Information Technology Reference
In-Depth Information
DREQ[7:5,3:0]
DACK[7:5.3:0]
T
REFRESH#
DMA
PIIX3
Figure 5.7
DMA interface
5.1.9 Interval timer
The PIIX3 contains three 8251-compatible counters. The three counters are contained in one
PIIX3 timer unit, referred to as Timer 1. Each counter provides an essential system function.
The functions of the counters are:
•
Counter 0 - connects to the IRQ0 line and provides a system timer interrupt for a time-of-
day, diskette time-out, and so on. The input to the counter is a 14.218 18 MHz clock
(OSC). This is then used to increment a 16-bit register, which rolls over every 55 ms.
•
Counter 1 - generates a refresh request signal.
•
Counter 2 - generates the speaker tone.
5.1.10 Interrupt controller
The PXII3 incorporates two 8259-compatible interrupt controllers which provide an ISA-
compatible interrupt controller. These are cascaded to give 13 external and three internal in-
terrupts. The primary interrupt controller connects to IRQ0-IRQ7 and the secondary con-
nects 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 2.2 in Section 2.3.2).
•
IRQ13 - used by the math coprocessor, which is connected to the FERR pin on the
processor.
Figure 5.8 shows that the PC uses IRQ0 as the system timer and IRQ2 by the programmable
interrupt controller.
The interrupt unit also supports interrupt steering. The four PCI active low interrupts
(
PIRQ#[D:A]
) can be internally routed in the PIIX3 to one of 11 interrupts (IRQ15, IRQ14,
IRQ12-IRQ9, IRQ7-IRQ3).
5.1.11 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, 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 dis-
abled. This is normally done with a motherboard jumper (to enable or disable the mouse in-
terrupt). Figure 5.8 shows an example of a mouse using IRQ12.
