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Motherboard design


11.13 Interrupt controller

The PXII3 incorporates two 8259-compatible interrupt controllers and provides an ISA- compatible interrupt controller. These are cascaded to give 13 external and 3 internal interrupts. The primary interrupt controller connects to IRQ0–IRQ7 and the secondary connects to IRQ8–IRQ15. The three internal interrupts are:

  • IRQ0. Used by the system timer and is connected to Timer 1, Counter 0.

  • IRQ2. Used by the primary and secondary controller (see Figure 8.7 in Section

      • 8.5.2)


  • IRQ13. Used by the math co-processor, which is connected to the FERR pin on the processor.

The interrupt unit also supports interrupt steering. PIIX3 also supports interrupt steering where the four PCI active low interrupts (PIRQA PIRQD ) can be internally routed to one

of 11 interrupts (IRQ15, IRQ14, IRQ12IRQ9, IRQ7IRQ3).

11.14 Mouse function

The mouse normally either connects to one of the serial ports (COM1: or COM2:) or a PS/2-type connector. If they connect to the PS/2-type connector then IRQ12 is used (see Figure 11.9), else a serial port connected mouse uses the serial interrupts (such as IRQ4 for COM1 and IRQ3 for COM2). Thus, a system with a serial connected mouse must have the IRQ12/M interrupt disabled. This is typically done with a motherboard jumper (to enable or disable the mouse interrupt) or by BIOS steering.

11.15 Power management

PIIX3 has extensive power management capabilities and permits the system to operate in a low-power state without being powered-down. In a typical desktop PC, there are two states – Power On and Power Off. Leaving a system powered on, when not in use, wastes power. PIIX3 provides a fast on/off feature that creates a third state called Fast Off. When in the Fast Off state, the system consumes less power than the Power-On state. The PIIX3’s power management function is based on two modes:

  • System Management Mode (SMM). Software (called SMM code) controls the tran- sitions between the Power On state and the Fast Off state. PIIX3 invokes this soft- ware by generating an SMI to the CPU (asserting the SMI signal).

  • Advanced Power Management (APM).

    • 11.16

      Universal serial bus

PIIX3 contains a USB. The host controller includes the root hub and two USB ports. This allows up to two USB peripheral devices to be directly connected to the PIIX3 without an external hub. If more devices are required, an external bus can be connected to either of the built-in ports. The USB’s PCI configuration registers are located in func- tion 2, PCI configuration space.

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