fpga4fun.com

Hiya -

I have a small DC motor which will run at 1.5V / 16 to 32mA no problem - my question is, is this too big to connect to a Pluto II IO?

The idea is to have direction control too - so it will be connected between two IOs (with a series resistor) and switched.

So a few questions -
Is this suicidal for the IOs?
If so, what alternatives are available?
Am I right in deciding not to use PWM for speed control (but instead a series resistor) - or doesn't it matter?

Thanks
kierenj

The motor will probably be too much current and an inductive load. The back EMF alone could kill the FPGA.

Buffer the pin.

Even at ~24mA, crikey.

What sort of buffer would I need? For reversible motor driving - do I want a 'H bridge'?

I think that 24mA is kind of high but possible directly out of the FPGA, at least temporarily (during a prototyping phase). Make sure you use resistors or diodes to reduce the voltage from 3.3V to 1.5V. Also the EMF could effectively give voltage spikes, so more diodes have to be there to eliminate them. If that works, 2 pins of the FPGA could effectively be used as an H-bridge with such a small motor.

Google for "h-bridge". You can make a discrete one or buy a single chip solution.

You can use icl7660 to transform 3.3 V 16 mA to 1.6 V 32 mA. for H bridge use two chips, one for each leg. EMF clamp diodes to vcc/gnd are needed at least for 1.6 V side, and maybe for 3.3 as well

Here's just a thought... how about combining IO pins? Like sharing the load between 3 pins each side of the motor, say? Each trio sharing a state of 1 or 0... would it work?

(I'd rather do it this way than pay ~£10 for the delivery of ~£2 of components )

Ta

I believe that would work to share the load (current).
You'd still have to take care of the EMF.

if its a smallish motor use ULN2803 plutos drive these great!

Probably you want a single chip solution.
You can combine several I/Os to share the current - connect 4 I/Os together for each motor wire to be sure that the FPGA is not overloaded - it will work for such a small motor.
But, don't forget to take care of the back EMF - this is very important.