Low Pin Count

The Low Pin Count (LPC) bus is a computer bus used on IBM-compatible personal computers to connect low-bandwidth devices to the CPU, such as the BIOS ROM (BIOS ROM was moved to the Serial Peripheral Interface (SPI) bus in 2006), "legacy" I/O devices (integrated into Super I/O, Embedded Controller, CPLD, and/or IPMI chip), and Trusted Platform Module (TPM). "Legacy" I/O devices usually include serial and parallel ports, PS/2 keyboard, PS/2 mouse, and floppy disk controller. Most PC motherboards with an LPC bus have either a Platform Controller Hub (PCH) or a southbridge chip, which acts as the host and controls the LPC bus. All other devices connected to the physical wires of the LPC bus are peripherals. The LPC bus was introduced by Intel in 1998 as a software-compatible substitute for the Industry Standard Architecture (ISA) bus. It resembles ISA to software, although physically it is quite different. The ISA bus has a 16-bit data bus and a 24-bit address bus that can be used for both 16-bit I/O port addresses and 24-bit memory addresses; both run at speeds up to 8.33 MHz. The LPC bus uses a heavily multiplexed four-bit-wide bus operating at four times the clock speed (33.3 MHz) to transfer addresses and data with similar performance. LPC's main advantage is that the basic bus requires only seven signals, greatly reducing the number of pins required on peripheral chips. An integrated circuit using LPC will need 30 to 72 fewer pins than its ISA equivalent. It is also easier to route on modern motherboards, which are often quite crowded. The clock rate was chosen to match that of PCI in order to further ease integration. Also, LPC is intended to be a motherboard-only bus. There is no standardized connector in common use, though Intel defines one for use for debug modules, and few LPC peripheral daughterboards are available, including Trusted Platform Modules (TPMs) with a TPM daughterboard whose pinout is proprietary to the motherboard vendor as well as POST cards for displaying BIOS diagnostic codes and proprietary cards such as RS-232 and Parallel port devices for industrial use.