Page 316 - Engineering Digital Design

6.11 FURTHER READING 287

Value Description Value Description

'U 1 Uninitialized 'X' Unknown
'0' Logic 0 (driven) 'V Logic 1 (driven)
1' Logic 0 (read) 'H' Logic 1 (read)
Don't care 'Z' High impedance
FIGURE 6.45
Eight logic data types supported by the IEEE 1076-1164 standard.

arithmetic, shift, and miscellaneous operators. Be aware that some of the operators have
different meanings depending on the synthesis tool used.
The IEEE standard 1164 supports standard data types that allow multiple I/O values
to be represented. As an example, the standard data type having eight values permit the
accurate modeling of a digital circuit during simulation and is presented in Fig. 6.45. The
word "driven" used in the description of data type characters '0' and ' 1' indicates that these
logic values are assigned (or forced) to a signal (e.g., an output). The word "read" would
apply to input logic values that must be read by a device. Note that each data type character
must be enclosed in single quotes as, for example, 'X'.
VHDL is a large and complex language that is easy to learn at the beginning but difficult
to master. It is particularly well suited to the design of very large systems, perhaps more so
than any other HDL. Libraries of circuit elements can be easily built, used, and reused in a
very effective and efficient manner, and this can be done at different levels of abstraction
ranging from the block diagram level to the transistor level. In fact, one of VHDL's strengths
is that it offers nearly unlimited use of reusable components and access to standard libraries
such as the built-in IEEE 1076-1993 Standard and the IEEE 1076-1164 Standard. Used in
the hands of a skilled designer, VHDL can greatly increase productivity as well as facilitate
the move into more advanced tools (for example, simulators) and advanced target systems.
Further Reading contains essential references for continued development in VHDL.

6.11 FURTHER READING

Most textbooks on digital design cover one or more of the performance characteristics re-
lated to digital design. Typical among these are the texts by Comer, McCluskey, Tinder,
and Wakerly. The text by Wakerly covers these subjects in considerable detail and consid-
ers various logic families. The performance characteristics covering several of the more
common logic families can be found in the Electrical Engineering Handbook (R. C. Dorf,
Editor-in-Chief).

[1] D. J. Comer, Digital Logic and State Machine Design, 3rd ed. Sanders College Publishing, Fort
Worth, TX, 1995.
[2] R. C. Dorf, Editor-inChief, Electrical Engineering Handbook, 2nd ed., CRC Press, Boca Raton,
FL, 1997, pp. 1769-1790.
[3] E. J. McCluskey, Logic Design Principles. Prentice-Hall, Englewood Cliffs, NJ, 1986.
[4] R. F. Tinder, Digital Engineering Design: A Modern Approach. Prentice-Hall, Englewood Cliffs,
NJ, 1991.