -12V) and a separate 5V Suspend rail.
The 5V Suspend rail is present whenever the ATX supply has AC input power. The other rails are
on only when a control signal from the PC hardware known as PS_ON# is held low by the
motherboard, allowing software control of the power supply. The PC motherboard may
implement several mechanisms for controlling the AC power, including a push button switch that
switches a low voltage logic signal rather than the AC main power. Other options may be
implemented, including the capability to turn on the main power on events such as a keyboard
press, mouse activity, etc.
AT power supplies do not have a Suspend rail and do not allow software control of the power
supply. An AT supply is on when the supply is connected to the AC main and the power switch
that is in series with the AC main input is on. AT supplies are extinct in the commercial PC
market, but the term lives on as a reference to a power supply that does not allow software
control.
An ATX supply may be converted to AT style operation by simply holding the ATX PS_ON# input
low all the time.
3.2.2. Power States
Power states are described by the following terms:
Table 51: Power States
State Description Comment
G3 Mechanical Off AC power to system is removed by a mechanical switch. System power
consumption is near zero – the only power consumption is that of the RTC circuits,
which are powered by a backup battery.
S5 Soft Off System is off except for a small subset that is powered by the 5V Suspend rail.
There is no system context preserved. VCC_5V_SBY current consumption is
system dependent, and it may be from tens of milliamps up to several hundred
milliamps.
S4 Suspend to Disk System is off except for a small subset that is powered by the 5V Suspend rail.
System context is preserved on a non-volatile disk media (that is powered off).
VCC_5V_SBY current consumption is system dependent, and it may be from tens
of milliamps up to several hundred milliamps.
S3 Suspend to RAM System is off except for system subset that includes the RAM. Suspend power is
provided by the 5V Suspend rail. System context is preserved in the RAM.
VCC_5V_SBY current consumption is system dependent, and it may be from
several hundred milliamps up to a maximum of 2A.
S0 On System is on.
COM Express signals SUS_S5#, SUS_S4# and SUS_S3# have the following behavior in the
Power States:
Table 52: Power State Behavior
State SUS_S5# SUS_S4# SUS_S3#
G3 NA NA NA
S5 Low Low Low
S4 High Low Low
S3 High High Low
S0 High High High
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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