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Line 29: − − Some notes on nomenclature and notation: − * The term '''signal''' is used to generically refer to a line, pin, or bus. − * Signals come in '''active-high''' and '''active-low''' varieties. Active-high means that a logical 1 is "on" and a logical 0 is "off". It is also known as "positive logic". Active-low is the exact opposite; logical 0 is "on" and logical 1 is "off", and it is alternately known as "negative logic". − * Some signals serve double-duty. As active-high logic they perform one operation, but as active-low logic they perform another operation. This links these operations as complimentary pairs. For example a shift register may shift its output or it may load a new value. If it's not shifting then it's loading. So the name would be something like "Shift/Load" to signify that shifting is active-high and loading is active-low. Carrying over this notation, any signal that is written "/Name" is an active-low signal. This notation is not always used, but it is quite common and I shall attempt to maintain this practice throughout the tutorial. When writing on paper, an active-low signal is frequently denoted by an overbar instead of a leading slash. − * A '''pin''' is an input, output, or inout. A '''line''' is a single pin or a single bit of data flowing along a wire. A '''bus''' is properly a multidrop line, but through common usage (due to the way circuits are commonly designed), in digital logic at this level the term has come to mean multiple lines bound together into a bundle. On a diagram, a bus appears as a thick line with a slash through it. Near the slash will be a number denoting how many lines are bundled into that bus.
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* '''out''': An ''out'' pin can be written to but never read from.
* '''out''': An ''out'' pin can be written to but never read from.
* '''inout''': An ''inout'' pin can be both read from and written to, providing the flexibility to allow bidirectional communication on a single line. At first this seems the ideal choice and that you would always want inout pins; in actual fact you want to avoid inout pins unless you absolutely need them for bidirectional communication.
* '''inout''': An ''inout'' pin can be both read from and written to, providing the flexibility to allow bidirectional communication on a single line. At first this seems the ideal choice and that you would always want inout pins; in actual fact you want to avoid inout pins unless you absolutely need them for bidirectional communication.
Some notes on nomenclature and notation:
* The term '''signal''' is used to generically refer to a line, pin, or bus.
* Signals come in '''active-high''' and '''active-low''' varieties. Active-high means that a logical 1 is "on" and a logical 0 is "off". It is also known as "positive logic". Active-low is the exact opposite; logical 0 is "on" and logical 1 is "off", and it is alternately known as "negative logic".
* Some signals serve double-duty. As active-high logic they perform one operation, but as active-low logic they perform another operation. This links these operations as complimentary pairs. For example a shift register may shift its output or it may load a new value. If it's not shifting then it's loading. So the name would be something like "Shift/Load" to signify that shifting is active-high and loading is active-low. Carrying over this notation, any signal that is written "/Name" is an active-low signal. This notation is not always used, but it is quite common and I shall attempt to maintain this practice throughout the tutorial. When writing on paper, an active-low signal is frequently denoted by an overbar instead of a leading slash. Unless otherwise specified, a signal that is not marked as active-low is assumed to be active-high by default. Some designs assume active-low as the default, but that will either be marked or implied by context (i.e. all active-high lines marked as such).
* A '''pin''' is an input, output, or inout. A '''line''' is a single pin or a single bit of data flowing along a wire. A '''bus''' is properly a multidrop line, but through common usage (due to the way circuits are commonly designed), in digital logic at this level the term has come to mean multiple lines bound together into a bundle. On a diagram, a bus appears as a thick line with a slash through it. Near the slash will be a number denoting how many lines are bundled into that bus.
=== Example: the block box ===
=== Example: the block box ===
Our outputs are not so clearly defined. This emulates the circuit itself, so the output of the black box is purely for our benefit to aid in testing. So I decided to define as outputs 32 channels, each 14-bits wide, which display the value being fed to each DAC at any given time.
Our outputs are not so clearly defined. This emulates the circuit itself, so the output of the black box is purely for our benefit to aid in testing. So I decided to define as outputs 32 channels, each 14-bits wide, which display the value being fed to each DAC at any given time.
=== The block diagram ===
=== The block diagram ===