Silicon Laboratories SI4421 - Manual (Page 16)

Si4421
Logic connections between power control bits:
Edge
detector
et
er
es
ebb
ex
enable crystal
oscillator
enable baseband
circuits
enable
RF front end
enable
RF synthesizer
start TX
clear TX latch
enable
power amplifier
VCO and
PLL
Crystal
oscillator
Digital signal
processing
TX latch
enable baseband
circuits
enable
RF front end
enable
power amplifier
enable
RF synthesizer
start TX
clear TX latch
enable
crystal oscillator
clock and data out
I/Q
demod
Note:
 If both et and er bits are set the chip goes to receive mode.
 FSK / nFFS input are equipped with internal pull-up resistor. To achieve minimum current consumption do not pull this input
to logic low in sleep mode.
 To enable the RF synthesizer, the crystal oscillator must be turned on
 To turn on the baseband circuits, the RF synthesizer (and this way the crystal oscillator) must be enabled.
 Setting the er bit automatically turns on the crystal oscillator, the synthesizer, the baseband circuits and the RF fronted.
 Setting the et bit automatically turns on the crystal oscillator, the synthesizer and the RF power amplifier.
Clock tail feature: When the clock output (pin 8) used to provide clock signal for the microcontroller (dc bit is set to 0), it is possible
to use the clock tail feature. This means that the crystal oscillator turn off is delayed, after issuing the command (clearing the ex bit)
192 more clock pulses are provided. This ensures that the microcontroller can switch itself to low power consumption mode. In order
to use this feature, a Status Read Command (page 27) must be issued before the ex bit set to zero. If status read was not performed
then the clock output shuts down immediately leaving the microcontroller in unknown state.
Automatic crystal oscillator enable/disable feature: When an interrupt occurs, the crystal oscillator automatically turns on –
regardless to the setting of the ex bit – to supply clock signal to the microcontroller. After clearing all interrupts by handling them
properly (see the Interrupt Handling section, page 28) and performing Status Read Command, the crystal oscillator is automatically
turned off. The clock tail feature provides enough clock pulses for the microcontroller to go to low power mode. Due to this automatic
feature, it is not possible to turn off the crystal by clearing the ex bit if any interrupt is active. For example, after power on the POR
interrupt must be cleared by a status read then writing zero to the ex bit will put the part into sleep mode. Very important to clear all
interrupts before turning the ex bit off because the extra current required by running crystal oscillator can shorten the battery life
significantly.
Disabling the clock output (bit dc=1) turns off both the clock tail and the automatic crystal oscillator enable/disable feature, only the
ex bit controls the crystal oscillator (supposing that both the er and et bits are cleared), the interrupts have no effect on it.
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