DP8570AV datasheet(11/26 Pages) NSC

11 / 26 page

Functional Description (Continued)

TLF8638 – 8

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

and Lock-Out Circuits (Lower Right)

The user may choose to have this power failed signal lock-

out the TCP’s data bus within 30 ms min63 ms max or to

delay the lock-out to enable mP access after power failure is

detected This delay is enabled by setting the delay enable

bit in the Routing Register Also if the lock-out delay was

not enabled the TCP will disconnect itself from the bus with-

in 30 ms min

x 63 ms max 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 signal is asserted the lock-out will

be forced If a lock-out delay is enabled the DP8570A will

remain active for 480 ms after power fail is detected This

will enable the mP to perform last minute bookkeeping be-

fore total system collapse When the host CPU is finished

accessing the TCP it may force the bus lock-out before

480 ms has elapsed by resetting 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 TCP 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 TCP 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

TCK G0 G1

Not Isolated

Locked Out

PFAIL

Not Isolated

INTR MFO

Not Isolated

Open Drain

The Timer and Interrupt Power Fail Operation bits in the

Real-Time Mode Register determine whether or not the tim-

ers and 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

DP8570AV Datasheet(PDF) 11 Page - National Semiconductor (TI)

DP8570AV Datasheet(HTML) 11 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Datasheet Download

Link URL