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The result is counters that run easily at hundred of MHz.
CORNER CASE FOR COUNTER FPGA SIMULATION SOFTWARE
That adds a bit of constraint on the mapping, but the software takes care of it.įPGA manufacturers also make sure that logic elements are heavily optimized for speed along the carry chain path.
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This local routing is often used as a carry chain.Įvery time you ask the FPGA software to implement a binary counter, it maps the bits next to each other so that it can use the local routing as a carry chain. So FPGA designers made sure that logic elements placed side by side have an extra local routing signal. Logic elements can communicate with their surroundings through general-purpose routing structures, but that's slow. and FPGAs have one nice trick to keep it fast.įPGAs are made of "logic elements", each containing one LUT and one D flip-flop.Įach logic element can implement one counter bit (a 32bit counter needs 32 logic elements). So it is important to keep this path fast. It is efficient in term of hardware but the problem is speed.įor example, a 32bit counter would need 30 chained AND gates.Īnd this chain is the main part of the counter "critical path" (which sets the maximum counter clock speed).
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However we can easily redraw our counter this way (we made a 6bit counter this time).īasically instead of having AND gates grow in size, we keep them small and chain them. Our example 4bit counter only needs two AND gates (plus the flip-flops obviously) so it's pretty efficient.įor a 32bit counter, we would need 30 AND gates, the last one having 31 inputs. It's good as long as the counter is small. So our synchronous counter takes shape by using a few AND gates. Now, if we look at the way a binary counter counts, we see that the bit0 always toggles and that for any higher bit to toggle, all the bits of lower order need to be 1. In a synchronous counter, the clock feeds all the flip-flop simultaneously, so there is only one clock domain. It's very efficient in terms of hardware, but it's not great for FPGAs as we now have as many clock domains as there are bits in the counter.įPGAs are optimized for synchronous circuits, so we need something where all the counter bits toggle at the same time. The smallest binary counter is a ripple counter.īasically each T flip-flop output drives the clock of the next flip-flop. So we are using T flip-flops on this page, knowing that FPGA software can easily map them in the FPGA. Let's learn more about carry chains using counters.Ĭounters are easily built using T flip-flops.Īt the clock rising edge, its Q output toggles if the T input is high, and doesn't change if T is low.įPGAs use D flip-flops internally, but D and T flip-flops are easily interchangeable with a bit of logic around them. That will prevent any other always block from being able to see these variables, which will prevent the potential simulation races that would occur if some other always block could see them.The carry chain is the feature allowing FPGAs to be efficient at arithmetic operations (counters, adders.). If you would prefer variable1 and variable2 to be combinational logic then you must use blocking assignments: always_ff sysclk) begin So, in your example code, variable1, variable2 and variable3 will all behave like flip-flops in simulation and hence will become flip-flops after synthesis. A non-blocking assignment within an always_ff block infers a flip-flop.