RFM22B/23B
8.2. Microcontroller Clock
The 30 MHz 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. The default setting for GPIO2 is to output the microcontroller
clock signal with a frequency of 1 MHz.
D7
D6
D5
D4
D3
D2
D1
D0
POR Def.
Add R/W
Function/Description
0A R/W
Microcontroller Output Clock
clkt[1] clkt[0] enlfc mclk[2] mclk[1] mclk[0]
06h
mclk[2:0]
000
Clock Frequency
30 MHz
001
15 MHz
010
10 MHz
011
4 MHz
100
3 MHz
101
2 MHz
110
1 MHz
111
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 RFM22B/23B 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 in “Register 0Ah. Microcontroller Output
Clock." When enlfc = 1 and the chip 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 chip enters SLEEP mode, the system clock will automatically switch to 32.768 kHz from the RC
oscillator or 32.768 XTAL.
Another available feature for the microcontroller clock is the clock tail, clkt[1:0] in “Register 0Ah. Microcontroller
Output Clock." If the 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]
Clock Tail
0 cycles
00
01
10
11
128 cycles
256 cycles
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. The minimum current consumption
will not be achieved until the interrupt is read. For instance, if the chip is commanded to SLEEP mode but an
interrupt has occurred the 30 MHz XTAL will not be disabled until the interrupt has been cleared.
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