![]() Although SystemVerilog allows you to mix signed variable with unsigned, similar to C/C++, it leaves a potential bug that cannot be caught by the compiler (some tools may produce a warning), we shall never mix signed and unsigned arithmetics and any assumption about the how the automatic conversion work is grounded for future errors that is difficult to debug. To declare a signed variable, use signed keyword such as logic signed a. In this book we will use big-endian and high:low bit-ordering.īy default, every declared variable is unsigned. It is highly recommend to use a consist bit-ordering throughout the design to avoid potential bugs. Typically bit-ordering follows the same endianness as the byte order. The difference is bit-ordering, similar to endianness for byte-ordering. To declare a multi-bit signal, we can do logic a, which declares a 16-bit signal. However, since the actual silicon has z and x values, we will see a discrepancy after the synthesis, which leads to a potential bug.īy default, a signal declared as logic only has one bit value. For instance, if a signal is declared as 2-state value, the simulator will happily compute with 0 and 1s. This is to ensure that the simulator and synthesis tool agree with each other, avoiding critical bugs that can only been discovered through gate-level simulation or even post-silicon tests. A rule of thumb is that any signals in your synthesizable design should be declared as logic (a few exceptions apply, which will be discussed). It can either represent a combinational signal or a sequential signal, since the downstream tools such simulator and synthesis tools will determine whether to instantiate a flip-flop based on the usage. Logic is the most commonly used basic data types in SystemVerilog. This can be useful when dealing with x-prorogation.ĭata types that only use 0 or 1 have 2-state values. Any 4-state value OR with 1 is 1, any 4-state value AND with 0 is 0, and any 4-state value XOR with x is x. Notice that the truth table for 4-state values are slightly different than the normal 2-state values. Keep in mind that x can propagate through your circuit if not taken care of, since any logic operation on unknown values results in unknown values. ![]() Any signal coming out from the shut-down region will be x. Another common scenario is low-power designs where some part of circuit is “shut-down”, i.e. no supply voltage. One common situation for x to appear in simulation is usage of uninitialized memory cells. Unknown value x means the system is able to determine the value, which may happen in various condition. It commonly appears in designs where a pin used as a bi-directional bus, e.g. tri-state. ![]() High-impedance value z typically implies a physically disconnected state, i.e. an infinitely high resistance. Values 0 and 1 serve the same purpose as in languages such as C/C++, but x and z are hardware specific.
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