CC2520_11 Datasheet, PDF (67/133 Pages) Texas Instruments – 2.4 GHZ IEEE 802.15.4/ZIGBEE RF TRANSCEIVER

CC2520 DATASHEET
2.4 GHZ IEEE 802.15.4/ZIGBEE® RF TRANSCEIVER
SWRS068 – DECEMBER 2007
19 Transmit Mode
This section describes how to control the transmitter, the integrated frame processing and how to use the
TX FIFO.
19.1 TX Control
CC2520 has many built in features for frame processing and status reporting. Note that CC2520 provides
features that make it easy for the microcontroller to have precise control of the timing of outgoing frames.
This is very important in an IEEE 802.15.4/ZigBee system, because there are strict timing requirements to
such systems.
Frame transmission will be started by the following actions:
• The STXON command strobe
o The SAMPLED_CCA signal is not updated.
• The STXONCCA command strobe, provided that the CCA signal is high.
o Aborts ongoing transmission/reception and forces a TX calibration followed by transmission.
o The SAMPLED_CCA signal is updated
Clear channel assessment is described in detail in section 19.7.
Frame transmission will be aborted by the following command actions:
• The SRXON command strobe
o Aborts ongoing transmission and forces a RX calibration
• The SRFOFF command strobe
o Aborts ongoing transmission/reception and forces the FSM to the IDLE state.
• The STXON command strobe
o See above.
To enable the receiver after transmission with STXON, the FRMCTRL1.SET_RXENMASK_ON_TX bit
should be set. This will set bit 14 in RXENABLE when STXON is executed. When transmitting with
STXONCCA, the receiver would be on before the transmission and will be turned back on afterwards
(unless the RXENABLE registers have been cleared in the mean time).
19.2 TX State Timing
Transmission of preamble begins 192 us after the STXON or STXONCCA command strobe. This is referred
to as "TX turnaround time" in [2]. There is an equal delay when returning to receive mode.
When returning to idle or receive mode, there is a 2 us delay while the modulator ramps down the signals to
the DACs. The down ramping happens automatically after the complete MPDU (as defined by the length
byte) has been transmitted or if TX underflow occurs. This affects:
• The SFD signal, which is stretched by 2 us.
• The radio FSM transition to the IDLE state, which is delayed by 2 us.
19.3 TX FIFO Access
The TX FIFO can hold 128 bytes and only one frame at a time. The frame can be buffered before or after
the TX command strobe is executed, as long as it does not generate TX underflow (see the error conditions
listed below).
Figure 16 illustrates what needs to be written to the TX FIFO (marked blue). Additional bytes are ignored,
unless TX overflow occurs (see the error conditions listed below).
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