Silicon Laboratories SI4421 manual Si4421, Logic connections between power control bits

Si4421

Logic connections between power control bits:

		enable
		power amplifier
et		start TX	enable
			power amplifier
					enable
	Edge
					RF front end
	detector	clear TX latch
		enable	enable
es		RF synthesizer		VCO and
			RF synthesizer
				PLL
			start TX
		enable		TX latch
er		RF front end	clear TX latch
			enable		enable baseband
				Crystal	circuits
			crystal oscillator
		enable baseband		oscillator	I/Q
					demod
		circuits
ebb
					Digital signal
		enable crystal			processing
		oscillator
ex
					clock and data out

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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