Kontron COMe Starterkit Eval T2 - Manual (Page 66)

COM Express Interfaces
2.6. Mobile PCI Express Module (MXM)
Mobile PCI Express Module is an interconnect standard for GPUs defined by the MXM-SIG,
mainly used in laptops. The goal of this standard was to enable a user an easy way to upgrade
the graphic card of a laptop without having to buy a hole new system or rely on proprietary
vendor upgrades. This goal was achieved with a non-proprietary standard socket.
At this writing, the current generation of MXM is MXM3. Two Module sizes, designated as A and
B, are allowed by MXM3 for different power envelopes and use cases.
Table 14: available MXM 3 Types
MXM Type Width Length Module Compatility Max. Power GPU memory bus
MXM-A 82mm 70mm A 55W 64-bit or 128-bit
MXM-B 82mm 105mm A,B 100W 256-bit
The MXM3-COM Express interface is over the COM Express PEG lines. The MXM3 outputs are
MXM3 Module dependent and can include DisplayPort, HDMI, DVI or TVout.
2.6.1. Signal Definitions
The primary interface between the COM Express Module and the MXM is a x16 PCI Express
bus. The Carrier contains the AC coupling caps for the PEG_RX[0:15] signals, the COM
Express Module contains the AC coupling caps for the PEG_TX[0:15] signals.
Three power rails are supplied to the MXM 3.3V@1A, [email protected] and 12V@ up to 10A. The
power rails are powered during S0. In the schematics below the main power rail to the MXM3
Module is shown as a fixed 12V (VCC_12V). An MXM3 Module can actually accept power over 7
to 20V range on this rail. Some COM Express Modules accept power over a similar range and if
you are designing a battery powered system you may be able to take advantage of this wide
range capability.
PEG_CLK_REQ# is used to enable the PCI Express clock PCIE_CLKPEG when a MXM is
installed. The clock does not run when a MXM is not installed, reducing emissions.
The SMBus is connected between the COM Express Module and the MXM. Zero ohm resistors
R115 and R116 are used to allow the SMBus to be disconnected from the MXM in the event
there are address or other conflicts.
Table 15: special MXM signals
Signal Signal Name Signal Description
PEX_STD_SW# PCI Express swing
select Pull-down resistor to ground determines the PCI
Express voltage. PCI Express Gen1 and GEN2 select
the correct resistor for short (resistor not installed),
medium short (147K Ohm to ground), medium long
(7.15K Ohm to ground), and long PCI Express channel
length (0 Ohm to ground). Refer to the MXM
specification for further information.
TH_OVERT# Thermal shutdown
request The carrier must power down the MXM Module within
500 ms of assertion.
TH_PWM Thermal PWM May be used to control a fan on the MXM thermal
solution.
PWR_OK Power OK Asserted by MXM card when all supplies are within
tolerance. There is also a COM Express signal named
“PWR_OK” which is not meant here.
PRSNT_R#/L# MXM card
presence detect Tied to ground on the MXM card. Can be used by the
carrier to detect that an MXM card is inserted
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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