Power Control Operation
The power control circuit accepts a VDD and a VBAT
input. Many types of batteries can be used with Intersil
RTC products. For example, 3.0V or 3.6V Lithium
batteries are appropriate, and battery sizes are available
that can power the ISL12022MA for up to 10 years.
Another option is to use a supercapacitor for applications
where VDD is interrupted for up to a month. See the
‚ÄúApplication Section‚ÄĚ on page 26 for more information.
Normal Mode (VDD) to Battery
Backup Mode (VBAT)
To transition from the VDD to VBAT mode, both of the
following conditions must be met:
VDD < VBAT - VBATHYS
where VBATHYS ‚Čą 50mV
VDD < VTRIP
where VTRIP ‚Čą 2.2V
Battery Backup Mode (VBAT) to
Normal Mode (VDD)
The ISL12022MA device will switch from the VBAT to VDD
mode when one of the following conditions occurs:
VDD > VBAT + VBATHYS
where VBATHYS ‚Čą 50mV
VDD > VTRIP + VTRIPHYS
where VTRIPHYS ‚Čą 30mV
These power control situations are illustrated in
Figures 12 and 13.
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 ISL12022MA are active during battery backup mode
unless disabled via the control register.
VBAT - VBATHYS
VBAT + VBATHYS
FIGURE 12. BATTERY SWITCHOVER WHEN
VBAT < VTRIP
VTRIP + VTRIPHYS
FIGURE 13. BATTERY SWITCHOVER WHEN
VBAT > VTRIP
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 the 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 ‚ÄúBETA
Register (BETA)‚ÄĚ on page 19.
Power Failure Detection
The ISL12022MA 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).
The ISL12022MA 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 ‚Äú1‚ÄĚ when brownout is detected. Note that
the I2C serial bus remains active unless the Battery
VTRIP levels are reached.
Battery Level Monitor
The ISL12022MA has a built-in warning feature once the
backup battery level drops first to 85% and then to 75%
of the battery‚Äôs 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
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
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
July 9, 2010