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ISL12020M
12
FN6667.5
December 13, 2011
The I2C bus is deactivated in battery-backup mode to reduce
power consumption. Aside from this, all RTC functions are
operational during battery-backup mode. Except for SCL and SDA,
all the inputs and outputs of the ISL12020M are active during
battery-backup mode unless disabled via the control register.
The device Time Stamps the switchover from VDD to VBAT and
VBAT to VDD, and the time is stored in tSV2B and tSB2V registers
respectively. If multiple VDD power-down sequences occur before
status is read, the earliest VDD to VBAT power-down time is stored
and the most recent VBAT to VDD time is stored.
Temperature conversion and compensation can be enabled in
battery-backup mode. Bit BTSE in the BETA register controls this
operation, as described in “BETA Register (BETA)” on page 20.
Power Failure Detection
The ISL12020M provides a Real Time Clock Failure Bit (RTCF) to
detect total power failure. It allows users to determine if the
device has powered up after having lost all power to the device
(both VDD and VBAT).
Brownout Detection
The ISL12020M monitors the VDD level continuously and
provides warning if the VDD level drops below prescribed levels.
There are six (6) levels that can be selected for the trip level.
These values are 85% below popular VDD levels. The LVDD bit in
the Status Register will be set to “1” when brownout is detected.
Note that the I2C serial bus remains active unless the Battery
VTRIP levels are reached.
Battery Level Monitor
The ISL12020M has a built in warning feature once the Back Up
battery level drops first to 85% and then to 75% of the battery’s
nominal VBAT level. When the battery voltage drops to between
85% and 75%, the LBAT85 bit is set in the status register. When
the level drops below 75%, both LBAT85 and LBAT75 bits are set
in the status register.
The battery level monitor is not functional in battery backup mode.
In order to read the monitor bits after powering up VDD, instigate a
battery level measurement, which is set by setting the TSE bit to "1"
(BETA register), and then read the bits.
There is a Battery Time Stamp Function available. Once the VDD is
low enough to enable switchover to the battery, the RTC time/date
are written into the TSV2B register. This information can be read
from the TSV2B registers to discover the point in time of the VDD
power-down. If there are multiple power-down cycles before
reading these registers, the first values stored in these registers
will be retained. These registers will hold the original power-down
value until they are cleared by setting CLRTS = 1 to clear the
registers.
The normal power switching of the ISL12020M is designed to
switch into battery-backup mode only if the VDD power is lost.
This will ensure that the device can accept a wide range of
backup voltages from many types of sources while reliably
switching into backup mode.
Note that the ISL12020M is not guaranteed to operate with VBAT
< 1.8V. If the battery voltage is expected to drop lower than this
minimum, correct operation of the device, especially after a VDD
power-down cycle, is not guaranteed.
The minimum VBAT to insure SRAM is stable is 1.0V. Below that,
the SRAM may be corrupted when VDD power resumes.
Real Time Clock Operation
The Real Time Clock (RTC) uses an integrated 32.768kHz quartz
crystal to maintain an accurate internal representation of
second, minute, hour, day of week, date, month, and year. The
RTC also has leap-year correction. The clock also corrects for
months having fewer than 31 days and has a bit that controls 24-
hour or AM/PM format. When the ISL12020M powers up after
the loss of both VDD and VBAT, the clock will not begin
incrementing until at least one byte is written to the clock
register.
Single Event and Interrupt
The alarm mode is enabled via the MSB bit. Choosing Single
Event or interrupt alarm mode is selected via the IM bit. Note that
when the frequency output function is enabled, the alarm
function is disabled.
The standard alarm allows for alarms of time, date, day of the
week, month, and year. When a time alarm occurs in single
event mode, the IRQ/FOUT pin will be pulled low and the alarm
status bit (ALM) will be set to “1”.
The pulsed Interrupt mode allows for repetitive or recurring alarm
functionality. Hence, once the alarm is set, the device will
continue to alarm for each occurring match of the alarm and
present time. Thus, it will alarm as often as every minute (if only
the nth second is set) or as infrequently as once a year (if at least
the nth month is set). During pulsed Interrupt mode, the
IRQ/FOUT pin will be pulled low for 250ms and the alarm status
bit (ALM) will be set to “1”.
VBAT - VBATHYS
VBAT
VBAT + VBATHYS
BATTERY-BACKUP
MODE
VDD
VTRIP
2.2V
1.8V
FIGURE 13. BATTERY SWITCHOVER WHEN VBAT <VTRIP
FIGURE 14. BATTERY SWITCHOVER WHEN VBAT >VTRIP
VTRIP
VBAT
VTRIP + VTRIPHYS
BATTERY-BACKUP
MODE
VDD
VTRIP
3.0V
2.2V
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