Proper power up of the device:

1. nSHUTDOWN must be low.
2. Turn on power supplies:
   1. Turn on VDD_IN (3.3V, max range is 1.8V to 4.8V)
   2. Turn on VDD_IO (1.8V, max range is 1.62V to 1.92V)
   3. VDD_IO and VDD_IN must be stable before releasing nSHUTDOWN.
3. Slow Clock must be stable within 2ms of releasing nSHUTDOWN.
4. Fast Clock must be stable within 20ms of releasing nSHUTDOWN.
5. The CC256x device will indicate a complete proper power-up sequence by pulling HCI_RTS low.

Figure 1-1 Proper Power Up Sequence

No signals are allowed on the I/O pins if VDD_IO power is not present. The only exceptions on this are the SLOW_CLK, XTALP, XTALM, and AUD_xxx pins that are fail-safe and can tolerate external voltages without VDD_IO and VDD_IN present.