Datasheet

System Maintenance Interrupt

(SMI) Recovery Stack

An SMI recovery register stack is located in the extend-

ed register bank, locations 4Eh and 4Fh. This register

stack, shown below, can be used by the BIOS to recov-

er from an SMI occurring during an RTC read or write.

The RTC address is latched on the falling edge of the

ALE signal. Each time an RTC address is latched, the

registers deep, holding the three previous RTC

addresses in addition to the current RTC address being

accessed. Figure 8 illustrates how the BIOS could

recover the RTC address when an SMI occurs.

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.

DS17285/DS17287/DS17485/DS17487/DS17885/DS17887

Real-Time Clocks

____________________________________________________________________ 27

Figure 8. ALE Waveform

RTC ADDRESS

RTC ADDRESS-1

4Eh RTC ADDRESS-2

4Fh RTC ADDRESS-3

SMI Recovery Stack

76543210

DV0 AD6 AD5 AD4 AD3 AD2 AD1 AD0

ALE