Silicon Laboratories SI4421 Si4421, TX Configuration Control Command, Frequency Setting Command

Si4421

There are four operation modes:

1.(a1=0, a0=0) Automatic operation of the AFC is off. Strobe bit can be controlled by the microcontroller.

2.(a1=0, a0=1) The circuit measures the frequency offset only once after power up. This way, extended TX-RX distance can be achieved. In the final application, when the user inserts the battery, the circuit measures and compensates for the frequency offset caused by the crystal tolerances. This method allows for the use of cheaper quartz in the application and provides protection against tracking an interferer.

3.(a1=1, a0=0) The frequency offset is calculated automatically and the center frequency is corrected when the VDI is high. The calculated value is dropped when the VDI goes low. To improve the efficiency of the AFC calculation two methods are recommended:

a.The transmit package should start with a low effective baud rate pattern (i.e.: 00110011) because it is easier to receive. The circuit automatically measures the frequency offset during this initial pattern and changes the receiving frequency accordingly. The further part of the package will be received by the corrected frequency settings.

b.The transmitter sends the first part of the packet with a step higher deviation than required during normal operation to ease the receiving. After the frequency shift was corrected, the deviation can be reduced.

In both cases (3a and 3b), when the VDI indicates poor receiving conditions (VDI goes low), the output register is automatically cleared. Use this “drop offset” mode when the receiver communicates with more than one transmitter.

4.(a1=1, a0=1) It is similar to mode 3, but suggested to use when a receiver operates with only one transmitter. After a complete measuring cycle, the measured value is kept independently of the state of the VDI signal. When the receiver is paired with only one transmitter, it is possible to use this “keep offset” mode. In this case, the DRSSI limit should be selected carefully to minimize the range hysteresis.

11. TX Configuration Control Command

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0	POR
	1	0	0	1	1	0	0	mp	m3	m2	m1	m0	0	p2	p1	p0	9800h

Bits 8-4 (mp, m3 to m0): FSK modulation parameters:

The resulting output frequency can be calculated as:

	fout = f0 + (-1)SIGN· (M + 1) · (15 kHz)					Pout
where:								df fsk		df fsk
	f0 is the channel center frequency (see the
	Frequency Setting Command)
	M is the four bit binary number <m3 : m0>
	SIGN = (mp) XOR FSK											f out
									f	0
Note: For the optimal FSK modulation settings at different
	data rates see the table on page 37.						mp=0 and FSK=0				mp=0 and FSK=1
Bits 2-0 (p2 to p0): Output power:							or				or
							mp=1 and FSK=1				mp=1 and FSK=0
	p2	p1	p0	Relative Output Power [dB]
	0	0	0		0	Note: FSK represents the value of the actual data bit.
	0	0	1		-2.5
	0	1	0		-5
	0	1	1		-7.5
	1	0	0		-10
	1	0	1		-12.5
	1	1	0		-15
	1	1	1		-17.5

Note: The output power given in the table is relative to the maximum available power, which depends on the actual antenna impedance. (See: Antenna Application Note: IA ISM-AN1)

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