Forum Discussion
Occasional ContributorTiming analysis error on Asynchronous control signals
Hello,
In my design, there is an asynchronous bus between FPGA and Microcontroller (Master). FPGA uses the edges on the signals WRN (Write active) and ALE (Address Enable) to classify if the signals on the bus indicate Data or Address .
As the logic is checking for the edges on WRN and ALE in a process, Quartus thinks they are clocks and gives timing errors: "WRN and ALE are clocks and they are not constrained".
How do I solve this issue having known that WRN and ALE are not clocks?
23 Replies
sstrellSuper Contributor
Set the signals as false paths.
NShan12Yes, I had set false paths on them. But Timing analyzer report still shows these two signals as unconstrained clocks in the clock status summary.
My Code:
--somewhere in the architecture
PROCESS ( ale, adbus )
BEGIN
IF ( ale'event AND ale = '0' ) THEN
address <= adbus;
END IF ;
END PROCESS;PROCESS ( wrn, clearn, adbus )
BEGIN
IF clearn = '0' THEN
data<= "0000000000000000" ;
ELSIF ( wrn'event AND wrn = '1' ) THEN
data <= adbus( 15 DOWNTO 0 );
END IF ;END PROCESS
I am using the edges of ALE and WRN to identify if it is address or data on the adbus, and the tool thinks these are clocks.
There are also warnings:
Warning (332060): Node: wrn was determined to be a clock but was found without an associated clock assignment.
Warning (332060): Node: ale was determined to be a clock but was found without an associated clock assignment.
- sstrell
You're coding them like clocks so the tool will think they're clocks.
What do your false path constraints look like?
- NShan12
The false path constraints are as follows:
set_false_path -from [get_ports WRN]
set_false_path -from [get_ports ALE]
- sstrell
I think as long as you are using edges instead of logic levels for these control signals, the tool will still look at these as clocks.
Is there a reason why you must do this and can't just turn these processes into combinatorial logic instead of registers?
- NShan12
This is a legacy design and I am using the same.
I anyways gave a try by changing the code as below:
PROCESS ( ale, adbus )
BEGIN
IF ( ale = '1' ) THEN
address <= adbus;
END IF ;
END PROCESS;PROCESS ( wrn, clearn, adbus )
BEGIN
IF clearn = '0' THEN
data<= "0000000000000000" ;
ELSIF ( wrn = '0' ) THEN
data <= adbus( 15 DOWNTO 0 );
END IF ;END PROCESS
Surprisingly, timing analyzer still shows that the clocks ALE and WRN are unconstrained.
I see in the RTL viewer that ALE and WRN are used as the latch enable signals which concurs with the code above. I do not understand why the timing analyzer still complains that these are unconstrained clocks.
Is it critical to proceed with these timing errors? Can I go back to existing legacy solution where edges of ALE and WRN are used?
In any case, how can I solve the unconstrained clock warning by the timing analyzer?
Thank you very much!
SyafieqSHi Naveen,
"In any case, how can I solve the unconstrained clock warning by the timing analyzer"
- Assuming I understand your question, have you able to constrain all the clocks in sdc? This should reflect in Timing Analyzer
- NShan12
Hi,
Yes, all the clocks (on IO ports, generated clocks, virtual clocks etc.) are constrained in the SDC file. Since WRN and ALE are not clocks only false path constraints exist on them.
- SyafieqS
Naveen,
If thats the case, my suggestion is to check Ignored Constrain Report in Timing Analyzer. I suspect that few clock constraint has been neglected for some reason. Once you identified it, make sure the clock is valid, you can use node finder and check the node if it exist.
- NShan12
Hello,
I checked the ignored constraints report in Timing analyser. It shows no constraints are ignored. See screenshot below:
But the warning: "Warning (332060): Node: WRN was determined to be a clock but was found without an associated clock assignment." still remains. Just to remind you, I am not using edges of WRN to latch data. I do not understand why WRN is considered as a clock even though I am not coding it as a clock.
Thank you!
- SyafieqS
Hi Naveen,
Speaking of legacy, you must be using the snippet to previous Quartus. What Quartus you are using now and prior to this? Can you try to use latest release and see if there is clock warning?
- NShan12
Hi SyafieqS_Intel,
I am using Quartus 20.1.1 Standard. All the information/code and screenshots I have shared are from the same version.
The legacy design was developed in MAXPLUS II.
Thank you.
- SyafieqS
Naveen,
May I know if there is any update/concern from my previous reply?
- SyafieqS
I mean 21.1 STD version, the latest Quartus release.
- SyafieqS
Naveen,
Have you able to resolve this at your end?
- SyafieqS
Hi Naveen,
Can you click on check timing report and see if there is no clock report. You can see details of the no clock ignored as the warning mentioned that. You can see the target node of the clock with reason.
- NShan12
Hello @SyafieqS ,
Sorry for a late reply. I could not check with 21.1 STD.
But with QP20.1.1 STD, I find these results:
1. The two unconstrained clocks are ALE and WRN.
2. The check timing report: I guess the number 75 indicates the total number of cells which are controlled by ALE and WRN .
