clock gate conversion issue

[ymiler]

Hi

My  FPGA desgin is prototype which use for ASIC RTL , 

Part of my desgin includes clock gate which choose 1 clock from several synchronized clocks (generated from same source clock )

Additionally, the clock gate instance includes a solution for glitch clock by enabling/disabling the clock at the falling edge (using a NOT logic element) :

The current synthesis & mappin are performed  by Synplify Premier tool and P & R by Quartus ( version 22.3 ) 

The clock gate solution is :

Same clock for all the FF's enabling the selection of the appropriate rate via a multiplexer (mux).

Regarding the glitch mux soultion which has 'not' logic element  - Synplify Premier create a new output clock from the PLL referred to as <clock_name>_inv_proxy and create relevant constraint for Quartus - all these worked fine without any timing issues  ,

Now I'd like to do all the process in Quartus ( for same design )

Quartus doesn't automatically generate a new inverted clock from the PLL. Instead, it retains the 'not' logic element."

this status cause to setup vilolation since the clock routing is as other logic elements routing 

please advice 

[sstrell]

Can't you just invert the clock in the PLL output settings?

Side note: none of what you mentioned is recommended for FPGA design (latches and gated clocks), though you do mention this is for prototyping an ASIC design.

[ymiler]

There are 2 paths for same output source clock 

1) posedge edge ( FF's )

2) negedge edge ( for glitchless mux code )

If I simply invert the PLL clock, I can change the functional code

Synplify Premier didn't change the functionality by adding 1 more output clock from PLL (  specific to glitch mux code ) and removing  the 'not' gate in the "glitch mux" code 

[ShengN_Intel]

Hi,

How about adding clock inversion primitive from PLL to glitch mux which has 'not' logic element?

Thanks,

Regards,

Sheng

[ymiler]

This is an option, but I prefer to do not change the code manually.

The preferred option is for the tool to be able to solve the problem on its own

[ShengN_Intel]

Hi,

Probably this is something to be enhanced. Could you provide a minimal test case for duplicating the limitation?

Thanks,

Regards,

Sheng

[ymiler]

I'll try to generate minimal test case

[ShengN_Intel]

Sure, will wait for the minimal test case.

[ymiler]

Hi

For the beginning, I create a test case with minimal design:

PLL - > clock gate -> counter (with no inverse clock)

I have the flag : 

set_global_assignment -name SYNTH_GATED_CLOCK_CONVERSION ON

Why Quartus did not convert the clock gate?

[ShengN_Intel]

Hi,

You need this further assignment set_global_assignment -name SYNTH_GATED_CLOCK_CONVERSION_BASE_CLOCK ON -entity bonsai_top

Report screenshot:

[ymiler]

Indeed , It's working !!

Thanks 

Now , I added 1 more clock gate .

Meaning the output clock of CG A is connecting to CG B input clock 

( attached new archive file )

I have same case in my prototype RTL release 

In this case , Quartus didn't convert the clock gates

please advice

[ShengN_Intel]

Hi,

When you cascade CG A and CG B, it forms a gated clock tree. Latch (CPEN) is an unsupported gate in the gated clock tree. May be can use flip-flop or combinational logic for the clock gating cascading.

Thanks,

Regards,

Sheng