User`s manual

PK220080 s Power Management
Power Failure Detection Circuitry
Figure C-1 shows the power failure detection circuitry of the PK2200.
Figure C-1. PK2200 Power-Fail Circuit
Power Failure Sequence of Events
The following events occur as the input power fails:.
1. The 691 power-management IC first triggers a power-failure /NMI
(non-maskable interrupt) when the unregulated DC input voltage falls
below approximately 7.9 V (as determined by the voltage divider R1/
R2).
2. At some point, the raw input voltage level will not exceed the required
regulated voltage level by the regulators dropout voltage whereupon
the regulated output begins to droop.
3. The 691 next triggers a system reset when the regulated +5 V supply
falls below ∼4.75 V, allowing your power-failure routine the holdup
interval, t
H
, to store your important state data.
4. The 691 forces the chip enable of the SRAM high (standby mode).
5. The time/date clock and SRAM switches to the lithium backup battery
when the regulated voltage falls below the battery voltage of approxi-
mately 3 V.
6. The 691 keeps the reset asserted until the regulated voltage drops
below 1 V.
7. At this point the 691 ceases operating. By this time, the portion of the
circuitry not battery-backed has long since ceased functioning.
/PFI
/PFO
/NMI
DCIN
+5V
R1
R2
IC691
Micro-
processor
R4
D6
3B
3Y
74HC257
U18
U11
U15
D6
D6
/RES
/RESET