syn_encoding VHDL Synthesis Attribute

A VHDL synthesis attribute that specifies encodings for the states modeled by an enumeration type. The attribute takes a string value that consists of an encoding style, such as "sequential" or "one-hot", or a space-separated list of bit strings to be used as an encoding. It can also take the value "safe", which directs the Quartus^® Prime software to add extra logic to recover from illegal states. You may combine the "safe" value with an encoding style by separating the values with a comma, for example, "safe, one-hot".

To use the syn_encoding attribute in a VHDL Design File (.vhd) Definition, you must first declare the attribute in the local scope or import its declaration from the altera_syn_attributes package in the altera library. You can then use the attribute specification to associate the attribute with an enumeration type. The syn_encoding attribute must follow the enumeration type definition but precede its use.

An arbitrary user encoding consists of a space-delimited list of encodings. The list must contain as many encodings as there are enumeration literals in your Enumeration Type. In addition, the encodings must all have the same length, and each encoding must consist solely of values from the std_ulogic type declared by the std_logic_1164 package in the IEEE library. For example, the syn_encoding attribute specifies an arbitrary user encoding for the enumeration type state_t in the following code sample:

type state_t is (S0, S1, S2, S3); attribute syn_encoding : string; attribute syn_encoding of state_t : type is "11 01 10 00";

In this example, the states are encoded as follows:

S0 = "11" S1 = "01" S2 = "10" S3 = "00"

If an enumeration type has a large number of states, it may be cumbersome to specify manual encodings for all states. For convenience, the Quartus^® Prime software accepts six encoding styles:

• "default"— Choose an encoding based on the number of states in the enumeration type. If there are fewer than five states, use the "sequential" encoding. If there are more than five but fewer than 50 states, use a "one-hot" encoding. Otherwise, use a "gray" encoding.
• "one-hot"— The default encoding style requiring N bits, where N is the number of states in the enumeration type.
• "sequential"— Use a binary encoding in which the first state in the enumeration type has encoding 0, the second 1, and so on.
• "gray"— Use an M-bit encoding in which the encodings for adjacent states differ by exactly one bit. An M-bit "gray" code can represent 2M states.
• "johnson"—Use an M-bit encoding in which the encodings for adjacent states differ by exactly one bit. An M-bit "johnson" code can represent at most 2 times M states but requires less logic than a "gray" encoding.
• "compact"— Use an encoding with fewest bits.

In the previous example, the syn_encoding attribute manually specified a "gray" encoding for the enumeration type state_t. This example could be written more concisely by specifying the "gray" encoding style instead of a manual encoding:

type state_t is (apple, orange, pear, mango); attribute syn_encoding : string; attribute syn_encoding of state_t : type is "gray";

The keyword "safe" indicates that the Quartus^® Prime software should insert extra logic to detect an illegal state (an unreachable state) and force the state machine into the reset state. You cannot implement a safe state machine by specifying manual recovery logic in your HDL source; the Quartus^® Prime software eliminates this logic while optimizing your design.

-- The "safe" option can be used alone attribute syn_encoding of state_t : type is "safe"; -- or with an encoding style attribute syn_encoding of state_t : type is "sequential, safe";