Generic timers

Providing a SysTick timer is not mandatory for low-end microcontrollers. Some targets may not have a system timer, but all of them expose some kind of interface to program several general-purpose timers for the program to be able to implement time-driven operations. Timers in general are very flexible and easy to configure and are generally capable of triggering interrupts at regular intervals. The STM32F4 provides up to 17 timers, each with different characteristics. Timers are in general independent from each other, as each of them has its own interrupt line and a separate peripheral clock gate. On the STM32F4, for example, these are the steps needed to enable the clock source and the interrupt line for timer 2. The timer interface is based on a counter that is incremented or decremented at every tick. The interface exposed on this platform is very flexible and supports several features, including the selection of a different clock source for input, the possibility...