My advice to you is to find a different tutorial. The code in your question uses force as a signal name, when this is also a reserved language keyword since VHDL2008.The code in your question uses rising_edge as an identifier, when this is actually already defined as a function, assuming you are including standard IEEE libraries newer than I believe VHDL93.The code in your question uses after 10ns, which is not valid you need a space between the value and the units (as in my code).It is unusual to model signal assignment delays if your code is going to be implemented in a real hardware device.This assumes that input and force are either signals, or inputs to the entity.Working from the idea that this is supposed to be a beginners tutorial, and with the lack of any explanation as to why such an unusual style has been used, a much more conventional implementation would be: process : (clk) It should also be mentioned again that the usage of rising_edge as a label is a terrible idea, as is the use of force for a signal name(in VHDL 2008, force is a reserved keyword that can be used to force a signal to a value). The usage of clk'event and clk = '1' has been discouraged since 1993(see here). For one thing, as others have stated, the usage of block is very rare and they are generally only used in quite advanced designs. I will say though, this is a terrible example. Thus, for all intents and purposes this block has the same behaviour as process(clk) in your case input or force after 10ns) will only be executed when the guard condition evaluates to true, i.e. The entire statement that has been guarded(i.e. Inside a guarded block, signals that are declared guarded (like in your example) will only be assigned if the guard condition evaluates to true To my (limited) knowledge, the use of blocks has no other advantage than readability.īecause your block statement contains a guard condition( clk'event and clk='1' is the guard condition here), it is a guarded block. Blocks are resonably rarely used and frequently not synthesis supported(see here). It can be used to improve readability(see this question) and really not much else. In terms of practical usage, it is very similar to a process, only it has a limited scope wich allows component-style signal mapping(with port and port map). Can someone please gently explain its purpose?Ī block is essentially a grouping of concurrent statements. Now I've read this over and over and searched on the net but have come up blanks as to what this is actually doing. So, in this case, the assignment of the signal result is only executed if the guard signal is actually true, and in our example it means that the assignment of the expression, which is input or force, will only happen on the rising_edge of the clock because that's on guard condition." All we're doing is we're assigning the result of the evaluation of input or force, and we're doing it in a guarded setup. We're looking for the event where the clock goes from 0 to 1, and if it does, then we can conditionally assign the results, so you'll see that the result variable here says that it is a guarded input or force after 10 ns might seem a bit confusing, but consider it without the guarded keyword. "Essentially I have a block called rising_edge, and it's a block with a guard condition which does the following, it checks that we have an event on the clock, and that the clock is equal to one, so we're effectively looking for the so called rising_edge. Result <= guarded input or force after 10ns I have the following from a beginner's VHDL tutorial: rising_edge: block(clk’event and clk = ‘1’)
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