User`s manual

Am486
®
Microprocessor PCI Customer Development Platform
2-27
Real-Time Clock (G17)
The real-time clock (RTC) function in the chipset’s 1487 chip is initially disabled
on the PCI CDP. Instead, a separate DS1685 year-2000 (Y2K)-compliant RTC is
enabled.
The DS1685 provides the features of widely-used, non-Y2K-compliant RTCs such
as the DS1287. In addition, the DS1685 provides a byte for storing century
information, plus other registers and features not found in older RTC chips.
Because of its extra features, the DS1685 RTC is not completely compatible with
the DS1287. The provided BIOS operates correctly with the DS1685 RTC, but the
extra RTC features can cause unexpected behavior if a customer-supplied BIOS is
used in PC-AT compatible systems. One example is that the DS1685 provides some
interrupt sources that are not present in the DS1287 RTC. Customer-written
initialization software can take the following steps to disable the extended RTC
interrupts:
1. Set the DV0 bit in CMOS register A to enable access to the extended register
bank in the DS1685.
2. Write a value of 60h to CMOS address 4Bh to disable the extended interrupt
addresses.
3. Clear the Dv0 bit in CMOS register a to disable access to the extended register
bank (so that legacy software behaves as expected).
4. Read CMOS register C to clear any pending interrupts that were triggered while
the extended interrupts were enabled.
For DS1685 RTC programming details, see the DS1685/DS1687 3 Volt/5 Volt Real
Time Clock Data Sheet, and Application Note 77: DS1585/87, DS1685/87, and
DS17x85/87 Accessing Extended User RAM via Software, available from Dallas
Semiconductor, www.dalsemi.com.
If desired, the M1487’s internal RTC can be enabled instead of the DS1685 by
removing resistor part R58 from the back side of the board, beneath location I14.
(See Sheet 16 of the schematics in Appendix B.)