RFM22
8.5. Low Battery Detector
A low battery detector (LBD) with digital read-out is integrated into the module. A digital threshold may be
programmed into the lbdt[4:0] field in "Register 1Ah. Low Battery Detector Threshold". When the digitized battery
voltage reaches this threshold an interrupt will be generated on the nIRQ pin to the microcontroller. The
microcontroller will then need to verify the interrupt by reading "Register 03h. Interrupt/Status 1" and ―Register 04h.
Interrupt/Status 2,‖. If the LBD is enabled while the module is in SLEEP mode, it will automatically enable the RC
oscillator which will periodically turn on the LBD circuit to measure the battery voltage. The battery voltage may also
be read out through "Register 1Bh. Battery Voltage Level" at any time when the LBD is enabled. The Low Battery
Detect function is enabled by setting enlbd=1 in "Register 07h. Operating Mode and Function Control 1".
POR
Def.
Function/Descri
ption
D7
D6
D5
D4
D3
D2
D1
D0
Ad
1A
1B
R/W
R/W
R
Low Battery
Detector Threshold
Battery Voltage
Level
14h
lbdt[4]
lbdt[3]
lbdt[2]
lbdt[1]
lbdt[0]
—
0
0
0
vbat[4] vbat[3]
vbat[2] vbat[1] vbat[0]
The LBD output is digitized by a 5-bit ADC. When the LBD function is enabled, enlbd = 1 in "Register 07h.
Operating Mode and Function Control 1", the battery voltage may be read at anytime by reading "Register 1Bh.
Battery Voltage Level". A Battery Voltage Threshold may be programmed to register 1Ah. When the battery voltage
level drops below the battery voltage threshold an interrupt will be generated on nIRQ pin to the microcontroller if
the LBD interrupt is enabled in ―Register 06h. Interrupt Enable 2,‖ . The microcontroller will then need to verify the
interrupt by reading the interrupt status register, Addresses 03 and 04H. The LSB step size for the LBD ADC is 50
mV, with the ADC range demonstrated in the table below. If the LBD is enabled the LBD and ADC will automatically
be enabled every 1 s for approximately 250 μs to measure the voltage which minimizes the current consumption in
Sensor mode. Before an interrupt is activated four consecutive readings are required.
BatteryVoltage =1.7+50mV × ADCValue
ADC Value
VDD Voltage [V]
< 1.7
0
1
1.7–1.75
2
……
29
1.75–1.8
……
3.1–3.15
3.15–3.2
>3.2
30
31
57
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