Variable Voltage Using PWM and Low Pass Filter
Here's one viable approach.. the buffer simulates the micro output. The output in this case is 0-Vdd = 0-5V and that is filtered and buffered and amplified by 1.6 to give 8V. I used 1kHz for this, but you can adjust resistor and cap values to suit. For precision, you can use an actual buffer and give it a separate reference supply. The op-amp type will depend on your requirements, but probably would be a rail-to-rail in and out capable of handling the desired output voltage. 

Edit: If you're actually wanting a 1 ohm load, you'll also need a hefty power stage followingt hte op-amp. This is a separate question, really. The PWM gets you the analog voltage, but you want a really high current, so that voltage requires a power analog buffer stage.  

P.S. You should be able to run the simulation on this. 







simulate this circuit &ndash; Schematic created using CircuitLab