Parallel Shift Crack



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Directed by Dawn L. Hall, Daniel Patterson. Parallel Shift is a glimpse into an ethereal reality without special effects. Divine inspiration and a dark paradoxical dream. The experience requires the audience to pause and be mindful of the layers of meaning and explore an individual interpretation. It can be used to perform input/output operations in both serial and parallel modes. Unidirectional shift registers and bidirectional shift registers are combined together to get the design of the universal shift register. It is also known as a parallel-in-parallel-out shift register or shift register with the parallel load.


Every modern communication protocol uses one or more error detection algorithms. Cyclic Redundancy Check, or CRC, is by far the most popular one. CRC properties are defined by the generator polynomial length and coefficients. The protocol specification usually defines CRC in hex or polynomial notation. For example, CRC5 used in USB 2.0 protocol is represented as 0x5 in hex notation or as G(x)=x5+x2+1 in the polynomial. This CRC is implemented in hardware as a shift register as shown in the following picture.

The problem is that in many cases shift register implementation is suboptimal. It only allows the calculation of one bit every clock. If a design has 32-bit wide datapath, meaning that every clock CRC module has to calculate CRC on 32-bit of data, this scheme will not work. Somehow this serial shift register implementation has to be converted into a parallel N-bit wide circuit, where N is the design datapath width, so that every clock N bits are processed.

I started researching the available literature on parallel CRC calculation methods and found only a handful of papers ([2], [3]) that deal with this issue. Most sources are academic and focus on the theoretical aspect of the problem. They are too impractical to implement in software or hardware for a quick code generation.

I came up with the following scheme that I’ve used to build an online Parallel CRC Generator tool. Sinkr download for mac. Here is a description of the steps in which I make use USB CRC5 mentioned above.

(1) Let’s denote N=data width, M=CRC width. For example, if we want to generate parallel USB CRC5 for 4-bit datapath, N=4, M=5.

(2) Implement serial CRC generator routine using given polynomial or hex notation. It’s easy to do in any programming language or script: C, Java, Perl, Verilog, etc.

(3) Parallel CRC implementation is a function of N-bit data input as well as M-bit current state CRC, as shown in the above figure. We’re going to build two matrices: Mout (next state CRC) as a function of Min(current state CRC) when N=0 and Mout as a function of Nin when M=0.

(4) Using the routine from (2) calculate CRC for the N values when Min=0. Each value is one-hot encoded, that is there is only one bit set. For N=4 the values are 0x1, 0x2, 0x4, 0x8. Mout = F(Nin,Min=0)

(5) Build NxM matrix, Each row contains the results from (3) in increasing order. For example, 1’st row contains the result of input=0x1, 2’nd row is input=0x2, etc. The output is M-bit wide, which the desired CRC width. Here is the matrix for USB CRC5 with N=4.

(6) Each column in this matrix, and that’s the interesting part, represents an output bit Mout[i] as a function of Nin.

(7) Using the routine from (3) calculate CRC for the M values when Nin=0. Each value is one-hot encoded, that is there is only one bit set. For M=5 the values are 0x1, 0x2, 0x4, 0x8, 0x10. Mout = F(Nin=0,Min)

(8) Build MxM matrix, Each row contains the results from (7) in increasing order. Here is the matrix for USB CRC5 with N=4

(9) Now, build an equation for each Mout[i] bit: all Nin[j] and Min[k] bits in column [i] participate in the equation. The participating inputs are XORed together.

Mout[0] = Min[1]^Min[4]^Nin[0]^Nin[3]

Mout[1] = Min[2]^Nin[1]

Mout[2] = Min[1]^Min[3]^Min[4]^Nin[0]^Nin[2]^Nin[3]

Mout[3] = Min[2]^Min[4]^Nin[1]^Nin[3]

Mout[4] = Min[0]^Min[3]^Nin[2]

That is our parallel CRC.

I presume since the invention of the CRC algorithm more than 40 years ago, somebody has already came up with this approach. I just coulnd’t find it and “reinvented the wheel”.

Keep me posted if the CRC Generation tool works for you, or you need more clarifications on the algorithm.


[September 29th, 2010] Users frequently ask why their implementation of serial CRC doesn’t match the generated parallel CRC with the same polynomial. There are few reasons for that:
– bit order into serial CRC isn’t the same as how the data is fed into the parallel CRC
– input data bits are inverted
– LFSR is not initialized the same way. A lot of protocols initialize it with F-s, and that’s what is done in the parallel CRC.


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References

  1. G. Campobello, G Patane, M Russo, “Parallel CRC Realization”
  2. W Lu, S. Wong, “A Fast CRC Update Implementation”


Home » Structural » Basements »

Basement wall cracks

Horizontal, vertical, and diagonal basement wall cracks may be a structural issue, a leaking issue or both, as well as costly to repair. Therefore, it is important to know which ones are major, and which ones are minor.

The seriousness partially depends on factors like the size of the crack, the location, the shape and direction of the crack.

It is not unusual for cracks to appear in basement walls, regardless of the type of wall you have.

Poured in place concrete walls – these are the most common type of basement walls built across the US today. They’re usually constructed by pouring concrete into metal or wood forms and then the forms are removed once the concrete has set-up or hardened.

Masonry block walls – built with rectangular block often called concrete block or cinder block. This type of wall is the second most common wall used for basements today.

Precast concrete walls – are concrete walls built offsite, trucked to the building site, and then set in place.

Brick, stone & clay tile – these types of walls are often found in historic and older homes. Due to the age and construction designs of these walls, they tend to have more cracks and leaks than other walls.

Cracks in masonry block walls often appear at the grout joints which are usually weaker than the block itself. The cracks may be straight, angled or stair stepped.

Of the two most common types of walls, poured in place concrete walls are often considered a little stronger and masonry block walls may be considered more economical to build; however, both types are thought of as good or acceptable walls for basement construction.

Basic causes of basement wall cracks

The majority of basement wall cracks occur because of the following reasons.

  1. Lateral loads and pressures pushing against the basement wall.
  2. Foundation movement resulting from soil or other issues.
  3. Hydrostatic pressures involving wet soils, high water tables and other water issues.
  4. The “Shrinkage” process of the concrete, especially when first poured.

Shrinkage cracks

Shrinkage cracks are very common, especially in poured in place concrete walls. They’re a thin, hairline type of crack that does not run all the way though the wall but are very shallow or just on the surface. They normally do not run continuously or in straight lines but tend to meander and have interruptions in a number of places.

When concrete (basically a mixture of sand, aggregate, cement and water) is first mixed together, more water is added then is required for hydration, the chemical curing process, often thought of as the hardening process. This extra water dissipates or evaporates causing stress in the concrete which causes the concrete to shrink. Thus, shrinkage cracks.

Concrete poured when the temperature is cool will have less shrinkage cracks than concrete poured when it is hot; and overall the more water added when first mixed, the more shrinkage cracking there will be. Approximately a 1/3 of the shrinkage cracks occur in the first 10 days, 1/2 in the first month and the majority will occur within a year.

Engineers generally feel that shrinkage cracks are more cosmetic than structural in nature. Dlc quest free.

Vertical cracks

Vertical cracks in a basement wall may be caused by several common problems.

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There are two types of vertical cracks that are generally not considered structural issues. They are:

  • Cold joints
  • Shrinkage cracks

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Cold joints in a basement wall are not structural cracks

Homeowners often think that a cold joint, which at first glance may look like a crack but actually is not. A cold joint is where one batch of concrete is poured and has begun to set up or has hardened and then later another batch of concrete is poured against the previous batch.

The time between the two pours may be several hours, a day, or even weeks or months. Contractors often plan cold joints at specific locations in a concrete wall or slab.

In basement walls cold joints have a propensity to leak unless proper waterproofing has been done

Control joints – Contraction joints

Newer poured in place concrete basements walls may have a straight vertical control joint (sometimes called a contraction joint) running from the bottom of the wall to the top of the wall. These control joints are purposely put there for contraction purposes; so that when the concrete contracts or shrinks, it will crack at that control joint. You might say that they want to control where a crack will appear if it does.

These control joints may appear about every 20 to 30 lineal feet of a perimeter wall and are designed to relive internal stress. Like cold joints, they’re generally not a structural concern but may have a propensity to leak unless proper waterproofing was done

Vertical shrinkage cracks

As a concrete poured in place wall shrinks it develops shrinkage cracks, most of which are small, however, if the concrete mix had too much water in it or if a wall is long wall, you may get a few vertical shrinkage cracks, up to a 1/16 or an 1/8 inch wide. If there are control joints, the cracks may appear in them.

If the footing of basement wall is cracked, shifting or rotating

If the footing of the basement wall is cracked, then there may be a vertical crack in the basement wall near or in line with the crack in the footing. This crack may run partially or all the way from the bottom of the wall to the top of the wall.

If the footing is rotating or tilting, then there may be vertical cracks. If one side of the wall crack is offset from the other side of the crack, then that is more of a concern.

Vertical cracks wider at the top than the bottom

Vertical cracks that are wider at the top than the bottom may be an indication of a cracked footing and differential settling. These types of crack are generally thought to be of a greater concern than some of the other types of vertical cracks. ( Read about differential settling )

If the footing is cracked and one side had shifted downward, then the bottom of the crack is usually smaller than the width of the crack at the top and if the crack is offset, there is even more of a concern.

Excessive force that causes a wall to break away from a corner

Should a stretch of basement wall have more pressure against it than deigned for, it may occasionally cause a crack near the basement corner because the perpendicular wall at the corner provides extra strength to the corner area.

Vertical cracks at a basement wall corner due to water and freezing

Gutter downspouts often dump water at the corners of a house and this may lead to excessive water build-up of the soils at these areas. Should there be vertical cracks near these corners and no horizontal cracking; and there is freezing of this water laden soil, then vertical cracking may occur. ( Read about how freezing water in soil damages foundations and basement walls )

Should this be the case, improving the drainage and the downspout discharge area may help reduce further cracking.

Horizontal cracks

As a general rule horizontal cracks are not caused by settling issues or the footing heaving upward but by issues that create lateral pressure on a basement wall.

Five basic causes of horizontal cracks:

  1. Back filling soil right after the basement has been poured
  2. Loading – weight of equipment or vehicles near the basement wall
  3. Soil loading issues
  4. Moisture loading and hydrostatic pressure
  5. Frost heaving and ice lenses
  6. Rebar issues

Back-fill issues

When building a new house, contractors are usually on a tight time schedule and occasionally they get in too big of a hurry. A good example of this is when a contractor backfills the gap between a new basement wall and the edge of the hole dug for the basement before the wall is strong enough to handle the weight of the soil against it.

A concrete basement wall generally does not gain its full structural strength for about 28 days. If a contractor backfills soil against the wall too soon, the pressure of the soil pushing against the wall creates lateral stress, resulting in cracks. These cracks are more likely be to be horizontal than vertical.

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Compaction too soon or too forceful

When back filling the space next to a basement wall it is important to get good compaction. Several of the problems with poor compaction is that loose compaction fill holds more water than well compacted soil. This adds to the hydrostatic pressure against basement walls and causes more damage when the soil freezes or develops ice lenses.

Loading at the top area of the wall

Where there is a driveway or parking area next to or near a homes foundation there may be a loading issue that results in the basement walls developing horizontal cracks. The weight of vehicles or heavy equipment sitting or driving near a foundation puts downward pressure on the soil which then adds to the existing lateral pressure against the basement wall in that area. This extra pressure, depending on the amount of it may lead to horizontal cracks.

These cracks will normally occur in the top 1/3 area of the basement wall.

Horizontal cracks due to soil and water issue

Soil loading

The weight of soil pressing against a basement wall may cause horizontal cracking at times. Evidence of this being an issue would be that the wall is bowed or tilting inwardly. In the case of soil loading, the greatest pressure on a wall is generally in the bottom third area of the wall.

Should the home be on a hillside, the uphill side will frequently have more lateral pressure than the downhill side.

Hydrostatic pressure / Water loading

Soil heavily laden with water may exert significant pressure on a basement wall. Not only is hydrostatic pressure one of the main causes for a basement to leak, but it is one of the key factors in crating lateral pressure. High water tables and poor drainage are two of the main culprits; also, sewer and water line leaks may likewise contribute or cause hydrostatic pressure.

Expansive soils

Homes that have clayey types of soils against their basement wall will often experience significant cracking when the soil gets wet; for these soils expand in volume when they are wet and place tremendous lateral pressures on a basement wall. ( Read about expansive soils and what you can do to decrease damage from them )

Look for efflorescence – it’s an indication of moisture on the exterior side of the wall

At times if you see a white powdery or sugary looking substance on a wall, it may be an indication that the there is a moisture issue. This white material is called efflorescence and will often brush off or fall off if you rub your hand across it. Normally it will be just a thin layer but in severe cases may be an 1/8 of an inch or thicker. ( Read more about efflorescence problems )

Remember – if a wall bows from pressure exerted on the outside portion of the wall and then the pressure is removed or stops, the wall will normally stay bowed, because as the wall moves inward, soil fills this area which keeps the bow inward; thus, the wall does not shift outwardly to straighten back up when the pressure is relieved.

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Horizontal cracks due to Frost Heaving and Ice Lenses

Much of the United States has climate conditions where the soils next to a basement wall will experience freezing: at times this soil may freeze and thaw several times in a year. Soils that are wet or saturated with water will expand significantly when they freeze.

This freezing process exerts tremendous pressures on basement walls, so much that they may crack, tilt or bow and generally when they crack, it will be a horizontal crack. In a few instances there may be vertical cracks, and if so, they tend to be in the corner areas of a basement. (Read more about how and why freezing soil damages basements and foundations )

Block wall horizontal cracks

Horizontal cracks in a block (masonry) basement wall are caused by many of the same issues and forces as those of a poured in place concrete wall.

When looking at a block wall there are several of things to watch for, including horizontal cracks in the mid 1/3 area and also horizontal cracks near the bottom course of blocks. In the mid-section, check for cracks that have opened up an 1/8 inch or more and if the wall has bowed in this mid-section. If the wall has bowed out 3/8 of an inch or more, a number of engineers would suggest further investigation.

Another area of concern may be a crack at the gout line above the first few bottom courses of the block wall or if the bottom course of blocks appears to have shifted; if the concrete floor has a parallel crack to the horizontal cracks in the wall, then there are additional concerns.

Diagonal cracks in a poured in place wall may be more related to shifting soils, settlement, expansive soils or even footing compaction issues.

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A diagonal crack running from the top of a poured in place wall that runs down to the bottom of a wall corner may be from lateral pressure against the top and / or midway section of the wall, especially if there is evidence that the wall tilts inwardly at the top area near the top area of the crack. Also, the wall may have a damaged first floor sill plate or the exterior framed house wall may be offset (not squarely sitting on the top of the foundation wall.

Another scenario could be that the basement wall was backfilled before the first floor of the house was framed, which helps prevent an inward or tilting movement of the basement wall causing a diagonal crack.

Excessive lateral loads accompanied by differential settlement may likewise lead to diagonal cracking.

Cracks at basement window and door corners

Shrinkage issues are one the primary causes of cracks at window and door corners, especially in poured in place concrete walls. A second cause of cracks at door and window corners is stress, for there can be considerable stress that may be concentrated at these areas.

The stress on these corner areas will often develop when there’s a shifting or movement of the footings and / or basement walls. Very small cracks at these areas will generally not be of a major concern to many engineers, however, wide cracks increase an engineer’s concerns.

Small chunks of concrete falling off wall

Should there be small chunks of concrete falling off a basement wall there may be a problem with the steel rebar in the wall or with anchor bolts embedded in the wall at the top of the wall. Over time rusting rebar will deteriorate the structural strength of a wall, but moisture penetrating the wall is normally what creates the problem in the beginning.

Concrete contractors often refer this breaking off of concrete chunks as spalling.

Earthquakes, floods and geotechnical events

When basement walls are exposed to earthquakes or flooding, they may move or crack. Structurally, they can only withstand so much pressure from these types of events. If buying a home that has experienced unusual movement or flooding, it is wise to look more carefully for damage. ( Read what to look for if a home has experienced a flood )

Homes that are in an area with slope creep or expansive soils may warrant extra attention if cracks start appearing or the top of the basement wall is not level. ( Read about slope creep )

Structural Engineers

It can be difficult and confusing when analyzing what is causing cracks, leaks and damage to a basement wall because there are so many different conditions, variables and circumstances that affect a wall. Therefore, consulting with a qualified structural engineer would help in understanding about existing cracks and what is taking place with the home.

Key homeowner tip on prevention and reduction of wall cracking

Many of the common issues when it comes to basement wall cracks and leaks are moisture issues. Poor and improper drainage is probably the most important thing that most homeowners can deal with. The less moisture near the foundation and basement wall, the better.

Following are a few ideas that homeowners may want to check or improve.

  • Check to see if the soil slopes away from the home. A basic rule is that the soil slopes away from the home and drops 6 inches in 10 feet; further is better. There should not be any areas where water will stand or puddle.
  • Do all patios, walks and drives drain away from the house, not toward it? Be sure water is channeled away from the house.
  • Roof. Where does the water go from the roof when it rains or the snow melts on the roof? Are there gutters and are they in good repair: is the size of the gutter large enough to carry the water off? Are the gutters clean?
  • Down spouts. Downspouts should be connected to drain piping that will carry the water a good distance from the basement wall. Preferably to the street. No water should be discharged next to or near the foundation.
  • Check yard and planter sprinkler systems. Avoid over watering and repair any leaking pipes. Do any planters trap water?

The less moisture and the further away moisture sources are from basement walls, the better.

Crack

1. It is not uncommon for basements walls to have cracks and in general, the older the home, the more likelihood of cracks. Many cracks are small, and some are even normal; i.e. shrinkage cracks.

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2. Just looking at a crack without looking at the big picture makes it hard to fully understand the seriousness of a crack or cracks. Therefore, keep in mind that you should step back at look at the entire home as well.

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3. Since many wall and crack issues relate to moisture and drainage, it would be wise to review how good the drainage is at your home or if it needs improving.

Additional Resources

Shift
  • If your foundation has a crack or cracks know what to look for to determine if they are major or minor.

  • Foundation damage and basement wall cracks may result from frost heaving and ice lenses, especially homes with poor drainage.

  • Settlement may cause damage to a house, but some settlement can be prevented or reduced.

  • Foundation and structural damage can result from slope creep and settlement. Know why.

  • Efflorescence can be ignored at tines or may be an indication of moisture issues and occasionally a structural warning sign.

  • One of the most common problems homes have is expansive soils, however the damage can be minimized.

  • Molds, like Serpula lacrymans, can destroy the structural strength of a building over time.

  • Can you name the type of foundation that you have?

  • People are surprised about beds in igloos and how they are made. Also, they didn't realize the time it takes to build one and about the entrance feature.