The blinking light demo also utilizes a “timer tick” to measure the
passage of time. While this timer tick utilizes interrupts, which we will not
be discussing until Chapter 11, the actual use here can be treated simply as
an idiom. The Cortex-M3 core used in the STM32 processors has a dedicated
timer for this function. The timer as a multiple of the system clock (which
is defined in ticks/second) – here we configure it for 1 msec interrupts (the
constant SystemCoreClock is defined in the firmware library to be the number
of system clock cycles per second):