Rainbow Electronics DS1680 - Manual (Page 8)

DS1680
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AIE (Alarm Interrupt Enable) – When set to a logic 1, this bit permits the interrupt request flag (IRQF)
bit in the status register to assert INT. When the AIE bit is set to logic 0, the IRQF bit does not initiate the
INT signal.
STATUS REGISTER – 0Ch
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
0 LOBAT 0 0 0 0 0 IRQF
LOBAT (Low Battery Flag) – This bit reflects the status of the backup power source connected to the
V
BAT
pin. When V
BAT
is greater than 2.5V, LOBAT is set to a logic 0. When V
BAT
is less than 2.3V,
LOBAT is set to a logic 1.
IRQF (Interrupt Request Flag) – A logic 1 in the interrupt request flag bit indicates that the current
time has matched the time of day alarm registers. If the AIE bit is also a logic 1, the INT pin will go high.
IRQF is cleared by reading or writing to any of the alarm registers.
POWER-UP/POWER-DOWN CONSIDERATIONS
When V
CC
is applied to the DS1680 and reaches a level greater than V
CCTP
(trip point), the device
becomes fully accessible after t
RPU
(250ms typical). Before t
RPU
elapses, some inputs are disabled. When
V
CC
drops below V
CCSW
, the device is switched over to the V
BAT
supply.
During power- up, when V
CC
returns to an in-tolerance condition, the RST pin is kept in the active state
for 250ms (typical) to allow the power supply and microprocessor to stabilize.
NONVOLATILE SRAM CONTROLLER
The DS1680 provides automatic backup and write protection for an external SRAM. This function is
provided by gating the chip-enable signal and by providing a constant power supply through the V
CCO
pin.
The DS1680 nonvolatizes the external SRAM by write-protecting the SRAM and by providing a backup
power supply in the absence of V
CC
. When V
CC
falls below V
CCTP
, access to the external SRAM is
prohibited by forcing CE0 high regardless of the level of CEI . Upon power-up, access is prohibited until
the end of t
RPU
.
POWER-FAIL COMPARATOR
The PFI input is connected to an internal reference. If PFI is less than 1.25V, PFO goes low. The power-
fail comparator can be used as an undervoltage detector to signal an impending power supply failure.
PFO can be used as a
µ
P interrupt input to prepare for power-down. For battery conservation, the
comparator is turned off and PFO is held low when in battery-backup mode.
ADDING HYSTERESIS TO THE POWER-FAIL COMPARATOR
Hysteresis adds a noise margin to the power-fail comparator and prevents PFO from oscillating when
VIN is near the power- fail comparator trip point. Figure 6 shows how to add hysteresis to the power- fail
comparator. Select the ratio of R1 and R2 such that PFI sees 1.25V when VIN falls to the desired trip
point (VTRIP). Resistors R2 and R3 adds hysteresis. R3 will typically be an order of magnitude greater
than R1 or R2. R3 should be chosen so it does not load down the
PFO pin. Capacitor C1 adds noise
filtering and has a value of typically 1.0µF (See Figure 6 for a schematic diagram and equations.)