Kontron COMe Starterkit Eval T2 - Manual (Page 170)

BIOS Considerations
4. BIOS Considerations
4.1. Legacy versus Legacy-Free
For the purposes of this document, “legacy” refers to a set of peripherals provided in desktop
PCs and associated chipsets that are no longer in production, including PS/2 keyboard and
mouse, parallel port (LPT), and UART serial ports. The COM Express standard was created with
newer chipsets in mind. As a result, COM Express is “legacy-free”, which means that legacy
peripherals are not directly supported by the Module. Such peripherals have been replaced by
space-efficient high-speed interfaces such as USB 2.0.
To facilitate the market’s transition toward newer peripherals, the Low Pin Count (LPC) interface
was created as a space-efficient replacement for the Industry Standard Architecture (ISA) bus. In
addition to firmware devices such as BIOS flash, low-speed super I/O controllers were developed
for the LPC bus to fill the gap until the momentum could build for new high-speed-serial-based
peripherals.
4.2. Super I/O
Within the COM Express modular architecture, super I/O controllers could be placed on Carrier
Boards according to unique application requirements. However, LPC super I/O devices are
closely coupled to the BIOS firmware that initializes them and performs setup-based interrupt
assignments. The BIOS flash generally resides on the COM Express Modules in order for the
Modules to be self-booting. This tight coupling of LPC super I/O to the BIOS presents a
multitude of problems in a legacy-free modular environment.
Normally the BIOS vendor supplies to the BIOS developer the choice of different super I/O
Modules that can be plugged-in at the source level during the BIOS build process. The BIOS
super I/O code Modules often require considerable adaptation work by the BIOS developer to be
able to be “plugged-in”. The supported super I/O device would be determined by the Module
vendor, and other device support would involve a custom BIOS for each super I/O device.
Consequently, PICMG recommends using USB peripherals or PCI or PCI Express super I/O
devices on Carrier Boards for customers wishing to use UART serial ports (COM1, COM2, etc.)
or other legacy peripherals. Plug-and-play based interrupt assignments are automatic, and
drivers initialize devices after the operating system is loaded. A USB keyboard can be used to
enter BIOS setup prior to power-on self-test.
PICMG recommends against using LPC super I/O devices on the Carrier Board, as such usage
creates BIOS customization requirements and can greatly restrict Module interoperability. PCI,
PCI Express, and/or USB devices should be used instead.
PICMG suggests that alternate BIOS firmware support on the Carrier Board as well as port 0x80
implementations are appropriate uses of the LPC interface on the Carrier Board.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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