The minidriver can run multiple handler functions. For devices that must do something every n milliseconds, you could attach two handler functions:
Since the timer minidriver will be polled (i.e. not invoked at a constant interval) during startup, the driver will need to use something to measure the time between the calls to get the proper interval.
This architecture will allow device drivers to start very early in the system startup and allow the device to continue to function during all boot phases. If a full driver doesn't choose to take over device control, the minidriver will continue to run while the system is operational.