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Cyclone V PLL : "Normal" mode compensation is not working.

Honored Contributor II

It seems I have encountered a Problem with PLLs in Cyclone V devices, 

when the PLL is operating in "normal" compensation mode. 


According to the Cyclone V manual, a PLL in normal mode should compensate the 

delay of a global clock network in such a way, that the clock routed via the global 

network should reach all the destination registers (almost) co-incident to the 

external reference input of the PLL. 


For Cyclone 1 and Cyclone IV devices, that seems to work as expected. 

But for Cyclone V devices, the TimeQuest analysis says that this does not work at all. 


Even worse some tests with real Cyclone V hardware suggest that TimeQuest is not 

(completely) wrong: It seems that somehow the "normal" compensation mode does 

not work as expected. 


I put two mini example designs for Cyclone IV and Cyclone V here: 


Both designs were compiled with Quartus II 16.1.0 (Build 196 10/24/2016). 

My guess is, that the Quartus II version, does not really matter 

(I expect the problem to appear with any Quartus II version). 


For both designs you can get a timing analysis, by opening TimeQuest  

and then running "Script->report_timing.tcl". 

I am particularly interested in the 

slow and fast corner "WITH PLL" panels. 


The part of the design I am interested in does this: 


GLOBAL CLK NET iClk -> PLL ----------> I/O Register ----> Pin rDataB oDataB  


I am measuring the delay from a rising edge on 

iClk to the output on the Pin ( oDataB ) ; 

so the clock-to-output time.  


iClk is supposed to have 50Mhz, the PLL output is 

also 50Mhz; the PLL is just used to compensate the  

delay of the global clock network. 



The essence (the part which I am worried about) is this: 


For Cyclone IV devices TimeQuest reports a clock path delay 

(from the input clock reference pin to an I/O register) of: 

- fast corner: -0.298 

- slow corner: 0.214 

This is what I would expect:  

The compensation keeps the delay close to 0ns. For the fast corner 

you get a negative "delay", because the PLL over-compensates. 


For Cyclone V devices TimeQuest reports the same kind of clock path delay as: 

- fast corner: 0.591 

- slow corner: 3.219 

Especially the slow corner seems to indicate, that the PLL  

is not compensating much. 


Now I am wondering what am I doing wrong ? 

Is this a known issue with Cyclone V devices ? 




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