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P R E L I M I N A R Y
54
Am79C930
CCA Result
UBDCS URSSI Baud Detect Carrier RSSI >= RSSI (CHBSY Bit
TCR28:1 TCR28:0 Sense Decision Lower Limit of TIR26)
0 0 don’t care don’t care CHBSY = TRUE
0 1 don’t care yes CHBSY = TRUE
0 1 don’t care no CHBSY = FALSE
1 0 TRUE don’t care CHBSY = TRUE
1 0 FALSE don’t care CHBSY = FALSE
1 1 TRUE yes CHBSY = TRUE
1 1 TRUE no CHBSY = FALSE
1 1 FALSE yes CHBSY = FALSE
1 1 FALSE no CHBSY = FALSE
The current CCA result is reported in the CHBSY bit
of TIR26.
A rising edge of CHBSY will set the Busy Channel
Found (BCF) bit of TIR5. This bit may serve as an inter-
rupt to the 80188 core, or the interrupt due to this bit may
be masked and the bit can be polled by the 80188 core.
The CCA result is also reported in the CHBSYC bit of
TIR4. This bit reports a change in state of the carrier
sense. This bit may serve as an interrupt to the 80188
core, or the interrupt due to this bit may be masked and
the bit can be polled by the 80188 core.
The current RSSI limit comparison test result may be
read from the RSALT bit (RSSI Above Limit) of TIR28.
The CCA result has no effect on the Transmit state ma-
chine operation. That is, if the CCA result is CHBSY =
TRUE and the TXS bit (Transmit Start) of TIR8 has been
set to a 1, then the transmit state machine will proceed
with execution of its transmission sequence. Determina-
tion of exactly when to begin transmission is the respon-
sibility of the firmware that sets the TXS bit of TIR8,
based upon input derived from the CHBSY bit of TIR26
and other considerations (such as NAV value, backoff
timer, etc.).
Automatic Antenna Diversity Logic
The TAI contains automatic antenna diversity logic that
relies on carrier sense determination in order to select a
satisfactory antenna for frame reception. The general
function of the antenna diversity logic is as follows:
The automatic antenna diversity logic switches the
ANTSLT and ANTSLT pins between two different anten-
nas repeatedly at a programmable periodic rate. Meas-
urements of signal strength and Baud Determination
testing are performed on each antenna. Antenna Diver-
sity Switching and test measurements continue until the
combination of the test outcomes dictates a stop to di-
versity switching. At such a point, a satisfactory antenna
has been found, antenna switching will cease and the
selected antenna will be used for reception until the Re-
ceive RESET (RXRES) bit of TIR16 is set, or until the
Receive Start (RXS) bit of TIR16 is set, or a hard reset,
or a soft reset. Baud detection tests continue during
antenna lock time, using the same periodicity (i.e.,
the dwell timer), even though antenna switching has
stopped. These baud tests provide input for the
CCA logic.
An antenna is deemed “satisfactory” when the combina-
tion of inputs to the Stop Diversity decision logic is
TRUE. The Stop Diversity decision is based upon the
value of one or two input conditions, where the user may
choose which conditions are examined. The following
are the two inputs that may be used for Stop
Diversity decisions:
One possible input to the Stop Diversity decision logic is
the Baud Detect for Stop Diversity determination as
described in the
Baud Determination
section and
summarized here. The Baud Detect for Stop Diversity
determination is made by making multiple measure-
ments of the separation between adjacent (and same–
direction) bit stream edge transitions (baud detect
tests), then comparing the measured rate of edge transi-
tions against programmed limits and tallying the number
of passes and failures of these tests, checking to see
that the total number of passed tests exceeds a given
lower limit, and finally, checking that the ratio of passes
to fails exceeds a given threshold ratio. If this final result
is TRUE, then the Baud Detect for Stop Diversity is con-
sidered to be “TRUE.”
Note that Baud Determination of Stop Diversity meas-
urements are made on a periodic basis where the period
and duty cycle of the measurements depends upon the
settings of the Antenna Diversity Timer (TCR4) and the
Baud Detect Start timer (TCR16). The Dwell Timer
never stops running (unless set to 0).
The second possible input to the Stop Diversity decision
logic is the result of a comparison of the RSSI converted
value against a pre-programmed lower limit. If the
measured RSSI input value exceeds the programmed
lower limit, then the result of this test is considered to
be TRUE.
The two tests mentioned above may be separately
selected/deselected to serve as inputs to the Stop