RFM70library
RFM70 hardware interface
smd-pins.jpg

The RFM70 uses a standard 4-wire SPI interface. It also provides an active low interrupt pin, which could be used to avoid polling. This library does not use the interrupt pin. The RFM70 also has a CE (chip enable) input, which must be de-asserted to put the chip in standby or power-down mode, and must be cycled to switch between receive and transmit mode. Hence the interface uses 5 data pins, plus ground and power (3.3V):

When the microcontroller operates at 3.3 Volt (or lower, the RFM70 datasheet claims operation down to 1.9 Volt) all lines, including power, can be connected directly to the microcontroller. If you are experimenting and want to protect yourself against problems when you accidentally configure the pin connected to MISO as output, you could insert a suitable series resistor in this line. 2k2 seems to be a good choice.

When the microcontroller operates at 5 Volt there are three possible issues:

There are various ways to create a 3.3 Volt supply for the RFM70 from a 5 Volt supply. I prefer to use a low-drop low-quiecent current 3.3 Volt linear regulator. Read the datasheet of the regulator carefully: some put very stringent requirements on the value and impedance of the decoupling capacitors. My favourite is the Microchip MCP1702, which has a low quiecent current (5 uA maximum), and is stable with only 1 uF input an output capacitors of any type (ceramic, tantalum, aluminium electrolytic).

TBW: picture

A crude way to create the power for the RFM70 is to use a resistor divider. I would do this only in an experimental setup on my desk, never in a final product. The source impedance of the divider causes a drop in the voltage when the RFM70 uses more current. This drop can be reduced by lowering the resistor values, but at the cost of a higher current through the resistors. The RFM70 can operate down to 1.9 Volt, but at that level the microcontroller might not reliably recognise a logic 1 from the RFM70. Another issue is the dissipation in the resistors. The circuit below is a compromise. It uses three equal-valued resistors because I don't stock many different resistor values. The idle current through the resistors is 83 mA at 5.5 Volt, in this situation the RFM70 gets 3.7 Volt. That is slightly high, but probably not a big problem. When the RFM70 draws its maximum current of 23 mA when the microcontroller's power is at 4.5 Volt the RFM70 still gets 2.6 Volt. You might want to double-check that the microcontroller accepts ~ 2 Volt as a logic 1.

TBW: picture