How to solve setup and hold time violations in digital logic

Article By : Deekshith Krishnegowda

Here are the tips and tricks that IC design engineers can use in the back-end flow and solve the setup and hold time violations.

Typically, a production chip consists of several million flip-flops and billions of transistors. All these flops must strictly adhere to a couple of timing requirements called setup and hold time requirements. If any one of these flops fails to meet the setup and hold requirement, then the flop enters a metastable condition that propagates through the entire chip, making the chip useless.

In the chip manufacturing process, these timing criteria are not met on the first try. After several interactive back-end flow runs, the timing requirement is successfully met, and only then, the chip is taped out. In this article, we will discuss the methods that are used in back-end flow to solve setup and hold time violations.

In basic data path logic, the data from the launch flop is sampled by capture flop in the next clock edge. Hence, the setup time check occurs in the next active clock edge while the hold time check occurs in the same clock edge.

A detailed description of the setup and hold time requirement along with equations and waveform can be found in the article titled “Equations and impacts of setup and hold time”.

Ways to solve setup time violation

The fundamental idea behind solving setup violation is to make the data path logic quicker. If that doesn’t work, then making the clock path logic slower will also help solve the timing problem.

1. Improve the setup time constraint of the capture flip-flop: For any technology node, there will be several libraries that are provided by the fabrication vendor. These libraries have different processes, voltages, and temperature settings and hence they all have different timing requirements. So, the setup and hold time requirements are different for different types of flops available in the library. When there is a setup time violation on any path in design, the capture flop can be replaced with a flop that has a small setup time window so that the path can accommodate large data path delay.
2. Improve the drive strength of data path logic: The output capacitance of gate charges and discharges for the on and off operation of the transistor. Charging operation can be quicker if a flop with increased drive strength is used. This ultimately makes the data path logic quicker and hence eases the setup time requirement on capture flop.
3. Reduce the clock-q to delay launch flop: Same as setup time number, the clock-q delay depends on the kind of flop and on the library that is used. So, different flops will have a different clock-q delay. Since setup time violation can be solved by decreasing the data path logic delay, using a flop with a smaller clock-q delay for launch flip-flop will ease timing requirement.
4. Using a faster cell for launch flip-flop: Flip-flop comes with various threshold voltage (VT). Some flip-flops have less threshold voltage while some have higher threshold voltage. These flops are categorized as LVT (Low VT), SVT (Standard VT), and HVT (High VT). Using an LVT flop for launch flip-flop will result in less time for turning on and turning off of transistors inside the flip-flop that will ultimately result in faster data path logic. Other slow data path cells can also be substituted with a faster counterpart to decrease the data path delay. Hence, substituting an LVT cell instead of SVT or HVT can solve setup time violation.
5. Decreasing the logic delay: Another way to reduce logic delay apart from replacing SVT and HVT cells with LVT is to change the logic in register transfer level (RTL). This will most of the time result in increased latency, resources, and power consumption but will ease timing requirement and solves setup time violation.
6. Reducing long routing wire: In an ASIC, there will be different blocks scattered across the chip. For example, floor planning is done in such a way that memories are nearer to the power source—near the pad—and processors are placed in the center of the chip. In place and route phase, care should be taken to avoid long wire connections between different flops from different blocks. Long wire increases logic delay while a short direct connection will ease the timing requirement.
7. Increasing the clock latency on capture flop: Any toggling of net from launch flop should be sampled by the capture flop in the next active edge of the clock. It will ensure no data is lost. So, setup time check will occur in the next active edge of the clock when the capture flop is sampling the change in the net. Because of uneven buffers and inverters in the clock path, there will be a difference in ideal clock position and actual clock position. This difference is referred to as skew. If there is more skew on the clock path to capture flop, then the setup time check is delayed until the clock reaches the capture flop. It will ease timing requirement and help solve setup time violation on certain paths.
8. Increasing the clock period: All the approaches discussed till now are optimizations that will be done in the chip developmental phase. When the chip is already taped out, these optimizations are impossible to be done. Under such circumstances, as a last resort, the operating frequency can be reduced by increasing the clock period. It will delay the setup time check and will give sufficient time for data to settle to a stable value before the setup check.

Ways to solve hold time violation

The fundamental rule to solve hold time violation is to ensure slower data path logic than clock path logic. In other words, data should change after the active edge of the clock where the hold time check occurs.

1. Improving the hold time constraint of launch flip-flop: As discussed previously, different types of flip-flops are available based on different library parameters. These flip-flops have different hold time requirement that needs to be fulfilled. Using a flop with less hold time requirement as launch flop will ease timing requirement and will help solve hold time violation when there is a large skew on launch flop.
2. Decrease the drive strength of data path logic: By decreasing the drive strength of cells found in the data path, we can slow the data path signal to capture flop. It’s because the output gate capacitance will take a long time to charge and discharge. It will help solve any hold violations.
3. Increase the clock-q delay of launch flip-flop: Similar to the previous fix, by choosing a flop that has more clock-q delay, delay can be induced in data path logic. It will ease timing and help solve hold time violations.
4. Use a slower cell for launch flip-flop: As discussed previously, there will be different corners of library available for implementation and they are typically grouped under LVT, SVT, and HVT. By substituting LVT and SVT launch flop with an HVT one, we can introduce delay in signal connected to capture flop. The higher threshold voltage will ensure delay to turn on and turn off compared to a low threshold or typical threshold.
5. Increasing logic delay: Additional logic such as buffers can be added to data path logic to increase the delay on data path logic. It will make the data path slower than the clock path and will help with solving hold time violation.
6. Add long routing path: Re-routing data path to make the connection long between launch flop and capture flop will increase wire delay, thus increasing delay in data path logic.
7. Reduce the clock latency on launch flip-flop: As discussed previously, skew plays a major role in the gap between the ideal clock edge and actual clock edge. Increased skew on launch flop gives the data less time to settle to a stable value before the active edge of the clock. Hence, keeping skew to a value as low as possible is necessary to reduce the number of hold time violations.
8. Reduce operating voltage: After tape-out, if there is a setup time violation, it can be solved by reducing the operating frequency. The same cannot be done for hold time violations, and hence at most, precautions should be taken to avoid hold time violations. It’s done by running simulations for worst-case scenarios using fast corner libraries and higher threshold voltage. Despite all this, if a hold time violation is discovered after tape-out, it can sometimes be fixed by reducing the supply voltage.

Following the steps outlined in this article will help solve setup and hold time violation and will ensure no metastability propagation occurs inside the design.