SI4022-A0-FT【PDF 2/20 页】IC TX FSK UNI 868/915MHZ 16TSSOP

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Si4022

DETAILED FEATURE-LEVEL DESCRIPTION

The Si4022 FSK transmitter is designed to cover the unlicensed

frequency bands at 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 20 to 160 kHz with 20 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.

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.

Crystal Oscillator and Microcontroller Clock Output

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 transmitter can supply

the clock signal for the microcontroller, so accurate timing is

possible without the need for a second crystal. In normal

operation it is divided from the reference 10 MHz. During sleep

mode a low frequency (typical 32 kHz) output clock signal can

be switched on.

When the microcontroller turns the crystal oscillator off by

clearing the appropriate bit using the Power Management

Command, the chip provides a certain number (default is 128)

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

let it go to idle or sleep mode.

Low Battery Voltage Detector

The low battery detector circuit monitors periodically (typ. 8 ms)

the supply voltage and generates an interrupt if it falls below a

programmable threshold level.

Wake-Up Timer

The wake-up timer has very low current consumption (4 糀 max)

and can be programmed from 1 ms to several hours.

It calibrates itself to the crystal oscillator at every startup and

then at every 40 seconds with an accuracy of ?.5%. When the

crystal oscillator is switched off, the calibration circuit switches

it back on only long enough for a quick calibration (a few

milliseconds) to facilitate accurate wake-up timing. The periodic

autocalibration feature can be turned off.

Event Handling

In order to minimize current consumption, the transmitter

supports the sleep mode. Switching between the various modes

is controlled by the appropriate bits in the Power Management

Command (page 11).

Si4022 generates an interrupt signal on several events (wake-

up timer timeout, low supply voltage detection, on-chip FIFO

almost empty). This signal 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 receiver by the microcontroller through the

SDO pin.

Interface and Controller

An SPI compatible serial interface lets the user select the

frequency band, center frequency of the synthesizer, and the

output power. Division ratio for the microcontroller clock, wake-

up timer period, and low supply voltage 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.

The interface supports the read-out of a status register, providing

detailed information about the status of the transmitter.

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