Kontron COMe Starterkit Eval T2 - Manual (Page 166)

Power and Reset
The PWRBTN# signal is an input to the COM Express Module. Switch de-bouncing is done on
the Module. The falling edge of the PWRBTN# signal can initiate a state transition from S5 (soft
off) to S0 (full on). It may also cause the reverse transition, to S5, if the unit is in one of the 'on'
states.
The ATX supply is controlled by the net PS_ON# in the figure above. The main ATX supply rails
are on when PS_ON# is driven low. To turn the supply off, PS_ON# can be floated. This net is
usually derived from an inverted copy of the COM Express SUS_S3# signal. Jumper J53 offers
the possibility to override this signal with a fixed setup for debugging reasons.
Table 55: ATX Signal Names
ATX Signal Name Description
PS_ON# Active-low, TTL-level input to ATX supply that, when low, enables all power rails. If high
or floating, all ATX power rails are disabled except for the +5V Suspend rail.
PWR_OK Active-high, TTL-level output signal from the ATX supply that indicates that the +12V, +5V, +3.3V
and -12V outputs are all present and OK to use.
+12V1DC +12V power rail for use by all system components except for the CPU, controlled by PS_ON#
+12V2DC +12V power rail for use by the CPU, controlled by PS_ON#. This power rail appears on a
separate 2x2 connector for CPU use only.
+5VDC +5V power rail, controlled by PS_ON#
+3.3VDC +3.3V power rail, controlled by PS_ON#
-12VDC -12V power rail, controlled by PS_ON#
+5VSB +5V Suspend power rail, present whenever the ATX supply is connected to its AC power input
source.
COM Common return path – usually referred to as “ground” or GND.
ATX signals are summarized in Table 55 above. Note that there are two separate +12V outputs,
+12V1DC and +12V2DC. These are independent +12V sources. Each source is limited to
240W maximum output to meet UL safety requirements. The +12V2DC output is intended for
CPU use.
Contemporary ATX supplies have two power connectors on the motherboard:
● A 24-pin connector in a 2x12 array that includes all signals in Table 55 above except for
+12V2DC.
● A 4-pin connector in a 2x2 array for CPU power that includes +12V2DC and COM only.
Earlier ATX supplies used a 2x10 connector instead of a 2x12. The two connector versions have
compatible pin-outs. The 2x10 cable plug may be used with a 2x12 motherboard receptacle as
long as pin 1 of the 2x10 cable plug mates with pin 1 of the 2x12 Carrier Board receptacle.
Very early ATX supplies had a single +12V rail, on a 2x10 connector. The 2x2 CPU connector
was not present. ATX power supplies are designed for desktop systems, which often have
power-hungry CPUs and peripherals. CPUs that require 80W are common. Most Modules use
lower-power CPUs, and the ATX supply capacity may be overkill. In particular, two +12V
supplies are not necessary for many COM Express Modules.
3.4.1.1. Minimum Loads
ATX supplies may not start up if the loading on the +12V, +5V and +3.3V rails is too light. The
ATX12V Power Supply Design Guide shows suggested minimum loads in various configurations
but does not specify what the minimum loads are. The minimum loads required may vary with
different power supply vendors. Experience has shown that a dummy load on the order of at
least 400 mA is required on the +5V line in COM Express Carrier Boards that use little or no +5V
and are powered from ATX supplies.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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