Rainbow Electronics AT86RF231 - Manual (Page 143)

143
8111A–AVR–05/08
AT86RF231
Generally, the Antenna Diversity algorithm is enabled with register bit ANT_DIV_EN (register
0x0D, ANT_DIV) set. In this case the control of an antenna diversity switch must be enabled by
register bit ANT_EXT_SW_EN (register 0x0D, ANT_DIV). The internal connection to digital
ground of the control pins pin 9 (DIG1) and pin 10 (DIG2) is disabled (refer to section 4.2), and
they feed the antenna switch signal and its inverse to the differential inputs of the RF Switch
(SW1).
Upon reception of a frame the AT86RF231 selects one antenna during preamble field detection.
The selected antenna is then indicated by register bit ANT_SEL (register 0x0D, ANT_DIV). After
the frame reception is completed, the antenna selection continues searching for new frames on
both antennas. However, the register bit ANT_SEL maintains its previous value (from the last
received frame) until a new SHR has been found, and the selection algorithm locked into one
antenna again. At this time the register bit ANT_SEL is updated again.
For transmission the antenna defined by register bits ANT_CTRL (register 0x0D, ANT_DIV) is
selected. If for example the same antenna is to be used for transmission as being selected for
reception, the antenna must be set using register bits ANT_CTRL, based on the value read from
r e g i s t e r b i t A N T _ S E L . I t i s r e c o m m e n d e d t o r e a d r e g i s t e r b i t A N T _ S E L a f t e r
IRQ_2 (RX_START).
The autonomous search and selection allows the use of Antenna Diversity during reception
even if the microcontroller does currently not control the radio transceiver, for instance in
Extended Operating Mode.
A microcontroller defined selection of a certain antenna can be done by disabling the automated
Antenna Diversity algorithm (ANT_DIV_EN = 0) and selecting one antenna using register bit
ANT_CTRL.
If the AT86RF231 is not in a receive or transmit state, it is recommended to disable register bit
ANT_EXT_SW_EN to reduce the power consumption or avoid leakage current of an external RF
switch, especially during SLEEP state. If register bit ANT_EXT_SW_EN = 0, output pins
DIG1/DIG2 are pulled-down to digital ground.
11.4.3 Antenna Diversity Sensitivity Control
Due to a different receive algorithm used by the Antenna Diversity algorithm, the correlator
threshold of the receiver has to be adjusted. It is recommended to set register bits PDT_THRES
(register 0x0A, RX_CTRL) to 3.
11.4.4 Register Description
Register 0x0A (RX_CTRL):
The RX_CTRL controls the sensitivity of the Antenna Diversity Mode
• Bit [7:4] - Reserved
Bit 7 6 5 4 3 2 1 0
+0x0A Reserved PDT_THRES RX_CTRL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Reset Value 1 0 1 1 0 1 1 1