Glitches in combinational circuits

What is a glitch: As per definition, a glitch is any unwanted pulse at the output of a combinational gate. In other words, a glitch is a small spike that happens at the output of a gate. A glitch happens generally, if the delays to the combinational gate output are not balanced. For instance, consider an AND gate with one of its inputs getting inverted and delayed version of  its other input. It, then will produce a short pulse (or glitch) at the ouput whenever its input goes from zero to one.

As also said above, this is due to the fact that the delays to the AND gate through two paths are not balanced. Let us elaborate with the help of below waveform. When input goes from zero to one, the other input will go to zero after some time as there is a delay equal to that of an inverter. Due to this, there will be a glitch at the output of AND gate. It needs to be noted that lesser the delay difference between the two inputs at the input of AND gate, lower will be the duration of glitch.

How are glitches harmful? Glitches may be harmful in two ways:

• Timing/functional issue: A glitch can be an issue if it propagates to the resultant logic or gets captured by a flip-flop. There can be two cases here:

• Synchronous timing paths: These are timing paths wherein we are required to meet setup and hold timings. So, even if there is a glitch, it will be within the limits of minimum and maximum delays permissible from one flip-flop to another. So, there will be no timing issue provided that you have taken care of setup and hold timings.

• Asynchronous timing paths: If the launch and capture clocks do not have any relationship, setup and hold cannot be ensured. So, if there is a glitch in the data path, it can get captured, hence, can cause issue. To prevent this, synchronizers are used and there are certain rules to be followed for asynchronous paths. These are to be followed to ensure that no wrong data gets captured due to clock glitches. It should be better to call this as functional issue as it can be taken care of only architecturally.

• High power!!! Every toggling causes power dissipation due to charging and discharging of gate capacitance. So, a glitch causes power dissipation. Even if there is no timing/functional issue associated with the glitch propagation, power dissipation can be an issue. Larger the combinational path leading to a node, larger the number of probable toggles possible; greater is the expected power dissipation.

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How to fix min pulse width violation

In our previous posts, we discussed about the duty cycle, duty cycle variation and duty cycle degradation. Bad duty cycle impacts half cycle timing paths and has impact in meeting timing for minimum pulse width checks of flip-flops. However, there are certain techniques available that can help you in improving the duty cycle of the clock. We will discuss these techniques in this post as below:

1. Dual inversion in clock branch: A certain category of logic cells are more probable of having one of the rise or fall delays greater than the other. A chain of such cells will make either high pulse of clock shorter or low pulse of clock shorter. One can use an inverter in the middle of the chain as shown in figure below to tackle this. Doing this, what we are essentially doing is converting rise edges to fall and vice-versa. So, the shortening of pulse of first few elements is balanced with the rest of the elements. In the below figure, there are 20 buffers, each shortening the pulse by 10 ps. The output of 10th buffer will have a shorter pulse as compared to clock source. The inverter at the output of 10th buffer will feed an inverted clock to 11th buffer. This will have high pulse which is greater than low pulse. Rest of the chain will try to reduce this pulse. In the end, we get a pulse which is equal to what was available at the source.

One can also try an all-inverter clock tree. In an all-inverter clock tree, every element will change the sense of clock pulse; thereby minimizing the clock pulse distortion.

However, this kind of delay balancing will only work where there is inherent variation of delays in rise vs fall. It will not work in case of OCV variations. So, if the chain length is arbitrarily large, our second method will come to rescue.

2. Even division to tackle duty cycle degradation: Suppose there is source clock with very poor duty cycle (say 10%) and you divide down the clock by 2 with a flip-flop divider. What we observe is amazing. The resulting clock is having almost 50% duty cycle. So, whenever we need an output clock with perfect duty cycle, we can use a divider to divide down the clock. The only drawback of this method is that we need a source clock of frequency twice than what is required to be timed!!

There are a few things to be kept in mind for this method:

• This method will improve the duty cycle of clock at the output of the flop. Degradation in duty cycle happening after the divider, if any, will be there.

• Duty cycle of the input clock at flip-flop must be within the limits of what is required to be minimum pulse width at the flip-flop.

Also read:

• Interview questions related to clock jitter and duty cycle variations

Duty cycle degradation

In the post, we discussed about duty cycle variation of the clock source. However, this is not the only pain in half cycle timing paths. Along clock path also, duty cycle of the clock can degrade. This can effect timing of half cycle paths adversely. We will discuss this in some detail; and also discuss how to tackle this. 

How is there degradation in duty cycle of clock: In addition to source duty cycle variation, there can be assymmetry in rise delay vs fall delay of clock elements. For instance, a buffer may have nominal rise (0 -> 1) delay of 50 ns whereas 48 ns for fall delay (1 -> 0). So, if a clock pulse passes through it, it will eat a portion of this clock pulse as shown in figure 1 below. For more clarity, we have exaggerated the scenario with a fall delay of 30 ns.

There are a lot of delay elements in clock distribution networks (also called clock tree network) inside the SoC. So, this problem is bound to happen there. Let us say, clock path has 20 buffers, each having a rise delay 10 ps greater than fall delay. So, the high pulse will get shortened when the clock reaches its sink. See how the pulse gets shortened if there is asymmetry in rise vs fall delay of a delay element or logic gate in below figure.

Even if we assume that the delay element has rise delay equal to fall delay, still, there is possibility of duty cycle degradation. Normally, a buffer (or inverter) has a nominal delay with some delay variations (for instance, OCVs) to be taken into account. For instance, it may have a rise delay of 100 ps with OCV variation to be taken of 5%. So, depending upon the scenario, we need to take its delay as either 95 ps or 105 ps. Similarly, even if we say that fall delay is exactly equal to rise delay, even then because of OCV variations, fall delay to be taken into account will be different than rise delay. Let us suppose, there are 20 such buffers in clock path with an ideal clock source. Then, we will have uncertainty in arrival of both rise and fall edges. And the effect will be visible in timing paths' slack.

How duty cycle degradation impacts timing: Degradation in duty cycle impacts timing wherever both rising and falling edges of clock are involved. For instance, it will impact half cycle timing paths as well as minimum pulse width check. You can go through our post duty cycle of clock to have an idea of the impact.

Also read:

• Interview questions related to clock jitter and duty cycle variations

Computer bug!!

Logic design interview questions

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• 3-input AND gate using 4:1 mux

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• Reset basics

Duty cycle variation of inter-clock timing paths

In the post, duty cycle variation, we understood what duty cycle variation is, and how to apply for intra-clock timing paths. But of similar importance is duty cycle variation as applied to inter-clock timing paths. Let us discuss these cases one-by-one:

Root clock to root-inverted clock: Inverted clock is same as root clock in frequency, with phase inverted. So, duty cycle variation needs to be applied for following cases:

• Root rise edge -> generated rise edge
• Root fall edge -> generated fall edge
• Generated rise edge -> Root rise edge
• Generated fall edge -> Root fall edge

Following commands will be needed to be applied:

set_clock_uncertainty -rise_from root_clk -rise_to gen_clk <duty_cycle> 

set_clock_uncertainty -fall_from root_clk -fall_to gen_clk <duty_cycle>

set_clock_uncertainty -rise_from gen_clk -rise_to root_clk <duty_cycle>

set_clock_uncertainty -fall_from gen_clk -fall_to root_clk <duty_cycle>

Root clock to odd 50% divided clock: In this scenario, we need to apply extra uncertainty for the following cases:

• Root rise edge -> Generated fall edge
• Root fall edge -> Generated rise edge
• Generated rise edge -> Root fall edge
• Generated fall edge -> Root rise edge

Following commands will need to be applied for this case:

set_clock_uncertainty -rise_from root_clk -fall_to gen_clk <duty cycle>

set_clock_uncertainty -fall_from root_clk -rise_to gen_clk <duty cycle>

set_clock_uncertainty -rise_from gen_clk -fall_to root_clk <duty cycle>

set_clock_uncertainty -fall_from gen_clk -rise_to root_clk <duty cycle>

Root clock to even 50% divided clock: In this case, we need to apply duty cycle uncertainty for the following cases:

• Root fall edge -> Generated rise edge
• Root fall edge -> Generated fall edge
• Generated rise edge -> Root fall edge
• Generated fall edge -> Root fall edge

Below figure shows these cases for a 50% divided clock from root clock.

So, the rule of thumb is same. Wherever there is a timing path wherein both rising and falling edges of root clock are involved, duty cycle variation will come into play. If you just keep this basic thing into mind, duty cycle variation will never haunt you. :-)

Also read:

• Interview questions related to clock jitter and duty cycle variations

Duty cycle variation

Duty cycle variation: Similar to jitter in clock period, there can be variations in duty cycle of the clock source due to uncertainty in the relative timings of positive and negative edges. Duty cycle variation is always measured with respect to corresponding positive and negative edges. In other words, we can also say that duty cycle variation is the uncertainty in arrival of negative edge, given that positive edge has arrived at certain fixed point of time. Let us take an example. If we are given a clock with a period of 10 ns with ideal 50% duty cycle. Also, we are given that it has the clock has a duty cycle variation of +-5%. So, if we say that we saw positive edge of clock at 100 ns, we can expect to see negative edge of clock at any time between 14.5 ns and 15.5 ns. The timing waveform in figure 2 illustrates this. 

Similarly, if we know with certainty, the point of arrival of negative edge, there will be uncertainty in the time of arrival of positive edge of the clock.

Applying duty cycle variation: There may be specific command in STA tools to specify duty cycle variation of a clock. If that is available, you just need to specify duty cycle variation of the master clock source. And all the above discussed cases will be taken care automatically by the tool. If not, it can be applied with the help of SDC command "set_clock_uncertainty". For instance, to apply duty cycle variation for a clock named "clk" of 0.5 ns, we can apply following commands:

set_clock_uncertainty -rise_from clk -fall_to clk 0.5 -setup

set_clock_uncertainty -fall_from clk -rise_to clk 0.5 -setup

set_clock_uncertainty -rise_from clk -fall_to clk 0.5 -hold

set_clock_uncertainty -fall_from clk -rise_to clk 0.5 -hold 

Note that there are two commands that need to be applied as there are two categories of half cycle paths, rise-fall and fall-rise. 

Timing implication of duty cycle variation: The same way as clock period jitter impacts setup slack of full cycle timing paths; duty cycle variation plays a role in half cycle timing paths. That is why, duty cycle variation is also referred as half cycle jitter. Keeping in mind that there are a lot of cases available with divided and undivided clocks, we will discuss below how to apply duty cycle variation while calculating timing slack. We need to keep in mind that, simlar to full cycle jitter, duty cycle variation is the property of a clock source. With reference to duty cycle variation, there can be following categories of clocks.

• Master clock
• Even divided clocks from master clock with 50% duty cycle
• Odd divided clocks from master clock with 50% duty cycle
• Even divided clocks from inverted master clock with 50% duty cycle
• Odd divided clocks from inverted master clock with 50% duty cycle
• Non-50% divided clocks/arbitrary divided clocks

Corresponding to these clock categories, there will be multiple cases, some or all of which may be present in your design. And depending upon the scenario, you may need to apply clock uncertainty for that particular case. A simple rule of thumb is that we should apply uncertainty for that scenario wherein the timing path involves both rise and fall edges of master clock. Let us discuss all these one by one:

Intra-clock timing paths:

Duty cycle variation of master clock: For a master clock, there will be a duty cycle variation as specified by the specifications of the clock source. So, if there are half cycle timing paths being formed with respect to this clock, we need to apply clock uncertainty for rise->fall and fall->rise timing paths as suggested by figure below.

So, the commands that need to be applied are:

set_clock_uncertainty <duty_cycle_variation> -rise_from clk -fall_to clk (both for setup and hold)

set_clock_uncertainty <duty_cycle_variation> -fall_from clk -rise_to clk (both for setup and hold)

where <duty_cycle_variation> is the duty cycle variation of clock source.

Duty cycle variation of odd divided clock: A divided clock with odd division factor with respect to root clock will have its positive edges aligned with respect to positive edge of root clock; and negative edges aligned to negative edges of root clock. Figure below illustrates this. So, if we assume that the positive edge is fixed (and neglecting clock period jitter), we can say that its negative edges have an uncertainty which is equal to that of root clock.

So, we need to apply following commands for duty cycle variations of odd_div_clock:

set_clock_uncertainty <duty_cycle_variation> -rise_from odd_div_clk -fall_to odd_div_clk

set_clock_uncertainty <duty_cycle_variation> -fall_from odd_div_clk -rise_to odd_div_clk

Duty cycle variation of even 50% divided clock: A divided clock with even division factor with respect to root clock will have both its positive and negative edges aligned to positive edge of root clock. Figure below illustrates this. So, for all the intra-clock paths being formed at this clock, duty cycle variation does not apply. This is one of the reasons why emphasis is given to always have even divided clocks in your design.

Duty cycle variation of arbitrary divided clocks: From the basics we have developed so far, duty cycle variation will be applied to a divided clock if its adjacent edges involve both rising and falling edges of the master clock (as in odd divided clocks) and will not be applied if it involves either only positive or only negative edges of root clock.

Figure below shows an example wherein divide-by-2 clock is generated through clock gating cell.  It has a duty cycle of 25%. Here, we need to apply duty cycle variation even as the clock is even divided.

Similarly, if an odd divided clock involves only positive or only negative edges of root clock, duty cycle variation will not apply. Figure below shows an example wherein there is a divide-by-3 clock with 33% duty cycle. Here, we dont need to apply duty cycle variation even though the clock is odd divided.

Also read:

• Interview questions related to clock jitter and duty cycle variations