RFM22 Datasheet, PDF (51/150 Pages) –

RFM22
8.2. Microcontroller Clock
The crystal oscillator frequency is divided down internally and may be output to the microcontroller through GPIO2.
This feature is useful to lower BOM cost by using only one crystal in the system. The system clock frequency is
selectable from one of 8 options, as shown below. Except for the 32.768 kHz option, all other frequencies are
derived by dividing the Crystal Oscillator frequency. The 32.768 kHz clock signal is derived from an internal RC
Oscillator or an external 32 kHz Crystal, depending on which is selected. The GPIO2 default is the microcontroller
clock with a 1 MHz microcontroller clock output.
POR
Add R/W Function/Description D7 D6 D5
D4 D3
D2
D1
D0
Def.
0A R/W Microcontroller Output Clock
clkt[1] clkt[0] enlfc mclk[2] mclk[1] mclk[0] 0Bh
mclk[2:0]
000
001
010
011
100
101
110
111
Modulation Source
30 MHz
15 MHz
10 MHz
4 MHz
3 MHz
2 MHz
1 MHz
32.768 KHz
If the microcontroller clock option is being used there may be the need of a System Clock for the microcontroller
while the RFM22 is in SLEEP mode. Since the Crystal Oscillator is disabled in SLEEP mode in order to save
current, the low-power 32.768 kHz clock can be automatically switched to become the microcontroller clock. This
feature is called Enable Low Frequency Clock and is enabled by the enlfc bit. When enlfc = 1 and the module is in
SLEEP mode then the 32.768 kHz clock will be provided to the microcontroller as the System Clock, regardless of
the setting of mclk[2:0]. For example, if mclk[2:0] = 000, 30 MHz will be provided through the GPIO output pin to
the microcontroller as the System Clock in all IDLE, TX, or RX states. When the module is commanded to SLEEP
mode, the System Clock will become 32.768 kHz.
Another available feature for the microcontroller clock is the Clock Tail, clkt[1:0]. If the Enable Low Frequency
Clock feature is not enabled (enlfc = 0), then the System Clock to the microcontroller is disabled in SLEEP mode.
However, it may be useful to provide a few extra cycles for the microcontroller to complete its operation prior to the
shutdown of the System Clock signal. Setting the clkt[1:0] field will provide additional cycles of the System Clock
before it shuts off.
clkt[1:0]
00
Modulation Source
0 cycles
01
128 cycles
10
256 cycles
11
512 cycles
If an interrupt is triggered, the microcontroller clock will remain enabled regardless of the selected mode. As soon as the interrupt
is read the state machine will then move to the selected mode. For instance, if the module is commanded to Sleep mode but an
interrupt has occurred the 30 MHz XTAL will not disable until the interrupt has been cleared.
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