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15803 Discussions

Unconnected CLK input to DFF defaulting to VCC with no warning (AHDL design)

Honored Contributor II

I have just debugged an issue with an AHDL design where I had unintentionally not connected a CLK to a couple of DFF registers. 

Apparently Quartus synthesis does not give a warning and connects the CLK input to a fixed high and later the register get removed as output assumed stuck at GND. 

I thought CLK was a required input for a register and would have generated a warning or error on when compile. I have verified this is not the case on Quartus versions 9.1 and 12. 

Why no message. If I dig into the optimization results I can see the registers were removed because of stuck clock port but this is not obvious from message window.
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2 Replies
Honored Contributor II

Yes, the warning about removal because of stuck clock is about as good as it gets. 

Have you tried a newer version? If you're in 9/12 are you stuck with an old device (and AHDL for that matter!)?
Honored Contributor II

I thought a DFF primative in AHDL required a CLK input so was surprised that tool defaults unconnected CLK input to 1 and compiles normally.  

Yes I am still supporting some very old FPGA devices in legacy boards requiring Quartus 9 and using still AHDL in those cases. But going to VHDL on new designs.