How can I further reduce the output power level of Si4010?
The guaranteed output power level range of the Si4010 RF transmitter IC is between +10 and -13 dBm. This power output range can be covered by the settings of PA_Power_Level (bLevel) and PA_Max_Drive (bMaxDrv) API registers included in the vPA_Setup() function. The minimum, -13 dBm, output power level can be achieved with bLevel=0 and bMaxDrv=0.
However, the Si4010 RF chip is capable to transmit with a lower power output than -13 dBm. For this, the PA_LVL SFR has to be directly written. The address of the PA_LVL SFR is 0xCE. For a more detailed register description please refer to the "SFR Definition 12.1. PA_LVL" section in the Si4010 datasheet.
After the vPA_Setup() function in the code the PA slice and bias settings can be overwritten by writing the PA_LVL SFR. For reference, the bLevel=0 and bMaxDrv=0 API settings are equivalent with the PA_LVL SFR value of 0x32. Smaller value of this SFR will provide lower power output than -13 dBm.