CC112X/CC1175
SWRU295C
Page 34 of 108
5.7
Preamble Detection
CC112X
has a high performance preamble detector which can be turned on by setting
.
The preamble quality estimator uses an 8 bits wide correlation filter to find a valid preamble. A
preamble qualifier value is available through the
threshold is configured with the register field
A preamble is detected if the preamble qualifier value is less than the programmed PQT threshold. A
“Preamble Quality Reached” signal can be observed on one of the GPIO pins by setting
IOCFGx.GPIOx_CFG = PQT_REACHED
(11). It is also possible to determine if preamble quality is
bit in the
stay asserted as long as a preamble is present but will de-assert on sync found. If the preamble
disappears, the signal will de-assert after a timeout defined by the sync word length + 10 symbols
after preamble was lost. When
SYNC_CFG1.DEM_CFG = 010b
or
110b
, sync word search is only
gated if a preamble is detected.
The PQT can also be used as a qualifier for the optional RX termination timer (see Section 8.5.1 for
more details).
A PQT startup timer is available and the startup timer period is programmable through the
PREAMBLE_CFG0.PQT_VALID_TIMEOUT
register. When
, 16 bits must
be received before
is asserted and when
, 43 bits must be
received. This enables a tradeoff between speed and accuracy as preamble search will not be gated
before
can be monitored on a GPIO pin by setting
(12).
5.8
RSSI
The AGC module returns an estimate on the signal strength received at the antenna called RSSI
(Received Signal Strength Indicator). The RSSI is a 12 bits two's complement number with 0.0625 dB
resolution hence ranging from
–
128 to 127 dBm. A value of
–
128 dBm indicates that the RSSI is
invalid. The RSSI can be found by reading
. It should be
noted that for most applications using the 8 MSB bits of the RSSI, with 1 dB resolution, is good
enough.
To get a correct RSSI value a calibrated RSSI offset value should be subtracted from the value given
by
.
The RSSI offset value can be found by input a signal of known strength to the radio
AGC_GAIN_ADJUST.GAIN_ADJUSTMENT
The RSSI value is output from a configurable moving average filter in order to reduce uncertainty in
the RSSI estimates. It is as such possible to trade RSSI computation speed/update rate against RSSI
accuracy. This trade-off is determined by configuring the
register field gives the number of new input samples to the moving average filter (internal RSSI
estimates) that are required before the next update of the RSSI value
8
8
By
setting the
or
IOCFG2.GPIO2_CFG = RSSI_UPDATE
(14),
a pulse
will occur on GPIO3 or GPIO2 each time the RSSI value is updated
Example:
Assume a -70 dBm signal into the antenna and
RSSI[11:0] = 0x200
(32) when
AGC_GAIN_ADJUST.GAIN_ADJUSTMENT = 0x00
.
This means that the offset is 102 dB as 32 dBm
– 102 dB = –70 dBm.
When the offset is known it can be written to the
AGC_GAIN_ADJUST.GAIN_ADJUSTMENT
register field (
GAIN_ADJUSTMENT = 0x9A
(102)). When the same signal is input to the antenna,
register will be 0xBA0 (-70).