Which Pins are necessary to communicate with CC2420

Mario Tippelhofer said:

I wan to use the CC2420 with the MSP430F1222 MCU and here is my question:

What pins (minimum!!!)  do I need to transmit and receive data from/to the CC2420. Is it enough to interface the CC2420 with the three SPI Pins (SI/SO/SCLK) ?

The CC2420 datasheet indicates in Section 13.7 that register access, command strobes, FIFO access and RAM access may be issued continuously without setting CSn high, but FIFO and RAM access must be terminated by setting CSn high.
Therefore you need to control CSn.

Mario Tippelhofer said:

What happens to the other pins (FIFO,FIFOP,CCA,CSN...etc)  that I see in a all of the example applications in the datasheet?

Section 14 of the CC2420 datasheet indicates the microcontrooler must be able to interface to the 4-wire SPI interface (SI, SO, SCLK and CSn), interface to the RX and TX FIFOs (FIFO, FIFOP), interface to the CCA pin and interface to the SFD pin.  Therefore, I believe you need to use all 8 signals.

There are no register bits that implement these signals which could be read over the SPI interface.

Mario Tippelhofer said:

I don't want to use the internal voltage regulator of the CC2420.  What should I do with the VREG_EN, VREG_IN, VREG_OUT pins. should I leave them unconnected.

Section 29 of the CC2420 datasheet indicates that the register and RAM contents will be lost if the internal VREG is disabled.  Therefore, you need to control VREG_EN and turn it on.