Timing error if slack value is negative

Figure 1 shows the timing analysis results. The TimeQuest Timing Analyzer uses positive or negative slack values to determine whether timing constraints are met.

Figure 1. Part of the Timing Analysis Report

In Masuo's head, it was "timing error when slack value? is negative???". It was very difficult to understand the slack value. By understanding the following four parameters, I was able to understand the slack value.

・Data arrival time

・Clock Arrival

・Setup data required time (Setup Data Required)

・Hold data required time (Hold Data Required)

About the four parameters related to the slack value

In order to understand the slack value, we have kept four parameters.

Data Arrival

Time for data to arrive at the D input of the receive register.

 

From Fig. 2, (Equation 1) can be derived.

Figure 2. Schematic diagram of data arrival time

Clock Arrival

The arrival time of the clock at the clock input of the receiving register.

 

(Formula 2) can be derived from Fig. 3.

Figure 3. Schematic diagram of clock arrival time

Setup Data Required

The signal must arrive within this time for the receiving register to receive the data correctly.

 

(Formula 3) can be derived from Fig. 4.

Figure 4. Schematic diagram of setup data request time

Hold Data Required Time

The signal must arrive after this time for the receiving register to receive the data correctly. Anything faster than this interferes with the data from the previous cycle.

 

(Formula 4) can be derived from Fig. 5.

Figure 5. Schematic diagram of setup data request time

Slack value definition formula

I was able to understand the slack value from the definition formula of the operating frequency (Fmax) and the above four parameters. For the definition of the operating frequency (Fmax), see "Timing Analysis Series 1 Concept of Timing Analysis".

◆ Setup Slack

Setup slack is the timing margin within which setup data requirements are met. It guarantees that the data will arrive within that time to fill the latch.

setup slack = setup data request time - data arrival time

Here, substitute (formula 1) (formula 4)

= (clock arrival time - Tsu) - (transmission edge + TCLK1 + μTco + TdataDelay)

Here, substitute (Equation 2)

= (receive edge + TCLK2 - Tsu) - (transmit edge + TCLK1 + μTco + TdataDelay)

= (receive edge - transmit edge) + TdataDelay + (TCLK2 - TCLK1) - Tsu - μTco

Here, substitute (Equation 5)

= 1 clock cycle - 1/Fmax

*1 clock cycle is the desired operating frequency. 1/Fmax is the calculated operating frequency.

◆Hold Slack

Hold slack is the timing margin within which the hold data request time is met. Ensures latched data is not interfered with by data from another rising edge. It also prevents data penetration.

hold slack = data arrival time - hold data request time

= 1 clock cycle - 1/Fmax

*1 clock cycle is the desired operating frequency. 1/Fmax is the calculated operating frequency.

therefore,

From (Equation 6), the slack value is the difference between the "desired operating frequency" and the "operating frequency calculated for the feasibility of the actual user design".

In other words, the slack value is the gap between the “ideal” and “real” operating frequencies.

Every day, Masuo also struggles to face the gap between ideal and reality. We are working hard every day to get even one step closer to our ideal! !