The Spin Language
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We will not cover the sophisticated intricacies of the language in this book but rather concentrate on using the simplest of the commands in the language to perform the simplest tasks so that we can start to become comfortable with the language. We will move toward using the language in a more sophisticated context as your skills improve and your comfort level with the language increases. In the first few examples, I provide code written in Spin that looks as much like BASIC as I can make it so that you can more easily follow what is going on. For those who are not comfortable with BASIC, I suggest you either get a beginning BASIC text or just ignore the BASIC examples and concentrate on learning what we are trying to do with Spin. The first program is the ubiquitous blink one LED program that seems to be a requirement in most texts. Before we can write this program, we need to say something about how the code is specified in Spin. Spin is structured, meaning that it requires you to be more organized in the way you do things. It requires that constants and variables be specified before you start writing the body of your program. It means that routines have to be specified as being private or public, and it means that a program has to follow a certain general structure. It also means a host of other things, which we will discover as we go along. Spin uses a documentation scheme that allows you to do the following:
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You can embed documentation within the program in a way that allows you to sup-
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press two levels of the documentation.
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You can separate the documentation from the code when needed.
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These are extremely useful features, and every attempt should be made to follow the suggestions given in the Propeller Manual in regard to the use of these features. The Spin screen is color-coded so that each section of code is easy to recognize as a separate block. Although you can specify the color codes, we will not bother with that in this book. We will use the color scheme that comes with the Propeller Tool. Six separate block designations are used in the Spin language:
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CON Used for the specification of constants VAR Used for the specification of variables OBJ Used for the designation of objects to be called PUB Used for declaring methods as public PRI Used for declaring methods as private DATA Used for declaring data and data structures
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All programming segments must fit within these six blocks. However, comments and documentation can appear outside the blocks. We will cover the use of the blocks in more detail as we need them. Let s begin with a few instructions that will be used in almost every program we will ever write. The first ones have to do with declaring constants of all kinds.
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The CON statement is used to declare the constants that will be used in the program. In any program, the first thing we need to do is to declare and define the constants we are going to use within our program. Defining the constants also names the constants and thus makes it much easier to follow the program. It is best to define each constant on a new line, and every constant within the program should be defined and its use then described adequately. Doing it this way allows you to change a constant at this one location and it will be changed all over the program. This makes it easier to understand and maintain programs. In our case, we will be using the following two constants:
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Pin To identify the pin being used Delay To specify the delay between the state change of the LED
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In the listing, these constants will be typed in as follows (shown in the Parallax font):
Pin = Delay = 1 500 ; number of the pin connected to the LED ; delay in milliseconds (half a sec)
All the constants declared in the CON section are in the shared memory and are available to all the cogs. Only those constants actually declared in the CON section are available to all the cogs. Constants used by methods that you call from your program are local and are not shared. The value of a constant never changes within the program, across all methods.
Variables, of course, are those values that change from time to time within the methods. We define the variables up front so that they are all in one place and are easy to rename and redefine if that becomes necessary. Because Spin uses a number of different types of variables, we have to specify the name of the variable along with the byte, word, or long designation that will be used to contain that variable. In our first program, the variables will all have a value under 255, so we can use the byte variable specification for them. (One byte can hold a value of from 0 to 255 in its 8 bits.) Putting the variables right under the constants allows us to see all the variables together in one place so that we can add and remove them from the list as necessary. Keeping the variable list next to the constants is an advantage in that if a constant needs to become a variable or a variable needs to become a constant, it can be done from this one place with ease.