This is a little tip for LDC Rx mode operation on Si446x that could possibly save you some frustration. When LDC Rx is running do not attempt to change the state of the radio without stopping the LDC Rx operation first. These attempts may lead to unexpected and funny behavior.
After LDC Rx is enabled the radio state machine will automatically enter into LDC Rx mode and will stay there until LDC Rx is disabled. No START_RX command is needed to start the operation, and it follows than that the automatic state transitions as defined by the START_RX command will not take place either.
If an action including a state change (i.,e Tx operation) is required upon either packet reception or a timeout event in LDC Rx mode, disable first LDC Rx and then proceed.
For enabling LDC Rx operation the WUT (Wake Up Timer) must be enabled too. As both bit fields that enable LDC Rx (WUT_LDC_EN) and the WUT (WUT_EN) reside in the same property (GLOBAL_WUT_CONFIG) one write to this property that enables both is a legitimate way of starting LDC Rx.
For disabling LDC Rx it is enough to disable LDC Rx only (in field WUT_LDC_EN) but disabling the WUT too (in field WUT_EN) will not make any harm either.
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You say that there is no need for a START_RX command is needed once LDC Rx is enabled, however how then do you set the receive channel?