Digital Interface Design and Application

3Timer Subsystems

3.1 Timer Subsystems

The need for timing in I/O applications has already been encountered in Chapter 2 where the de-bounce time for a simple keyboard had to be accommodated. The solution taken for this issue was to simply waste time by locking the computer into a repeated loop. There are two reasons why this is not a good approach one is that the actual time is difficult to predict with any degree of accuracy and the second is that the computer resources are wasted during this delay and could be employed much more productively. A better method to deliver the required time interval is clearly desirable in practical application designs. It turns out that a simple hardware system can deliver an accurately defined time interval but it should be understood that this will not release the computer resources for more productive work unless other techniques are also employed. These will be discussed in Chapter 5.

The de-bounce application does not itself present a particularly demanding timer requirement but in many applications of embedded systems accurate and predictable time intervals are a high priority. In a control system for example, the response to a change in demand must be delivered within a specific time interval or instability might result. So when the embedded application has a real time requirement, measuring time intervals or generating time-based events, this clearly has to be addressed. Accurate timing is also essential if the embedded application ...