RFM22
8.6. Wake-Up Timer
The RFM22 contains an integrated wake-up timer which periodically wakes the module from SLEEP mode.
Thewake-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:
32 X M X 2R-D
WUT =
ms
32.768
WUT Register
Description
wtr[3:0]
wtr[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
Def.
Function/Descri
D7
D6
D5
D4
D3
D2
D1
D0
Ad
14
15
16
17
18
R/W
R/W
R/W
R/W
R
ption
Wake-Up Timer
Period 1
wtr[3]
wtr[2]
wtr[1]
wtr[0]
wtd[1]
wtd[0]
00h
00h
00h
—
Wake-Up Timer
Period 2
wtm
[15]
wtm
[7]
wtm[14] wtm[13] wtm[12] wtm[11] wtm[10] wtm[9] wtm[8]
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]
Wake-Up Timer
Value 1
wtv[
15]
wtv[
7]
wtv[9]
wtv[1]
wtv[8]
wtv[0]
Wake-Up Timer
Value 2
R
—
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 module 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 module will not change state until
commanded by the microcontroller. The two different modes of operation of the WUT are demonstrated in Figure 28.
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.
58
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