![]() The flip flops are used to store the current state number (Ryu et al., 2011). This box has flip flops of a given number. In the design of a sequential circuit, there is also a box that is integrated inside that is referred to as a State. Unlike a combinational circuit, a sequential circuit has a clock as part of its design. ![]() The output levels of a sequential circuit not only depend on the values of the external inputs but also the current state of the memory device. This means that, to some extent, this circuit has some form of memory. This means that the output in a sequential circuit may depend on the values of past output values. These past input values will also include past output values. On the other hand, sequential logic circuits are the circuits where the output also depends on the values of past input values. While designing a combinational circuit, the output levels are always dependent on the input levels of that circuit (Nazarian, Fatemi, & Pedram, 2011). In this case, when the input values are changed, the previous input values are lost this shows that combinational logic circuits do have memories. Combination circuits are the circuits where the output is represented as a function of the present input values alone. And you don't need al lthe brackets in the VHDL, use rising_edge, etc.Combinational logic circuits are the circuits that are used to implement Boolean functions. and there's nothing wrong with latches, as long as your tool flow can cope with them (ie. Note also that your snippets #1 and #2 are useless, and are not equivalent to #3/#4 - there's no data input. Having said all that, you need to stick with conventional synthesis templates to get something that you know will synthesise as intended, rather than just simulating correctly. I don't buy this, and I'd like to see an example. One of the comments refers to an SO thread where someone suggests that a form which an explicit assignment may synthesise to a gated-clock rather than a gated-data implementation. It doesn't make any difference if the process is interpreted as being clocked or combinatorial. It doesn't make any difference if you explicitly add an assignment which says 'assign the signal to itself', because this is the same as not assigning to the signal at all (assuming that your code isn't sensitive to delta-delays, you haven't done anything fancy with manually-specified delays, and so on). It has to, because you haven't assigned to it. If you trace every possible path through your 'process' (and everything essentially decomposes to a 'process', in both VHDL and Verilog), and a 'signal' is not assigned to in one or more paths, then that signal holds its value in those paths. The basic answer to your question is true of both VHDL and Verilog. Often using a full cycle enable to operate a clock gate to generate the enable signal for the latch. This ensures the Latch hold the Output value at the time when the input is to be updated. If a latch is open and closes on near the updating data (posedge of a clock), timing uncertainties mean that you may latch the old data or the new data.Ī typical way to control this is to have open the latch on the first half of the clock and close it for the second half. A single latch when enabled is transparent. This means that only 1 of the 2 latches is open at a given point. Latches are not inherently bad but require a touch of caution therefore accidental ones are often cause for concern and many bugs.Ī flip-flop is two latches one after the other with a clock inversion. ![]() Flip-flop with asyn active-low reset and async setĪlways clk or negedge rst_n or posedge set) beginĪs Greg has also noted from at least 2001 comma, separated lists can be used instead of or.Ĭode-5 and Code-6 imply combinatorial blocks, which maintain state this implies a latch. When synthesised you will get the behaviour of always imply sequential or flip-flop use edge triggered ie // Flip-flop sync or no reset ![]() While learning there is sometimes the idea of purposefully giving an incomplete sensitivity list to get the simulation behaviour you want, Do not do this for RTL. You may want a combinatorial block always the * gives you an automatically compiled sensitivity list (less bugs, easier refactoring). The use of Clock or B or C) is messy because it combines edge triggered and level triggered. Code-5 and Code-6 are the same, and both contain the same mistake.Ĭode-1 and Code-2 update based Gregs comments.Brian Drummond has answered that Code-3 and Code-4 are the same.Code-1 and Code-2 are the same, but messy.How about the same thing about a combinatorial logic ? Can we say the followings are equivalent (Verilog example)?Ĭould we also say Code-5 and Code-6 are the same? Is this true to say that the following Code-1 and Code-2 are equivalent in Verilog:Ĭould we also say Code-3 and Code-4 are the same? ![]()
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