The first two weeks have introduced how to repair setup and hold violation, this time we will talk about another very important violation - the repair of drv.
First, let's understand the basic concept of drv. The full name of drv is design rule violation, which mainly includes max transition, maxcapacitance, max fanout, and max length. There are requirements for pin conversion time, capacitance, fan-out, and wire length respectively. Among them, the violation of max transition and capacity is what we must fix, because too large slew and load will cause the lookup table lookup table to have large and inaccurate values. Fanout and length do not have to be fixed, but if the value is too large, it will affect the first two. Next, let's see how to manually fix max transition and maxcapacitance during the ECO phase.
Let's first look at the max transition, usually we also call it slew, you can see the concept introduction.
[Introduction to Basic Concepts of Time Series Analysis]
There are many reasons for slew violation, but the main ones fall into the following categories:
1) The driving ability of the cell is too weak
2) The number of fanouts is too large
3) The net length is too long
Cell driving ability is too weak
This is the most common type of slew violation. If the driving ability of a cell is too weak to drive the next-level cell, it is easier to generate a relatively large delay.
How to fix it? It is also very simple. We can improve the driving capability by size up this cell, such as replacing the BUF of X1 with X4, X8 and so on.
Under normal circumstances, if we see that the output transition of a cell is much larger than the input transition, it means that the driver of this cell is not enough, we can try to size up. As shown in the figure below: the INV input transition of the first X1 is 18.051, and the output transition is 66.328. It is obvious that the INV of X1 cannot push the 14 fanouts below. Consider replacing the INV of X1 with a cell with a stronger driver.
The number of Fanouts is too large
This is also a common type of situation that is easy to cause slew violation. Excessive fanouts can significantly worsen pin transitions.
In this case, we can reduce the number of fanouts by inserting buffers. As shown below:
The output pin in the middle drives 15 instances, which are distributed on the left and right sides. If the driving ability of the cell is not strong and the overall net length is relatively long, it is easier to cause a max transition violation.
In this case, we can choose to insert a buffer on the right side of the instance. Because the drive of the buffer is strong, let it drive more than 10 instances on the right side, which can significantly reduce the number of fanout and fix the violation of max transition. . As shown in the figure below: the fanout of the original instance has changed from 15 to 4.
Here we recommend a more practical function. In the ECO window, we can select the frame next to Listed terminals, and then frame the pin of the cell that the added buffer needs to drive on the GUI interface, and the tool will automatically make the buffer drive. These pins are easier and faster than we grab these pins by command.
net length is too long
An excessively long net length will also worsen the slew, resulting in a violation of the max transition. As shown below:
In this case, we can insert a buffer in the middle of the net to interrupt the net, and we can solve the violation of the max transition.
maxcapacitance means that the load of the pin is too large, exceeding the constraints or the settings in the library, which will also cause inaccurate timing data. The repair method is the same as transition. For the maxcapacitance on the output pin, the number of fanouts is generally too large, or the next-level net is too long; for the maxcapacitance on the input pin, the driver of the upper-level cell is generally not enough. Or the input net is too long, etc. The basic situation is the same as max transition, so I won't introduce it too much here.
It is not difficult to repair drv. The first thing we have to do is to find out the cause of the violation and prescribe the right medicine to solve the problem better.
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