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SI4020 Datasheet(PDF) 2 Page - Silicon Laboratories |
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SI4020 Datasheet(HTML) 2 Page - Silicon Laboratories |
2 / 32 page Si4020 2 DETAILED DESCRIPTION The Si4020 FSK transmitter is designed to cover the unlicensed frequency bands at 315, 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 PLL’s high resolution allows the usage of multiple channels in any of the bands. The FSK deviation is selectable (from 30 to 240 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 uA 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. 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. 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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