![]() ![]() The Sequential Circuits are those whose output depends upon present as well as past input. They are easy to use and handle.The design part of the combinational circuits are really easy to implement.Įxamples – Encoder, Decoder, Multiplexer, Demultiplexer What is Sequential Circuit? These circuits will not have any memory element. The combinational circuit does not have a clock so there won’t be any triggering. in the combinational circuit, the time is independent. In case of the Combinational circuits, they don’t have the capability to store any state of their own hence there is no feedback between input and output. And It can be designed by anyone so easily.Įlementary building blocks are the Logic gates which are Used for the arithmetic as well as for the boolean operations. In Combinational Circuit the output depends only on the present input.The speed of this circuit is fast. The sequential circuits depend on the present as well as past inputs to generate the output. On the contrary, the Sequential circuits are the clock cycles dependent. The Combinational circuits can be termed as the time-independent circuits which do not depend on the previous inputs to generate any desired output. In digital electronics the circuits are classified into two categories they are combinational and sequential circuits. And you don't need al lthe brackets in the VHDL, use rising_edge, etc.Overview of combinational and sequential circuit ![]() 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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