Assertion for clock gating

Question

Hi,

Below are my requirements for clock gating assertion :

1. refclk toggles when both req and ack are asserted. I want to check for the refclk to be
gated or 0 during the other 3 situations,

ie ref = 0 and ack = 0;
ref = 0 and ack = 1;
ref = 1 and ack = 0;

2. I have another clock in my test bench to time this assertion, ie sclk. But my assertion
gets triggered @ every posedge of sclk and hence it fails until req and ack becomes zero.

property refclkDeassert;
          @(posedge sclk) 1 |-> !refclk throughout (!refClkReq && !refClkAck) [*1:$]; 
endproperty

Can someone please suggest a better way to write an assertion for this?

Thanks

Answer 1

ben@SystemVerilog.us

Full Access

2218 posts

March 27, 2021 at 1:25 pm

In reply to foxtrot:
Your requirements are ambiguous.
Can you create a table or a better description of the requirements?
For example,

If ref = 0 and ack = 0 then .. // or is it "if($fell(ref) and ack", or is it "if($fell(ack) ..
....If(..) then ..
...

You need to clarify the antecedents. It is OK (or even better) to have multiple small assertions.
With !refclk throughout (!refClkReq && !refClkAck) [*1:$];
are you trying to say

@(posedge sclk) $fell(req) |-> !refclk throughout (!Req ##[*1:$] !Ack);

The throughout operator specifies that a signal (expression) must hold throughout a sequence.
( b throughout R ) is equivalent to ( b [*0:$] intersect R )
For example, in the following handshake example, a req is honored with an acknowledge within the next 4 cycles. The ack is then followed at a later time by a done control signal. However, there is one additional requirement that throughout the time between the ack and done an enable signal must be asserted to drive a tri-state bus. That assertion can be expressed as follows. Figure 2.4.6 demonstrates a timing diagram that satisfies an assertion of this property.

ap_handshake : assert property (
$rose(req) |-> ##[1:4] enable throughout (ack ##[1:$] done));

Ben Cohen
http://www.systemverilog.us/
For training, consulting, services: contact http://cvcblr.com/home.html
** SVA Handbook 4th Edition, 2016 ISBN 978-1518681448
...
1) SVA Package: Dynamic and range delays and repeats https://rb.gy/a89jlh
2) Free books: Component Design by Example https://rb.gy/9tcbhl
Real Chip Design and Verification Using Verilog and VHDL($3) https://rb.gy/cwy7nb
3) Papers:
- Understanding the SVA Engine,
https://verificationacademy.com/verification-horizons/july-2020-volume-16-issue-2
- SVA Alternative for Complex Assertions
https://verificationacademy.com/news/verification-horizons-march-2018-issue
- SVA in a UVM Class-based Environment
https://verificationacademy.com/verification-horizons/february-2013-volume-9-issue-1/SVA-in-a-UVM-Class-based-Environment

Answer 2

nimitz_class

Full Access

41 posts

March 27, 2021 at 1:32 pm

In reply to ben@SystemVerilog.us:

Let me simplify and ignore other scenarios. I want the assertion to check for refclk to be gated when req and ack both are zero. Not at any edge, I want it to be level-triggered.

Btw, the assertion I posted earlier will not be useful in my case as it will get triggered at every posedge of sclk and I will see failures throughout my simulation. So, please suggest a better method.

Thanks

Answer 3

ben@SystemVerilog.us

Full Access

2218 posts

March 27, 2021 at 5:58 pm

In reply to foxtrot:
Below is my approach. Note my use of a delayed sysclk to be used in the concurrent assertion.

/* @(posedge sclk) $fell(req) |-> !refclk throughout (!Req ##[*1:$] !Ack);
I want the assertion to check for refclk to be gated when req and ack both are zero.
 Not at any edge, I want it to be level-triggered.
 req ack gate ref_clk
 0    0   0     0
 0    1   1     ==clk 
 1    0   1     == clk
 1    1   1     == clk
 */
module top;   
  `include "uvm_macros.svh"
  import uvm_pkg::*;
  logic req, ack, ref_clk, gate, sysclk=0, sysclk1; 
  // gate is the enable for the ref_clk to equal the sysclk 
  // sysclk is the system clock 
  // sysclk1 is a delayed sysclk by 1ns; it is used for the conccurrent assertion 
  // I delayed by 1ns to evaluate the signals after they settled from the gating. 
 
  initial forever #10 sysclk = !sysclk;
  assign #1 sysclk1= sysclk;  
  always_comb begin  // Operation of the ref_clk per the design 
      if(req==0 && ack==0) ref_clk=0; 
      else ref_clk = sysclk; 
  end
 
  always_comb begin
    if(req==0 && ack==0) gate=0; // logic for the gate, used in the verification 
    else gate = 1'b1; 
end
 
  always_comb begin  // check on the gate 
      p_ref: assert final (gate==(req || ack));
  end
  // concurrent assertions on the ref clk 
  ap_ref_clk1: assert property(@ (sysclk1) gate |-> ref_clk==sysclk);  
  ap_ref_clk1b: assert property(@ (sysclk1) (req || ack) |-> ref_clk==sysclk);  
  ap_ref_clk0: assert property(@ (sysclk1) !gate |-> ref_clk==0);  
 
  initial begin
    repeat (200) begin
      @(posedge sysclk);
      if (!randomize(req, ack) with {
        req dist {1'b1 := 1, 1'b0 := 1};
        ack dist {1'b1 := 1, 1'b0 := 2};
      })
        `uvm_error("MYERR", "This is a randomize error");
    end
    $finish;
  end
endmodule

Ben Cohen
http://www.systemverilog.us/
For training, consulting, services: contact http://cvcblr.com/home.html
** SVA Handbook 4th Edition, 2016 ISBN 978-1518681448
...
1) SVA Package: Dynamic and range delays and repeats https://rb.gy/a89jlh
2) Free books: Component Design by Example https://rb.gy/9tcbhl
Real Chip Design and Verification Using Verilog and VHDL($3) https://rb.gy/cwy7nb
3) Papers:
- Understanding the SVA Engine,
https://verificationacademy.com/verification-horizons/july-2020-volume-16-issue-2
- SVA Alternative for Complex Assertions
https://verificationacademy.com/news/verification-horizons-march-2018-issue
- SVA in a UVM Class-based Environment
https://verificationacademy.com/verification-horizons/february-2013-volume-9-issue-1/SVA-in-a-UVM-Class-based-Environment

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