DS1685/DS1687 3V/5V Real-Time Clocks
The RTC address is latched on the falling edge of the ALE signal. Each time an RTC address is latched, the
register address stack is pushed. The stack is only four registers deep, holding the three previous RTC addresses
in addition to the current RTC address being accessed. The following waveform illustrates how the BIOS could
recover the RTC address when an SMI occurs.
ALE
1
2
3
4
1) The RTC address is latched.
2) An SMI is generated before an RTC read or write occurs.
3) RTC address 0Ah is latched and the address from 1 is pushed to the RTC Address-1 stack location. This step
is necessary to change the bank select bit, DV0 = 1.
4) RTC address 4Eh is latched and the address from “1” is pushed to location “4Eh,” “RTC Address-2” while 0Ah
is pushed to the “RTC Address-1” location. The data in this register, 4Eh, is the RTC address lost due to the
SMI.
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DALLAS [ DALLAS SEMICONDUCTOR ]
