hi,

just try to test. Connect appropriate resistor (to provide the load needed) and then see the voltage. After all, try establish serial connection with resistor connected, and test it as well.

For me, I would use DC/DC inverter, for example, MAX889.

By the way, if you have free either one PCA channel or timer 2 (in clock-out mode) then it is possible to obtain about -3,5V just with two diodes and two capacitors:


If you need highter voltage then inductor should be used:


For both schematic I suggest you to use a p-n-p transistor as buffer between a pin and rest schematic. It increases current trought negative load as well.

Regards,
Oleg