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28.5.2 Battery-Backed Clock Current Consumption
For the battery-backed features of the Rabbit 4000 to perform while the processor is pow-
ered down, both the VBAT and BATIO pins need to be supplied properly. The VBAT pin
powers the internal real-time clock and the battery-backed SRAM, while VBATIO powers
the /RESET, /CS1, CLK32K, and RESOUT pins.
Note that the VBATIO pin can be powered at 1.8 V during powerdown even if the proces-
sor is running at 3.3 V normally. A circuit to switch between a 1.8–2.0 V battery and the
main power can use the RESOUT pin to switch the power source for the VBATIO pin. R
is a current-limiting resistor that should be adjusted for the battery voltage; a good value to
use for a 3.0 V battery is 150 k
Ω
.
Figure 28-7. Switching Circuit for VATIO Pin
Table 28-12 shows the typical current consumption for these pins while the remainder of
the Rabbit 4000 is powered down.
Table 28-12. Typical Battery-Backed Current Consumption
(-40°C to +85°C)
Pin
Voltage
Current
VBAT
1.8 V
1.7 µA
VBATIO
1.8 V
0.1 µA
Rabbit 4000
RESOUT
3.3 V
Main Power
(
p
channel)
FDV302P
2.2 nF
VBATIO
BAT54
R
Battery
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