ieee.std_logic_unsigned and similar libraries because they are not standardized.
Note: if you want to disable warnings for this in the Sigasi tool, select Window > Preferences > Sigasi> VHDL > Errors/Warnings and at the bottom of that page, set the severity of “Deprecated IEEE packages” to ignore.
This article was originally published at http://tams-www.informatik.uni-hamburg.de/vhdl/doc/faq/FAQ1.html#4.11
The IEEE did not, originally, define a standard set of types and overloaded functions to handle vectors which contained coded numeric values. This meant that individual vendors were free to define their own types and functions.
Synopsys produced three packages -
std_logic_unsigned operated on type
std_logic_vector, and gave an implicit meaning to the contents of the vector.
std_logic_arith, however, defined two new types, SIGNED and UNSIGNED, and operated on these types only. Unfortunately, Synopsys decided that these packages should be compiled into library IEEE. Other vendors, including Cadence and Mentor, now produced their own versions of
std_logic_arith, which were not the same as Synopsys’s. They also required their packages to be placed in library IEEE.
Finally, the IEEE decided to standardize this situation, and produced packages
numeric_std (see Section 4.8 on how to obtain
numeric_bit is based on type bit,
numeric_std on on type std_logic. Both packages followed Synopsys in defining new types, SIGNED and UNSIGNED. However, the package functions did not have the same names, or parameters, as the Synopsys functions.
Currently many vendors support
numeric_std. Hence, for maximum portability, avoid using a package called
std_logic_unsigned, and always use SIGNED or UNSIGNED types (or integers) for arithmetic. If you are using Synopsys, use
std_logic_arith, and if you are not using Synopsys, use
numeric_std (if it is supported). This is not completely portable, since the functions are still different (for example,
CONV_UNSIGNED), but it is a lot better than using different types in different environments.
Partially extracted from an article by Evan Shattock.