A lot of chopping driver chips don't directly support microstepping, however it can be done by varying the VREF input.
I made a simple circuit using a R2R DAC and a voltage follower between the PIC outputs and the VREF input of TEA3718 chips (used these TEA's because I had them here), it's working fine.
Below is the basic diagram.
With this it's possible to use recuperated or cheaper chips than those quite expensive LMD's.
If there's an interest I can make up the complete schematic and post it in the open forum.
There are no modifications needed to the PICSTEP firmware.
Alan, do you have objections or comments?
Luc.