Generate current by microcontroller
Constant current can be generated by a microcontroller by using feedback and designing a control system that can monitor the output current and adjust the output accordingly. Most microcontrollers will not be able to source 110 mA on any of the I/O so you would at least need an amplifier or switching element (such as a FET).

One common solution would be to use one of the PWM outputs of the microcontroller to switch an NFET at a high frequency with the load connected between the positive supply and the drain of the NFET. A shunt resistor is connected between the source terminal of the NFET and ground.







simulate this circuit &ndash; Schematic created using CircuitLab

The control system running on the microcontroller would then modulate the PWM duty cycle based on the current reading (measured as the voltage across the shunt resistor). The example shown above is a rudimentary outline of this design. Designing the control system software is non-trivial and will require some reading and experimentation if you have no previous experience.