General Interfacing
 

Both the ARMexpress and the ARMmite can be directly connected to 5V TTL devices.  The output voltage for these ARM devices ranges from 0.4V to 2.9V when driving up-to 4mA of current.  Most TTL devices will recognize these as valid logic levels (normally defined to be 0.8 and 2.0V) 

Inputs

The ARMexpress and ARMmite may also be directly connected to 5V TTL outputs.  If they are TTL compatible the voltage levels of the TTL output would normally be (0.4 and 3.4V), though they may go higher.  The inputs for these ARM devices are 5V compatible.

Tying to Supply lines

The ARMexpress and ARMmite inputs may be connected directly to a GND pin, but if connecting to a fixed voltage supply, then a 1K or greater resistor in series is recommended.  This is the same recommendation for any TTL compatible device.  The reason being is that the 5V supply may exceed the 5V at times, or if that voltage is available before the power supply to the CPU, large currents may flow through the protection diodes in the CPU.

Interfacing to higher voltages

A resistor divider may be used to connect the ARMexpress and ARMmite to voltages that go higher than 5V. The picture below shows a connection appropriate for a 24V signal.  A 100K resistor is connected from the input to IO(11) and then an 11K resistor connects IO(11) to GND.  This will divide that 24V input to vary between 0 and 2.4V. 

This resistor divider divides the 24V by 10 and also limits the current if that 24V goes higher.  The circuit below shows schematically the connection that was made.

The resistors can be varied to handle different voltages.  If the voltage to be sensed is susceptible to large spikes a 3V Zener diode can be connected in parallel with R2 to further protect the ARMmite IO.

Opto-Isolator

Another way to sense large voltages and to isolate the ARMmite from those voltages is to use an opto-isolator.  These devices consist of an LED and a photo-transistor in a single package.  They can provide isolation of 1000s of Volts.  Below is a sample circuit.  The D2 optional diode should be used if the isolated voltage to be sensed is an AC voltage.   The value of R1 should be chosen depending on the Opto-isolator spec, with the current through the opto-isolator diode typically being 10 mA.

Driving Transistors

The ARMexpress outputs are rated for 4mA, when more is required a common 2N3904 transistor can be used for 100-200 mA. The base of the transistor is driven from an IO with a series resistor.  When the IO is high the transistor is turned on.

Driving Relays

When higher currents or voltage are involved a relay can be used.  For mechanical relays a driving transistor with a catch diode are required.  The circuit starts as the above transistor circuit, which when on can either close or open the relay contacts.  When it turns off, current continues to flow in the coil of the relay as the magnetic field collapses, this current needs to go somewhere, that's what the catch diode provides is a path for that current to flow back into the supply of the relay.