![](http://datasheet.mmic.net.cn/120000/MAX6917EO50_datasheet_3558996/MAX6917EO50_20.png)
Power Control
VBATT provides power as a battery backup. VCC pro-
vides the primary power in dual-supply systems where
VBATT is connected as a backup source to maintain
timekeeping in the absence of primary power. When
VCC rises above the reset threshold, VRST, VCC powers
the MAX6917. When VCC falls below the reset thresh-
old, VRST, and is less than VTRD, VBATT powers the
MAX6917. If VCC falls below the reset threshold, VRST,
and is more than VTRU, VCC still powers the MAX6917.
VCC slew rate in power-down is limited to 10V/ms (max)
for proper data retention.
VOUT Function
VOUT is an output supply voltage for battery-backed-up
devices such as SRAM. When VCC rises above the
reset threshold or is greater than VBATT,VOUT connects
to VCC (Figure 19). When VCC falls below VRST and
VBATT, VOUT connects to VBATT. There is a typical
±100mV hysteresis associated with the switching
between VCC and VBATT on the VOUT output. Connect
a 0.1F capacitor from VOUT to GND.
Power-On Reset (POR)
The MAX6917 contains an integral POR circuit that
ensures all registers are reset to a known state on power-
up. Once either VCC or VBATT rises above 1.6V (typ), the
POR circuit releases the registers for normal operation.
When VCC or VBATT drops to less than 0.9V (typ), the
MAX6917 resets all register contents to the POR defaults.
Oscillator Start Time
The MAX6917 oscillator typically takes 1s to 2s to begin
oscillating. To ensure the oscillator is operating correct-
ly, the system software should validate proper time-
keeping. This is accomplished by reading the seconds
register. Any reading with more than 0s, from the POR
value of 0s, is a validation of proper startup.
Alarm-Generation Function
The alarm function is configured using the alarm-con-
figuration register and the alarm-threshold registers
(Table 1). Writing a one to D7 (ONE SEC) in the alarm-
configuration register sets the alarm function to occur
once every second, regardless of any other setting in
the alarm-configuration register or in any of the alarm-
threshold registers. When the alarm is triggered, D4
(ALM OUT) in the status register is set to one and the
open-drain alarm output ALM goes low. The alarm is
cleared by reading or writing to the alarm-configuration
register or by reading or writing to any of the alarm-
threshold registers. This resets the ALM output to a
high and the ALM OUT bit to zero.
When D7 (ONE SEC) is set to zero in the alarm-configu-
ration register, then the alarm function is set by the
remaining bits in the alarm-configuration register and
the contents of the respective alarm-threshold register.
For example, writing 01h (0000 0001) to the alarm-con-
figuration register causes the alarm to trigger every
time the seconds-timekeeping register matches the
seconds alarm-threshold register (i.e., once every
minute on a specific second). Writing 02h (0000 0010)
to the alarm configuration register causes the alarm to
trigger on a minutes match (i.e., once every hour).
Writing a 4Fh (0100 1111) to the alarm configuration
register causes the alarm to be triggered on a specific
second, of a specific minute, of a specific hour, of a
specific date, of a specific year.
When setting the alarm-threshold registers, ensure that
both the hour-timekeeping register and the hour-alarm-
threshold register are using the same-hour format
(either 12hr or 24hr format).
The alarm function, as well as the ALM output, is opera-
tional in both VCC and battery-backup mode.
MAX6917
I2C-Compatible RTC with Microprocessor
Supervisor, Alarm, and NV RAM Controller
20
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