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Registers
MDMCTRL0 (0x61A8) – Controls Modem
Bit Name Reset R/W Description
No.
7:6
DEM_NUM_ZEROS[1:0]
10 R/W Sets how many zero symbols must be detected before the sync word when
searching for sync. Note that only one is required to have a correlation value
above the correlation threshold set in the MDMCTRL1 register.
00: Reserved
01: 1 zero symbol
10: 2 zero symbols
11: 3 zero symbols
5
DEMOD_AVG_MODE
0 R/W Defines the behavior or the frequency offset averaging filter.
0: Lock average level after preamble match. Restart frequency offset
calibration when searching for the next frame.
1: Continuously update average level.
4:1
PREAMBLE_LENGTH [3:0]
0010 R/W The number of preamble bytes (two zero-symbols) to be sent in TX mode prior
to the SFD, encoded in steps of 2 symbols (1 byte). The reset value of 2 is
compliant with IEEE 802.15.4.
0000: 2 leading-zero bytes
0001: 3 leading-zero bytes
0010: 4 leading-zero bytes
… …
1111: 17 leading-zero bytes
0
TX_FILTER
1 R/W Defines the kind of TX filter that is used. The normal TX filter is as defined by
the IEEE802.15.4 standard. Extra filtering may be applied in order to lower the
out-of-band emissions.
0: Normal TX filtering
1: Enable extra filtering
MDMCTRL1 (0x61A9) – Controls Modem
Bit Name Reset R/W Description
No.
7:6 – 00 R0 Read as zero
5
CORR_THR_SFD
0 R/W Defines requirements for SFD detection:
0: The correlation value of one of the zero symbols of the preamble must
be above the correlation threshold.
1: The correlation value of one zero symbol of the preamble and both
symbols in the SFD must be above the correlation threshold.
4:0
CORR_THR[4:0]
0x14 R/W Demodulator correlator threshold value, required before SFD search.
Threshold value adjusts how the receiver synchronizes to data from the radio. If
the threshold is set too low, sync can more easily be found on noise. If set too
high, the sensitivity is reduced, but sync is not likely to be found on noise.
In combination with DEM_NUM_ZEROS, the system can be tuned so sensitivity
is high with less sync found on noise.
FREQEST (0x61AA) – Estimated RF Frequency Offset
Bit Name Reset R/W Description
No.
7:0
FREQEST[7:0]
0x00 R Signed 2s-complement value. Contains an estimate of the frequency offset
between carrier and the receiver LO. The offset frequency is
FREQEST × 7800 Hz. DEM_AVG_MODE controls when this estimate is updated.
If DEM_AVG_MODE = 0, it is updated until sync is found. Then the frequency
offset estimate is frozen until the end of the received frame. If DEM_AVG_MODE
= 1, it is updated as long as the demodulator is enabled. To calculate the
correct value, one must use an offset (FREQEST_offset), which can be found
in the device's data sheet (Appendix C). Real FREQEST value = FREQEST –
FREQEST_offset.
277
SWRU191C–April 2009–Revised January 2012 CC253x Radio
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