VHDL Types

In this section, we discuss some basic issues related to HDL coding for synthesis such as VHDL types, unwanted latches, variables and signals, and priority encoding. For a certain desired functionality, it is often possible to code HDL in a number of different ways. However, there are several guidelines that one can follow to develop a consistent coding style for synthesis. 

It is recommended that stdjogic types be used for port declarations in the entity. This convention avoids the need for conversion functions when integrating different levels of synthesized netlist back into the design hierarchy. The stdjogic type is declared in the IEEE 8tdJogic 1164 package. Some of the examples in this chapter use the type bit for the sake of simplicity and easy understanding. 

The type buffer can be used when an output must be used internally. The use of mode buffer is not recommended for synthesis. This is because ports of mode buffer can only be associated with ports of mode buffer and gate level VHDL simulation models from ASIC vendors never use the mode buffer. Once declared as a buffer, all 

Example 2.1 VHDL Coding Style to Avoid Buffer Data Types 

Example 2.1 shows an effective way to avoid the use of buffer types using internal signal declarations. 

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Directory and filename . /udcounter.vhdl Type of input VHDL Entity name UP DOWN Architecture name BEHAVIOUR Share resources yes Destination library generic... 

A hardware implementation can only represent values as groups of Is and Os. This is irrespective of the type - numeric, enumeration, etc. An integer value of 23, for example, will be represented by the binary pattern 10111. This does not mean that the designer can access individual bits of this string, as its VHDL type is INTEGER. To access these bits an object must be declared an appropriate type, such as BIT VECTOR. 

For the convenience of the reader, we have outlined the method of sequential flotation employed in our laboratory for separating chylomicrons VLDL, LDL, HDLa, HDLs, VHDL, and d> 1.25 bottom (Table 1). This method, the result of years of experience, has been highly reproducible in terms of the normal human population examined in this laboratory. Such a method may not necessarily apply to dyslipoproteinemic states, where modifications may be necessary, depending on the type of abnormality under consideration. It should also be stressed that any lipoprotein isolated is in need of purification this may be achieved by ultracentrifugation based on the assumption that contaminants are in loose association with the main complex. Whenever this purification is not achieved, other methods may be used as outlined below. For a discussion of the application of density gradient ultracentrifugation to the study of plasma lipoproteins, the reader is referred to a recent review (L3). 

The Reference is set to BLKl. This is similar to a part name such as R1 or C3 except that the reference will affect how nodes and parts within the block are named. For example, a resistor within BLKl named R1 on the schematic would be referred to as R BLK1 R1 in the PSpice netlist. A node labeled as vo in block 1 will be renamed as BLKl vo if that node does not connect to components inside another block. An Implementation Type of Schematic View was chosen because we will create the contents of the block using OrCAD Capture. You can also specify the function of a block using other methods such as writing a VHDL description of the block (not available in Oread Lite). The Implementation Name will become the name of the folder in the project tree where the schematic is located. When we look at the tree view... 

Due to the difficulty of extracting all three types of flow in one formalism, most existing approaches either model the whole simulation engine or select a VHDL subset that eliminates either the delta delay or the unit delay. The former approach results in an excessively complicated model. In some cases, the latter approach is justifiable - e.g. when it is intended for use with a synthesizable subset of VHDL that does not support delta delays. However,... 

As mentioned earlier, global formulas correspond to properties that must hold for the VHDL program as a whole. Four types of global formulas are needed initial state, control flow, signal persistence, and variable persistence. 

Local formulas correspond to each type of statement in the VHDL subset. In general, the formalization of a statement consists of two parts flow and function. The flow component captures the program flow implicit in the statement. In traditional procedural languages, flow proceeds from one statement to the next, except for loops and selection statements. In VHDL, the situation is complicated somewhat by the presence of a time dimension for certain statements (e.g. WAIT). The second component of a statement formalization is the functional component, and captures the semantics of the variable values affected by the statement. The functional component is complicated by the presence of statements that affect variable values at a different time (e.g. signal assignment). 

Vhdl was approved as an IEEE standard in 1987 and has gained considerable momentum in the last few years [18, 1]. The language model can be described as a network of interconnected components, each of which has an algorithmically described behavior. The expressive power of the language is very large all basic data types, including subranges, records, and arrays, are supported ... 

Column G4000SW+G3000SW. Sample serum from normal female (A) and hyperlipidemia type IV (B). Fraction 1, chylomicrons+VLOL 2, LDL 3, HDL2 4, HOL3 5, VHDL. Other HPLC conditions as in Fig. 14. 

Language characteristic VHDL is a rich and strongly typed Verbose is not that strong and it is not... 

VHDL behavioral and dataflow descriptions at the logic. Register Transfer, and algorithmic levels. Signals can be typed, can have a specified bit width, and can have clocking and sensitivity information specified. 

If one has the source library file (.lib file), one can write out VHDL models using the following dc shell command. One can control the type of VHDL model written out (that is, UDSM, FTBM, FTSM or FTGS) by setting the dc shell variable vhdilib.architecture. 

In VHDL, a component declaration is required before instantiating a component. This defines the template of the component being instantiated and includes information such as, port mode or direction, port names and the data type of the ports. These templates or component declarations are bound to sub-design entities using configuration declaration statements. Hence, when reading in a VHDL netlist you... 

When there are multiple drivers on a net, a resolution function is needed to resolve the final value on the net. In VHDL, one can create resolution functions and associate it with a datatype or a signal object. std ulogic is an example of an unresolved datatype, while stdjogic is an example of a resolved datatype. The resolution function is used to figure out the final value on a signal when there are more than one driver on it. In Verilog, one can t have user defined resolution functions. Instead one can use net types with built in resolution functions (pre-defined in Verilog) such as wand, wor, and so on. 

Example 10.1 shows the mechanism by which the predefined arithmetic operation, in VHDL is mapped to DesignWare components. The operation in the VHDL code for data type unsigned operands is mapped to the corresponding function in the std Jogic arith package. This function in turn calls the function mult which is mapped to the DesignWare synthetic operator MULT UNS OP. 

SUB" is an enumeration literal defined in type "CHARACTER" in the package "STANDARD" which is implicitly visible in all VHDL designs. Since this definition as well as the component declaration for "SUB" are both visible in the code, a homograph is reported. You can explicitly avoid this ambiguity as follows... 

ICDB is composed of two subsystems (1) a knowledge acquisition support system and (2) a component server. The component server provides two types of facilities. It (1) generates components from a given component specification and (2) answers queries about generated components. A generated component is represented in a VHDL netlist for logicl-level structure or CIF for layout. 

Finally, attention must be paid to the user interface. Unfortunately, more sophisticated graphic features usually hinder portability. So we chose to implement a text oriented, menu driven interface that allows automatic command sequence recording and use of command sequences ("scripts") to accommodate beginners and advanced users. Synthesis parameters (e.g., minimum savings for merging, number and type of functional units allowed, file names, etc.) are recorded as VHDL attributes and kept in separate files. 

VHDL is a strongly typed language. This is in contrast to many C variants, in which operations on mismatched data types can be supported. In VHDL, t5Tping is strongly enforced where variables of different types need to be assigned, explicit casting needs to be performed. This is very much like the situation in a real electronic system, in which, for example, an non-tristate output pin should not be connected to a bus line. 

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