SI4464 Datasheet, PDF (27/56 Pages) Silicon Laboratories – HIGH-PERFORMANCE

Si4464/63/61/60
FIFO memory. For further information on the configuration of the FIFOs for a specific application or packet size,
see "6. Data Handling and Packet Handler" on page 39. In RX mode, only the bytes of the received packet
structure that are considered to be “data bytes” are stored in FIFO memory. Which bytes of the received packet are
considered “data bytes” is determined by the Automatic Packet Handler (if enabled) in conjunction with the Packet
Handler configuration. If the Automatic Packet Handler is disabled, all bytes following the Sync word are
considered data bytes and are stored in FIFO memory. Thus, even if Automatic Packet Handling operation is not
desired, the preamble detection threshold and Sync word still need to be programmed so that the RX Modem
knows when to start filling data into the FIFO. When the FIFO is being used in RX mode, all of the received data
may still be observed directly (in realtime) by properly programming a GPIO pin as the RXDATA output pin; this can
be quite useful during application development. When in FIFO mode, the chip will automatically exit the TX or RX
State when either the PACKET_SENT or PACKET_RX interrupt occurs. The chip will return to the IDLE state
programmed in the argument of the “START TX” or “START RX” API command, TXCOMPLETE_STATE[3:0] or
RXVALID_STATE[3:0]. For example, the chip may be placed into TX mode by sending the “START TX” command
and by writing the 30h to the TXCOMPLETE_STATE[3:0] argument. The chip will transmit all of the contents of the
FIFO, and the ipksent interrupt will occur. When this event occurs, the chip will return to the ready state as defined
by TXCOMPLETE_STATE[3:0] = 30h.
4.3.2.2. Direct Mode
For legacy systems that perform packet handling within the host MCU or other baseband chip, it may not be
desirable to use the FIFO. For this scenario, a Direct mode is provided, which bypasses the FIFOs entirely. In TX
Direct mode, the TX modulation data is applied to an input pin of the chip and processed in “real time” (i.e., not
stored in a register for transmission at a later time). Any of the GPIOs may be configured for use as the TX Data
input function. Furthermore, an additional pin may be required for a TX Clock output function if GFSK modulation is
desired (only the TX Data input pin is required for FSK). To achieve direct mode, the GPIO must be configured in
the “GPIO_PIN_CFG” API command as well as the “MODEM_MOD_TYPE” API property. For GFSK,
“TX_DIRECT_MODE_TYPE” must be set to Synchronous. For 2FSK or OOK, the type can be set to asynchronous
or synchronous. The MOD_SOURCE[1:0] should be set to 01h for are all direct mode configurations. In RX Direct
mode, the RX Data and RX Clock can be programmed for direct (real-time) output to GPIO pins. The
microcontroller may then process the RX data without using the FIFO or packet handler functions of the RFIC.
4.4. Preamble Length
The preamble length requirement is only relevant if using the synchronous demodulator. If the asynchronous
demodulator is being used, then there is no requirement for a conventional 101010 pattern.
The preamble detection threshold determines the number of valid preamble bits the radio must receive to qualify a
valid preamble. The preamble threshold should be adjusted depending on the nature of the application. The
required preamble length threshold depends on when receive mode is entered in relation to the start of the
transmitted packet and the length of the transmit preamble. With a shorter than recommended preamble detection
threshold, the probability of false detection is directly related to how long the receiver operates on noise before the
transmit preamble is received. False detection on noise may cause the actual packet to be missed. The preamble
detection threshold may be adjusted in the modem calculator by modifying the “PM detection threshold” in the “RX
parameters tab” in the radio control panel. For most applications with a preamble length longer than 32 bits, the
default value of 20 is recommended for the preamble detection threshold. A shorter Preamble Detection Threshold
may be chosen if occasional false detections may be tolerated. When antenna diversity is enabled, a 20- bit
preamble detection threshold is recommended. When the receiver is synchronously enabled just before the start of
the packet, a shorter preamble detection threshold may be used. Table 12 demonstrates the recommended
preamble detection threshold and preamble length for various modes.
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