DP8572AM datasheet(13/22 Pages) NSC

13 / 22 page

Functional Description (Continued)

TLF9980 – 11

FIGURE 6 System-Battery Switchover (Upper Left) Power Fail

and Lock-Out Circuits (Lower Right)

If chip select is low when a power failure is detected a

safety circuit will ensure that if a read or write is held active

continuously for greater than 30 ms after the power fail sig-

nal is asserted the lock-out will be forced If a lock-out delay

is enabled the DP8572A will remain active for 480 ms after

power fail is detected This will enable the mP to perform

last minute bookkeeping before total system collapse

When the host CPU is finished accessing the RTC it may

force the bus lock-out before 480 ms has elapsed by reset-

ting the delay enable bit

The battery switch over circuitry is completely independent

of the PFAIL pin A separate circuit compares VCC to the

VBB voltage As the main supply fails the RTC will continue

to operate from the VCC pin until VCC falls below the VBB

voltage At this time the battery supply is switched in VCC is

disconnected and the device is now in the standby mode If

indeterminate operation of the battery switch over circuit is

to be avoided then the voltage at the VCC pin must not be

allowed to equal the voltage at the VBB pin

After the generation of a lock-out signal and eventual

switch in of the battery supply the pins of the RTC will be

configured as shown in Table II Outputs that have a pull-up

resistor should be connected to a voltage no greater than

VBB

TABLE II Pin Isolation during a Power Failure

PFAIL e

Standby Mode

Logic 0

VBB l VCC

CS RD WR

Locked Out

A0 – A4

Locked Out

D0 – D7

Locked Out

Oscillator

Not Isolated

PFAIL

Not Isolated

INTR MFO

Not Isolated

Open Drain

The Interrupt Power Fail Operation bit in the Real-Time

Mode Register determine whether or not the interrupts will

continue to function after a power fail event

As power returns to the system the battery switch over cir-

cuit will switch back to VCC power as soon as it becomes

greater than the battery voltage The chip will remain in the

locked out state as long as PFAIL e 0 When PFAIL e 1

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