Electronics > FPGA

Are GreenPack SPLDs logic prone to glitches?

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zapta:
When using GreenPak SPLD such as SLG46824, can the LUT generate glitches?  For example, if use a 3 input LUT as an AND gate and the input changes from 110 to 101, can the output have a short '1' during the transition?

I couldn't find any mentioning in the datasheet, compare for example to


ejeffrey:
Yes, but this isn't specific to these devices. Any asynchronous logic and especially programmable logic can have this. That is why we use flip flops, to only register stable values.

zapta:

--- Quote from: ejeffrey on November 25, 2023, 03:08:17 pm ---Yes, but this isn't specific to these devices. Any asynchronous logic and especially programmable logic can have this.

--- End quote ---

Is it?  Take for example the circuit below where a clock passes through a gate to a ripple counter. If the OR gate is implemented with a 74HC32, can a transition on the clock input induce multiple transitions?

In my case, I am asking for the SLG46826 GreenPAK/SPLD, where this schematic is taken from. Is this circuit is safe or can the LUT which implements the OR gate creates more than one transition on a clock transition?
 

ejeffrey:
That depends on clk1 and clk2?  Do they change at the same time?  If so consider if clk1 can transition 0->1 "at the same time" that clk2 transition 1->0.  If clk1 is delayed relative to clk2 the output of the OR gate can go low transiently.  You can fix this by delaying clk2 relative to clk1 but then the reverse transition is a problem.  Or you can fix it for both by making the falling edge slower than the rising edge, but now you have made a problem for AND gates.  You can work around the problem for a specific case you care about but there is no universal solution.  Since synthesis tools use logical equivalences to map your requested circuit onto the available primitives, it's much harder to solve this by hand for programmable logic.

zapta:
My question is for the case where clk2 is permanently grounded.

Also, in the case of greenpaks there are no degrees of freedom in the synthesis. The user dictates all the details including which lut instance to use.

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