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Technical Reference Guide
Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition - September 1998
4-27
4.4.5 INTERVAL TIMER
The interval timer generates pulses at software (programmable) intervals. A 8254-compatible
timer is integrated into the South Bridge chip. The timer function provides three counters, the
functions of which are listed in Table 4-15.
Table 4–15.
Interval Timer Functions
Table 4-15.
Interval Timer Functions
Counter Function Gate Clock In Clock Out
0 System Clock Always on 1.193 MHz IRQ0
1 Refresh Always on 1.193 MHz Refresh Req.
2 Speaker Tone Port 61, bit<0> 1.193 MHz Speaker Input
The interval timer is controlled through the I/O mapped registers listed in Table 4-16.
Table 4–16.
Interval Timer Control Registers
Table 4-16.
Interval Timer Control Registers
I/O Port Register
040h Read or write value, counter 0
041h Read or write value, counter 1
042h Read or write value, counter 2
043h Control Word
Interval timer operation follows standard AT-type protocol. For a detailed description of timer
registers and operation, refer to the Compaq Extended Industry Standard Architecture Expansion
Bus Technical Reference Guide.
4.4.6 ISA CONFIGURATION
The working relationship between the PCI and ISA buses requires that certain parameters be
configured. The PC/ISA bridge function of the South Bridge component includes configuration
registers to set parameters such as PCI IRQ routing and top-of-memory available to ISA/DMA
devices. These parameters are programmed by BIOS during power-up, using registers listed
previously in Table 4-6.