Notes:
The second method is an example of “ polling ”. It tends to waste a lot of your time by frequently checking the phone when there is nobody there. It also takes time to swap from one task to the other. This is what you would have to do if the phone had no ringtone. Polling is the method that is used when events occur frequently and must be handled in a timely manner. The third method is an example of “ multitasking ”. The first person can concentrate on their work on the computer. The second person could be checking (polling) the phone all the time and pass messages to you as and when needed. The problem is that there is not always a second person around. Don’t worry – operating systems such as Linux on the Raspberry Pi do make it possible for you to do multitasking, even though there is only one processor.
Embedded applications
An embedded application is where a computer is built into another device. For example, a Freeview box for a TV or a satnav in a car. If you compare a desktop PC with a Raspberry Pi, you can see that a desktop PC is not suitable as a component to build into small devices, unlike a Raspberry Pi. A standard PC does not normally have any GPIO interfaces fitted either! A Raspberry Pi is much more versatile in this regard than a desktop PC.
Real-time applications
A lot of real-world control applications are said to function in “real time”. For real- time applications, it is often necessary to be able to read inputs, process them and produce outputs thousands of times a second. The rate of this processing has to be predictable – one calculation in a fixed timeframe. For example, a CD has a sample rate of 44.1 kHz – that means that for each data sample point, you only have 1/44100 seconds (22.6 µS) to do all the processing! In a multitasking operating system, such as Linux on the Raspberry Pi, you cannot guarantee that your program will have full control of the CPU during those few microseconds – the operating system may be busy, communicating on the network port, for example. When there is an irregular sample rate on input data, this is called “ jitter ”. The predictable timing accuracy that is required needs either dedicated hardware or special real-time operating systems and low-level programming languages, such as C or Assembler. The operating systems currently available on the Raspberry Pi are not really suitable for real-time applications. Fortunately, the Raspberry Pi does include a C compiler (called GCC) if you want to learn how to write a lower- level program.
Human-computer interfacing
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