SI4021-A1-FT Silicon Laboratories Inc, SI4021-A1-FT Datasheet - Page 2



Manufacturer Part Number
Silicon Laboratories Inc

Specifications of SI4021-A1-FT

Package / Case
433MHz, 868MHz, 915MHz
Modulation Or Protocol
Data Rate - Maximum
Power - Output
Current - Transmitting
Data Interface
PCB, Surface Mount
Antenna Connector
PCB, Surface Mount
Voltage - Supply
2.2 V ~ 5.4 V
Operating Temperature
-40°C ~ 85°C
Operating Frequency
433 MHz to 915 MHz
Mounting Style
Operating Supply Voltage
2.2 V to 5.4 V
Supply Current
1.5 mA
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Memory Size
Lead Free Status / Rohs Status
Lead free / RoHS Compliant
Other names

Available stocks

Part Number
Part Number:
5 000
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.
The programmable PLL synthesizer determines the operating
frequency, while preserving accuracy based on the on-chip
crystal-controlled reference oscillator. The PLL’s 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
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
Wake-Up Timer
The wake-up timer has very low current consumption (1.5 μA
typical) and can be programmed from 1 ms to several days with
an accuracy of ±5%.
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
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.
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.
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.