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Friday, April 8, 2011

"Setup and Hold Time Violation" : Static Timing Analysis (STA) basic (Part 3b)





Static Timing analysis is divided into several parts:



 Here we will discuss how to calculate the Setup and Hold Violation for a design.

Till now we have discussed setup and hold violation with respect to the single flipflop, now lets extend this to 2 flip flop. In the following fig there are 2 flipflops (FF1 and FF2).



Single-Cycle Setup and Hold For Flip-Flops

Few important things to note down here-
  • Data is launching from FF1/D to FF1/Q at the positive clock edge at FF1/C.
  • At FF2/D , input data is coming from FF1/Q through a combinational logic.
  • Data is capturing at FF2/D, at the positive clock edge at FF2/C.
  • So I can say that Launching Flip-Flop is FF1 and Capturing Flip-Flop is FF2.
  • So Data path is FF1/C --> FF1/Q --> FF2/D
  • For a single cycle circuit- Signal has to be propagate through Data path in one clock cycle. Means if data is launched at time=0ns from FF1 then it should be captured at time=10ns by FF2.
So for Setup analysis at FF2, Data should be stable "Ts" time before the positive edge at FF2/C. Where "Ts" is the Setup time of FF2.
  • If Ts=0ns, then , data launched from FF1 at time=0ns should arrive at D of FF2 before or at time=10ns. If data takes too long ( greater then 10ns) to arrive (means it is not stable before clock edge at FF2) , it is reported as Setup Violation.
  • If Ts=1ns, then, data launched from FF1 at time=0ns should arrive at D of FF2 before or at time=(10ns-1ns)=9ns. If data takes too long (greater then 9ns) to arrive (means it is not stable before 1ns of clock edge at FF2), it is reported as Setup Violation.
For Hold Analysis at FF2, Data should be stable "Th" time after the positive edge at FF2/C. Where "Th" is the Hold time of FF2. Means there should not be any change in the Input data at FF2/D between positive edge of clock at FF2 at Time=10ns and Time=10ns+Th.
  • To satisfy the Hold Condition at FF2 for the Data launched by FF1 at 0ns, the data launched by FF1 at 10ns should not reach at FF2/D before 10ns+Th time.
  • If Th=0.5ns, then we can say that the data launched from FF1 at time 10ns does not get propagated so soon that it reaches at FF2 before time (10+0.5)=10.5ns ( Or say it should reach from FF1 to FF2 with in 0.5ns). If data arrive so soon (means with in 0.5ns from FF1 to FF2, data can't be stable at FF2 for time=0.5ns after the clock edge at FF2), its reported Hold violation.
With the above explanation I can say 2 important points:
  1. Setup is checked at next clock edge.
  2. Hold is checked at same clock edge.
Setup Check timing can be more clear for the above Flip-flop combination with the help of following explanation.

Setup Check Timing
In the above fig you can see that the data launched by FF1/D ( at launch edge) reaches at FF2/D after a specific delay ( CLK-to-Q delay + Conminational Logic Delay) well before the setup time requirement of Flip-Flop FF2, so there is no setup violation.
From the Fig its clear that if Slack= Required Time - Arrival time < 0 (-ive) , then there is a Setup violation at FF2.

Hold Check timing can be more clear with the help of following circuit and explanation.




Hold Check Timing

 In the above fig you can see that there is a delay in the CLK and CLKB because of the delay introduced by the series of buffer in the clock path. Now Flip-flop FF2 has a hold requirement and as per that data should be constant after the capture edge of CLKB at Flip-flop FF2.
You can see that desired data which suppose to capture by CLKB at FF2.D should be at Zero (0) logic state and be constant long enough after the CLKB capture edge to meet hold requirement but because of very short logic delay between FF1/Q and FF1/D, the change in the FF1/Q propagates very soon. As a result of that there occurs a Hold violation.
This type of violation (Hold Violation) can be fixed by shortening the delay in the clock line or by increasing the delay in the data path.

Setup and Hold violation calculation for the single clock cycle path is very easy to understand. But the complexity increases in case of multi-cycle path ,Gated clock, Flip-flop using different clocks, Latches in place of Flip-Flop. We will discuss all these later sometime.





Thursday, April 7, 2011

"Setup and Hold Time" : Static Timing Analysis (STA) basic (Part 3a)





Static Timing analysis is divided into several parts:



Its been long time, people are asking about Setup and Hold time blog. Finally time come for that. :)

The way we will discuss this concept in the following manner
  1. What is SetUp and Hold time?
  2. Definition of Setup and Hold.
  3. Setup and Hold Violation.
  4. How to calculate the Setup and Hold violation in a design?
I saw that lots of people are confused with respect to this concept. And the reason of this are
  1. They know the definition but don't know the origin or say concept behind Setup and Hold timing.
  2. They know the formula for calculating setup and hold violation but don't know how this formula come in picture.
  3. They become confuse by few of the terminology like capture path delay, launch path delay, previous clock cycle, current clock cycle, data path delay, slew, setup slew, hold slew, min and max concept, slowest path and fastest path, min and max corner, best and worst case etc during the explanation of Setup and Hold Timings/Violation.
I hope I can clarify your confusion. Let me explain this and if you face any problem let me know.

What is Setup and Hold time?

To understand the origin of the Setup and Hold time concepts first understand it with respect to a System as shown in the fig. An Input DIN and external clock CLK are buffered and passes through combinational logic before they reach a synchronous input and a clock input of a D flipflop (positive edge triggered). Now to capture the data correctly at D flip flop, data should be present at the time of positive edge of clock signal at the C pin ( to know the detail just read basis of D flipflop).
Note: here we are assuming D flip flop is ideal so Zero hold and setup time for this.

SetUp and Hold Time of a System

There may be only 2 condition.
  • Tpd DIN > Tpd Clk
    • For capture the data at the same time when Clock signal (positive clock edge) reaches at pin C, you have to apply the input Data at pin DIN "Ts(in)=(Tpd DIN) - (Tpd Clk)" time before the positive clock edge at pin CLK.
    • In other word, at DIN pin, Data should be stable "Ts(in)" time before the positive clock edge at CLK pin.
    • This Time "Ts(in)" is know as Setup time of the System.
  • Tpd DIN < Tpd Clk
    • For capture the data at the same time when clock signal (positive clock edge) reaches at pin C, input Data at pin DIN should not change before "Th(in)= (Tpd Clk) - (Tpd DIN)" time. If it will change, positive clock edge at pin C will capture the next data.
    • In other word, at DIN pin, Data should be stable "Th(in)" time after the positive clock edge at CLK pin.
    • This time "Th(in)" is know as Hold Time of the System.
From the above condition it looks like that both the condition can't exist at the same time and you are right. But we have to consider few more things in this.
  • Worst case and best case (Max delay and min delay)
    • Because of environment condition or because of PVT, we can do this analysis for the worst case ( max delay) and best case ( min delay) also.
  • Shortest Path or Longest path ( Min Delay and Max delay)
    • If combinational logic has multiple paths, the we have to do this analysis for the shortest path ( min delay) and longest path ( max delay) also.
So we can say that above condition can be like this.
  • Tpd DIN (max) > Tpd Clk (min)
    • SetUp time == Tpd DIN (max) - Tpd Clk (min)
  • Tpd DIN (min) < Tpd Clk (max)
    • Hold time == Tpd Clk (max) - Tpd DIN (min)
For example for combinational logic delays are
Data path (max, min) = (5ns, 4 ns)
Clock path (max, min) = (4.5ns, 4.1ns)
Then Setup time= 5-4.1=0.9ns
Hold time is = 4.5-4=0.5ns

Now similar type of explanation we can give for a D flip flop.  There is a combinational logic between C and Q , between D and Q of the Flipflop. There are different delays in those conbinational logic and based on there max and min value , a flipflop has Setup and Hold time. One circuitry of the positive edge triggered D flip is shown below.
Positive Edge Triggered D flip-flop

There are different ways for making the D flip flop. Like by JK flipflop, master slave flipflop, Using 2 D type latches etc. Since the internal circuitry is different for each type of Flipflop, the Setup and Hold time is different for every Flipflop.

Definition:
Setup Time:
  • Setup time is the minimum amount of time the data signal should be held steady before the clock event so that the data are reliably sampled by the clock. This applies to synchronous circuits such as the flip-flop.
  • Or In short I can say that the amount of time the Synchronous input (D) must be stable before the active edge of the Clock.
  • The Time when input data is available and stable before the clock pulse is applied is called Setup time.
Hold time: 
  • Hold time is the minimum amount of time the data signal should be held steady after the clock event so that the data are reliably sampled. This applies to synchronous circuits such as the flip-flop.
  • Or in short I can say that the amount of time the synchronous input (D) must be stable after the active edge of clock.
  • The Time after clock pulse where data input is held stable is called hold time.


Setup and Hold Violation:

In simple language-
If Setup time is Ts for a flip-flop and if data is not stable before Ts time from active edge of the clock, there is a Setup violation at that flipflop. So if data is changing in the non-shaded area ( in the above figure) before active clock edge, then it's a Setup violation.
And If hold time is Th for a flip flop and if data is not stable after Th time from active edge of the clock , there is a hold violation at that flipflop. So if data is changing in the non-shaded area ( in the above figure) after active clock edge, then it's a Hold violation.

How to calculate the setup and hold violation in a design.. please see the next blog.




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