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=== Busy Ctrl Line ===

Used to indicate the status of the internal state machine. The BUSY line is set back to zero once the chip has reached a stable mode and it's ready for a new command

* Sleep 模式下，始终为 HIGH (20 kΩ上拉）。唤醒后，BUSY 转 LOW
* In FS, BUSY goes low when the PLL is locked.
* In RX, BUSY goes low as soon as the RX is up and ready to receive data. 
* In TX, BUSY goes low when the PA has ramped-up and transmission of preamble starts.
* In handle its internal IRQ, the BUSY also goes high. In this scenario, it is essential to wait for the BUSY line to go low before sending an SPI command (either a “read” or “write” command).
* All “write” commands cause the BUSY line to be asserted high after time Tsw,as per the graph above. Tsw represents the time required for the internal state machine to wake-up and start processing the command.  
* Conversely, the “read” command is handled directly without the help of the internal state machine and thus the BUSY line remains low after a “read” command.

The max value for Tsw from NSS rising edge to the BUSY rising edge is, in all cases, 600ns.

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