RF42/43
7.6. 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:
R -D
32 X M X 2
WUT =
ms
32.768
WUT Register
Description
wtr[3:0]
wtd[1:0]
R Value in Formula
D Value in Formula
M Value in Formula
wtm[15:0]
Use of the D variable in the formula is only necessary if finer resolution is required than the R value gives.
POR
Function/Descri
ption
D7
D6
D5
D4
D3
D2
D1
D0
Ad
14
15
16
17
18
R/W
Def.
R/W
Wake-Up Timer
Period 1
wtr[3]
wtr[2]
wtr[1]
wtr[0]
wtd[1]
wtd[0]
00h
R/W
R/W
Wake-Up Timer
Period 2
wtm
[15]
wtm
[7]
wtm[14] wtm[13] wtm[12] wtm[11] wtm[10] wtm[9] wtm[8]
00h
00h
—
Wake-Up Timer
Period 3
wtm[6]
wtv[14]
wtv[6]
wtm[5]
wtv[13]
wtv[5]
wtm[4]
wtv[12]
wtv[4]
wtm[3]
wtv[11]
wtv[3]
wtm[2]
wtv[10]
wtv[2]
wtm[1] wtm[0]
Low Battery
wtv[
R
R
wtv[9]
wtv[1]
wtv[8]
wtv[0]
Detector Threshold 15]
Battery Voltage
Level
wtv[
7]
—
There are two different methods for utilizing the wake-up timer (WUT) depending on if the WUT interrupt is enabled
in ―Register 06h. Interrupt Enable 2,‖. If the WUT interrupt is enabled then nIRQ pin will go low when the timer expires.
The chip will also change state so that the 30 M XTAL is enabled so that the microcontroller clock output is available
for the microcontroller to use process the interrupt. The other method of use is to not enable the WUT interrupt and use
the WUT GPIO setting. In this mode of operation the chip will not change state until commanded by the microcontroller.
The two different modes of operation of the WUT are demonstrated in Figure 19.
A 32 kHz XTAL may also be used for better timing accuracy. By setting the x32 ksel bit in 07h, GPIO0 is automatically
reconfigured so that an external 32 kHz XTAL may be connected to this pin. In this mode, the GPIO0 is extremely
sensitive to parasitic capacitance, so only the XTAL should be connected to this pin and the XTAL should be physically
located as close to the pin as possible. Once the x32 ksel bit is set, all internal functions such as WUT, micro-controller
clock, and LDC mode will use the 32 K XTAL and not the 32 kHz RC oscillator.
43
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