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Transmit Mode
Frame-Check Sequence
A 2-byte frame-check sequence (FCS) follows the last MAC payload byte as shown in Figure 23-3. The
FCS is calculated over the MPDU, i.e., the frame-length field is not part of the FCS.
The FCS polynomial defined in [1] is
G(s) = x
16
+ x
12
+ x
5
+ 1
The radio supports automatic calculation/verification of the FCS. See Section 23.8.10 for details.
23.8 Transmit Mode
This section describes how to control the transmitter, how to control the integrated frame processing, and
how to use the TXFIFO.
23.8.1 TX Control
The radio has many built-in features for frame processing and status reporting. Note that the radio
provides features that make it easy 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 is started by the following actions:
• The STXON command strobe
– The SAMPLED_CCA signal is not updated.
• The STXONCCA command strobe, provided that the CCA signal is high.
– Aborts ongoing transmission/reception and forces a TX calibration followed by transmission.
– The SAMPLED_CCA signal is updated.
Clear channel assessment is described in detail in Section 23.8.12.
Frame transmission is aborted by the following command actions:
• The SRXON command strobe
– Aborts ongoing transmission and forces an RX calibration
• The SRFOFF command strobe
– Aborts ongoing transmission/reception and forces the FSM to the IDLE state.
• The STXON command strobe
– Aborts ongoing transmission and forces an RX calibration
To enable the receiver after transmission with STXON, the FRMCTRL1.SET_RXENMASK_ON_TX bit should
be set. This sets bit 6 in RXENABLE when STXON is executed. When transmitting with STXONCCA, the
receiver is on before the transmission and is turned back on afterwards (unless the RXENABLE registers
have been cleared in the meantime).
23.8.2 TX State Timing
Transmission of preamble begins 192 μs after the STXON or STXONCCA command strobe. This is
referred to as TX turnaround time in [1]. There is an equal delay when returning to receive mode.
When returning to idle or receive mode, there is a 2-μs 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 μs.
• The radio FSM transition to the IDLE state, which is delayed by 2 μs.
23.8.3 TXFIFO Access
The TXFIFO 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 in Section 23.8.5).
229
SWRU191C–April 2009–Revised January 2012 CC253x Radio
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