Rainbow Electronics RF22 - Manual (Page 39)

RF22
Version: 0.1 Date: 12/23/2008
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8.5. Low Battery Detector
A low battery detector (LBD) with digital read-out is integrated into the chip. 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 chip 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".
Ad R/W Function/Description D7 D6 D5 D4 D3 D2 D1 D0 POR Def.
1A R/W Low Battery Detector Threshold lbdt[4] lbdt[3] lbdt[2] lbdt[1] lbdt[0] 14h
1B R Battery Voltage Level 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.
ADCValue mV tage BatteryVol
× + =
50 7 . 1
ADC Value VDD Voltage [V]
0 < 1.7
1 1.7–1.75
2 1.75–1.8
… …
29 3.1–3.15
30 3.15–3.2
31 > 3.2
8.6. Wake-Up Timer
The chip contains an integrated wake-up timer which periodically wakes the chip from SLEEP mode. The wake-up
timer runs from the internal 32.768 kHz RC Oscillator. The wake-up timer can be configured to run when in SLEEP
mode. If enwt = 1 in "Register 07h. Operating Mode and Function Control 1" when entering SLEEP mode, the wake-up
timer will count for a time specified by the Wake-Up Timer Period in Registers 10h-12h. At the expiration of this period
an interrupt will be generated on the nIRQ pin if this interrupt is enabled. The microcontroller will then need to verify the
interrupt by reading the Interrupt Status Registers 03h-04h. The wake-up timer value may be read at any time by the
wtv[15:0] read only registers 13h-14h.
The formula for calculating the Wake-Up Period is the following:
ms
M
WUT
D R
768 . 32
2 32
− Ч Ч
=
WUT Register Description
wtr[3:0] R Value in Formula
wtd[1:0] D Value in Formula
wtm[15:0] M Value in Formula