Si1000/1/2/3/4/5
taken in setting the drive strength and loading on GPIO2 when the microcontroller clock is used. Excess
loading or inadequate drive may contribute to increased spurious emissions.
Pin 6, ANT may be used as an alternate to control a TR switch. Pin 6 is a hardwired version of GPIO set-
ting 11000, Antenna 2 Switch used for antenna diversity. It can be manually controlled by the antdiv[2:0]
bits in register 08h if antenna diversity is not used. See AN440, register 08h for more details.
23.8.9. Antenna Diversity
To mitigate the problem of frequency-selective fading due to multi-path propagation, some transceiver sys-
tems use a scheme known as antenna diversity. In this scheme, two antennas are used. Each time the
transceiver enters RX mode the receive signal strength from each antenna is evaluated. This evaluation
process takes place during the preamble portion of the packet. The antenna with the strongest received
signal is then used for the remainder of that RX packet. The same antenna will also be used for the next
corresponding TX packet.
This chip fully supports antenna diversity with an integrated antenna diversity control algorithm. The
required signals needed to control an external SPDT RF switch (such as PIN diode or GaAs switch) are
available on the GPIOx pins. The operation of these GPIO signals is programmable to allow for different
antenna diversity architectures and configurations. The antdiv[2:0] bits are found in register 08h “Operating
& Function Control 2.” The GPIO pins are capable of sourcing up to 5 mA of current, so it may be used
directly to forward-bias a PIN diode if desired.
The antenna diversity algorithm will automatically toggle back and forth between the antennas until the
packet starts to arrive. The recommended preamble length for optimal antenna selection is 8 bytes. A spe-
cial antenna diversity algorithm (antdiv[2:0] = 110 or 111) is included that allows for shorter preamble
lengths for beacon mode in TDMA-like systems where the arrival of the packet is synchronous to the
receiver enable. The recommended preamble length to obtain optimal antenna selection for synchronous
mode is 4 bytes.
D7
D6
D5
D4
D3
D2
D1
D0
POR
Def.
Add R/W Function/Description
08 R/W
Operating & Function
Control 2
antdiv[2] antdiv[1] antdiv[0] rxmpk autotx enldm ffclrrx
ffclrtx
00h
Table 23.8. Antenna Diversity Control
antdiv[2:0]
000
RX/TX State
Non RX/TX State
GPIO Ant1
GPIO Ant2
GPIO Ant1
GPIO Ant2
0
1
0
1
1
0
1
0
0
0
1
1
0
1
0
1
0
0
1
1
0
1
0
1
001
010
011
100
101
110
111
Antenna Diversity Algorithm
Antenna Diversity Algorithm
Antenna Diversity Algorithm in Beacon Mode
Antenna Diversity Algorithm in Beacon Mode
23.8.10. RSSI and Clear Channel Assessment
Received signal strength indicator (RSSI) is an estimate of the signal strength in the channel to which the
receiver is tuned. The RSSI value can be read from an 8-bit register with 0.5 dB resolution per bit.
Figure 23.25 demonstrates the relationship between input power level and RSSI value. The absolute value
of the RSSI will change slightly depending on the modem settings. The RSSI may be read at anytime, but
Rev. 1.0
279
SILICON [ SILICON ]
