Synchronization Header And Frame-Length Fields; Frame Filtering; Sfd Signal Timing - Texas Instruments CC253x User Manual

Receive Mode

23.9.4 Synchronization Header and Frame-Length Fields

Frame reception starts with detection of a start-of-frame delimiter (SFD), followed by the length byte,
which determines when the reception is complete. The SFD signal, which can be output on GPIO, can be
used to capture the start of received frames:
Received Frame Preamble
SFD (Accepted Frame)
SFD (Rejected Frame)
Preamble and SFD are not written to the RXFIFO.
The radio uses a correlator to detect the SFD. The correlation threshold value in MDMCTRL1.CORR_THR
determines how closely the received SFD must match an ideal SFD. The threshold must be adjusted with
care:
• If set too high, the radio misses many actual SFDs, effectively reducing the receiver sensitivity.
• If set too low, the radio detects many false SFDs. Although this does not reduce the receiver
sensitivity, the effect is similar, because false frames might overlap with the SFDs of actual frames. It
also increases the risk of receiving false frames with correct FCS.
In addition to SFD detection, it is also possible to require a number of valid preamble symbols (also above
the correlation threshold) prior to SFD detection. See the register descriptions of MDMCTRL0 and
MDMCTRL1 for available options and recommended settings.

23.9.5 Frame Filtering

The frame filtering function rejects nonintended frames as specified by [1], section 7.5.6.2, third filtering
level. In addition, it provides filtering on:
• The eight different frame types (see the FRMFILT1 register)
• The reserved bits in the frame control field (FCF)
The function is controlled by:
• The FRMFILT0 and FRMFILT1 registers
• The PAN_ID, SHORT_ADDR and EXT_ADDR values in RAM
Filtering Algorithm
The FRMFILT0.FRM_FILTER_EN bit controls whether frame filtering is applied or not. When disabled, the
radio accepts all received frames. When enabled (which is the default setting), the radio only accepts
frames that fulfill all of the following requirements:
• The length byte must be equal to or higher than the minimum frame length, which is derived from the
source- and destination-address mode and PAN ID compression subfields of the FCF.
• The reserved FCF bits [9:7] ANDed together with FRMFILT0.FCF_RESERVED_BITMASK must equal
000b.
• The value of the frame version subfield of the FCF cannot be higher than
FRMFILT0.MAX_FRAME_VERSION.
• The source and destination address modes cannot be reserved values (1).
• Destination address:
224
CC253x Radio
SFD LEN
Frame Rejected
Figure 23-11. SFD Signal Timing
Copyright © 2009–2014, Texas Instruments Incorporated
MPDU (LEN[6:0] Bytes)
SWRU191F – April 2009 – Revised April 2014
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