SI4021-A1-FT【PDF 2/34 页】IC TX FSK 915MHZ 5.4V 16-TSSOP

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Si4021

DETAILED DESCRIPTION

The Si4021 FSK transmitter is designed to cover the unlicensed

frequency bands at 433, 868, and 915 MHz. The device

facilitates compliance with FCC and ETSI requirements.

PLL

The programmable PLL synthesizer determines the operating

frequency, while preserving accuracy based on the on-chip

crystal-controlled reference oscillator. The PLLs high resolution

allows the usage of multiple channels in any of the bands. The

FSK deviation is selectable (from 30 to 210 kHz with 30 kHz

increments) to accommodate various bandwidth, data rate and

crystal tolerance requirements, and it is also highly accurate due

to the direct closed-loop modulation of the PLL. The transmitted

digital data can be sent asynchronously through the FSK pin or

over the control interface using the appropriate command.

The RF VCO in the PLL performs automatic calibration, which

requires only a few microseconds. To ensure proper operation in

the programmed frequency band, the RF VCO is automatically

calibrated upon activation of the synthesizer. If temperature or

supply voltage change significantly or operational band has

changed, VCO recalibration is recommended.. Recalibration can

be initiated at any time by switching the synthesizer off and back

on again.

RF Power Amplifier (PA)

The power amplifier has an open-collector differential output and

can directly drive a loop antenna with a programmable output

power level. An automatic antenna tuning circuit is built in to

avoid costly trimming procedures and the so-called hand effect.

The transmitters can operate in On-Off Keying (OOK) mode by

switching the power amplifier on and off. When the appropriate

control bit is set using the Power Setting Command, the FSK pin

becomes an enable input (active high) for the power amplifier.

Crystal Oscillator

The chip has a single-pin crystal oscillator circuit, which provides

a 10 MHz reference signal for the PLL. To reduce external parts

and simplify design, the crystal load capacitor is internal and

programmable. Guidelines for selecting the appropriate crystal

can be found later in this datasheet.

The transmitters can supply the clock signal for the

microcontroller, so accurate timing is possible without the need

for a second crystal. When the chip receives a Sleep Command

from the microcontroller and turns itself off, it provides several

further clock pulses (clock tail) for the microcontroller to be

able to go to idle or sleep mode. The length of the clock tail is

programmable.

Low Battery Voltage Detector

The low battery voltage detector circuit monitors the supply

voltage and generates an interrupt if it falls below a

programmable threshold level. The detector circuit has 50 mV

hysteresis.

Wake-Up Timer

The wake-up timer has very low current consumption (1.5 糀

typical) and can be programmed from 1 ms to several days with

an accuracy of ?%.

It calibrates itself to the crystal oscillator at every startup, and

then every 30 seconds. When the oscillator is switched off, the

calibration circuit switches on the crystal oscillator only long

enough for a quick calibration (a few milliseconds) to facilitate

accurate wake-up timing. The auto calibration feature can be

disabled by setting the a bit in the Low Battery Detector

Command.

Event Handling

In order to minimize current consumption, the device supports

sleep mode. Active mode can be initiated by several wake-up

events: timeout of wake-up timer, detection of low supply

voltage, pressing any of the four push-button inputs, or through

the serial interface. The push-button inputs can be driven by a

logic signal from a microcontroller or controlled directly by

normally open switches. Pull-up resistors are integrated.

If any wake-up event occurs, the wake-up logic generates an

interrupt, which can be used to wake up the microcontroller,

effectively reducing the period the microcontroller has to be

active. The cause of the interrupt can be read out from the

transmitters by the microcontroller through the nIRQ pin.

Interface

An SPI compatible serial interface lets the user select the

operating frequency band and center frequency of the

synthesizer, polarity and deviation of FSK modulation, and output

power level. Division ratio for the microcontroller clock, wake-up

timer period, and low battery detector threshold are also

programmable. Any of these auxiliary functions can be disabled

when not needed. All parameters are set to default after power-

on; the programmed values are retained during sleep mode.

EEPROM Mode

In simple applications, the on-chip digital controller provides the

transmitters with direct interface to a serial (SPI) EEPROM. In this

case, no external microcontroller is necessary. Wake-up events

initiate automatic readout of the assigned command sequence

from EEPROM memory. For every event, there is a dedicated

starting address available in the EEPROM.

Programming the EEPROM is very simple. Any control command

can be programmed in the EEPROM sequentially (same as in

microcontroller mode).

The internal power-on reset (POR) is a dedicated event, which

can be used to program the basic settings of the transmitters. In

this case the chip starts to read out the preprogrammed data

from the 00h address in EEPROM. Data can be transmitted with

the help of the Data Transmit Command, which tells the

transmitters how many bytes must be transmitted. The whole

process finishes with a Sleep Command.

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