DS17485S-5 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

MFG CO.

DS17485S-5

Real-Time Clocks

Maxim Integrated 

Real-Time Clocks

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

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. 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.

RTC ADDRESS

RTC ADDRESS-1

4Eh RTC ADDRESS-2

4Fh RTC ADDRESS-3

SMI Recovery Stack

7

6

5

4

3

2

1

0

DV0

AD6

AD5

AD4

AD3

AD2

AD1

AD0

REGISTER BIT DEFINITION

ALE

1

2

3

4

Figure 8. ALE Waveform

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