6.10LPC Bus

LPC (Low Pin Count) is a bus-like interface often used on laptops and increasingly used on desktops too. To find out if you have LPC type "lspci" and look for "LPC". There are other words next to "LPC" such as "ISA Bridge ... LPC Interface Controller" or "LPC Bridge", etc. LPC is not really ISA but it substitutes for an ISA bus.

The old ISA bus was slow and devices that needed more speed were put on the newer PCI but.


But devices that didn't need high speed were often implemented by chips on the motherboard and remained on the ISA bus even though there were no slots for any ISA cards. Then the LPC bus came along to replace what remained of the ISA bus. LPC is much smaller than ISA and just as fast since it runs at 4 times the clock speed of ISA. Its multiplexed bus for data/address and control is only 4 bits wide. To send a byte requires splitting the byte into 2 half-bytes and then putting them back together. So its clear why it's "Low Pin Count" = LPC. There's also a few other lines in the bus.

This small LPC interface is used for slow "legacy" devices such as serial ports, parallel ports, and floppy drives. So a computer using LPC will have all fast devices on the PCI bus, etc. and slow (legacy) devices on the LPC bus interface. All LPC devices will be on-board; there are no LPC slots.

LPC has no standards for Plug-and-Play configuring but says that the BIOS or ACPI should do the configuring. Devices on this bus sometimes use isapnp.

Linux support for LPC as of late 2004 was very much incomplete but Linux has some support for the configuring aspects of ACPI. Sometimes a BIOS menu lets one manually PnP-configure devices on the LPC bus but it may not tell you that the device resides on LPC.

A major chip on the LPC bus is the superio chip which contains legacy IO devices: serial and parallel ports, floppy controller, keyboard controller, mice, etc. BIOS data may also reside on the LPC bus. The keyboard and mouse (input devices) should be listed in /proc/bus/input/devices but instead of seeing "lpc" it seems to show "isa0060/serio0, etc. even though it's on the lpc bus and not the isa bus.

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