Programming Embedded Systems in C and C++

94 Chapter 7: Peripherals

When a software timer is started, the data members state, type, and length are

initialized and the timer is inserted into a linked list of active timers called the

timerList. The timers in the timer list are ordered so that the first timer to expire is

at the top of the list. In addition, each timer has a count associated with it. This

value represents the number of ticks that will be remaining in the software timer once

all previous timers in the list have expired. Taken together, these design choices favor

quick updates to the timer list at the price of slower insertions and deletions. Speed is

important during updates because the timer list will be updated every time the hard-

ware generates a clock tick interrupt—that’s every one millisecond.

Figure 7-1 shows the timer list in action. Remember that each software timer has its

own unique length and starting time, but once it has been inserted into the list, only

the count field matters for ordering. In the example shown, the first and second

timers were both started (the second might actually have been restarted, because it

is periodic) at the same time. Since the second is 5 ms longer, it will expire 5 clock

ticks after the first. The second and third timers in the list both happen to expire at

the same time, though the third timer will have been running for 10 times longer.

The code for the interrupt service routine ...