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Strength in Verilog

The strength is used to have more accurate modeling that specifies a value on a net.

The strength of a net is derived based on the strength of multiple drivers and the final strength will get the strength of the strongest driver.

Verilog provides three types of strength.

  • Driving strength
  • Capacitive strength
  • High impedance

Keywords used in Verilog strength

  • The default strength is the strong drive .
  • The default strength for pullup and pulldown gates is the pull drive .
  • The default strength for trireg is medium capacitive .
  • The default strength for supply nets is the supply driver .
  • A net can not be driven with a high impedance strength.
  • The (highz1, highz0) and (highz0, highz1) strength combinations are not allowed.
  • If a net is driven with 0 (low) and 1 (high) simultaneously, the result will be ‘x’.

strength0: When net drivers drive the value as 0, then strength is specified by strength0.

strength1:   When net drivers drive the value as 1, then strength is specified by strength1.

cap_strength:   For trireg nets, the only cap_strenth is applicable.

For example:

or (strong0, weak1) o1(out, i1, i2)

assign (weak0, strong1) out = i1 & i2;

trireg (large) tr1

Verilog strength Example

When below ‘OR’ and ‘AND’ operation drives the same output,  the final strength of the output be the strength of the strongest driver.

or (supply1, pull0) o1(out, i1, i2)

and (strong1, supply0) a1(out, i1, i2)

Verilog Tutorials

Nets have strength values to allow more accurate modelling.

Description:

Verilog has 4 driving strengths, 3 capacitive strengths and high impedance. This is not a strength. The strength of a net is derived dynamically from the strenght of the net driver(s) and will get the strength of the strongest driver. The words strenght0 specifies the strength when the net drivers drive the value 0; strength1 specifies the strength when the net drivers drive the value 1. The cap_strength is for trireg nets only.

The table below shows the strengths:

The default strength is strong drive . For pullup and pulldown gates, the default strength is pull drive ; for trireg the default strength is medium capacitive ; and for supply nets, the default strength is supply drive .

A net that is not being driven has a high impedance strength, except for tri0 and tri1 that have pull strength ; trireg hold their last strength; and supply nets have supply strength .

  • The strength combinations ( highz0 , highz1 ) and ( highz1 , highz0 ) are not allowed.

Continuous assignment , Net data type

Gate level modeling in Verilog

Designing circuits using basic logic gates is known as gate-level modeling. A digital circuit is implemented using logic gates and interconnections between these gates. The primitives (The most basic commands of a language) defined in Verilog have been set keeping the user requirements in mind making it easy to design bigger blocks. For example, for implementing AND gate or logic, the primitive is simply and(Yd, A, B) .

Gate level modeling also takes care of gate delays, which is not possible in a higher level of abstraction like behavioral modeling.

However, the designer should know the gate-level diagram of the circuit. In general, gate-level modeling is used for implementing lowest level modules in a design like full-adder , multiplexers , and other digital circuits.

In this post, we will take an in-depth look at the theory behind gate-level modeling in Verilog. This is the first modeling style that we will be studying in this Verilog course .

First, we view the logic values and ‘strengths.’ Then we talk about gate primitives. We will also have a look at the gate delays and, finally, see the subtle difference between gate-level modeling and switch-level modeling.

Logic values

A logic value is a state in which a data-type or a signal line in a chip can be present. In Verilog, there are four logic values, which means that a signal line can take one of the four values shown in the table below:

These values are either assigned in the hardware description or the output of a block.

Usually, in chip design, multiple signal paths are combined into one to save space. In this scenario, the logic values need to be assigned a higher value to facilitate this. Thus we have the strength values . These strength values assign a relative value to logic values. Strengths are used to resolve which value should appear on a net or gate output. The types of strengths are mentioned below:

supply, strong, pull, weak, highz strengths, large, medium, and small strengths.

Their value orders their strengths. The supply  strength is the strongest, and the  highz  strength is the weakest strength level. Strength values can be displayed by system tasks ($display, $monitor – by using the %v characters).

If two or more drivers drive a signal, then it will have the value of the strongest driver.

Gate primitives

Gate primitives are predefined modules in Verilog . There are two classes of gate primitives:

Single input gate primitives

Single input gate primitives have a single input and one or more outputs . The gate primitives notif1, bufif1, notif0, and bufif0 also have a control signal. The gates propagate only if the control signal is asserted, else the output is high impedance state (z).

not, buf gates

These gates have only one scalar input but may have multiple outputs .

buf stands for a buffer that transfers the input value to the output without any change.

not stands for an inverter that inverts the input signal. So a 0 at its input will produce a 1 and vice versa. Let’s view the implementation.

bufif/notif gates

bufif and notif primitives are buffers and inverters, respectively, with an added control signal to enable the output. The gates have a valid output only if the control signal is enabled else the output will be in high impedance state.

These gates are available in two flavors.

One with the normal polarity of control indicated by a 1 like bufif1  and  notif1 and second with the inverted polarity of control indicated by a 0 like  bufif0  and  notif0 .

bufif verilog tri-state buffer logic instantiation

Multiple input gate primitives

Multiple input gate primitives include AND, OR, NOR, XOR, and XNOR. They may have multiple inputs and a single output .

AND/OR/XOR gates

Both primitives we introduce here, an AND, OR, and an XOR gate, need multiple scalar inputs and produce a single scalar output. The first terminal in the argument list to these primitives is the output that is changed as any of the inputs shift. Here is the implementation to get a clear picture of the behavior of inputs and outputs.

NAND/NOR/XNOR gates

The inverse of all the above gates are nand ,  nor  and  xnor . The same design from above is reused only that the primitives are interchanged with their inverse versions.

All these gates may also have more than two inputs.

Gate Delays

In Verilog, the gate delays may be defined by a designer in a primitive instance. This allows the engineer to get the logic circuit to function in real-time.

It is equal to the time taken by a gate output transition to 1, from another value 0, x, or z.

It is equal to the time taken by a gate output transition to 0, from another value 1, x, or z.

Turn off delay

It is equal to the time taken by a gate output transition to a high impedance state, from another value 1, x, or z.

Some syntax rules to be kept in mind are:

  • If all the three values are specified then, they are considered as rise, fall, and turn-off delays.
  • If two values are specified then, they are considered as rise and fall delays.
  • If only one delay is specified then, it is used for all delays.

Lets us see some examples which can be used in Verilog:

Verilog empowers us to control the delays more extensively in the form of min: typ: max   values for each delay. Only one of the min/typ/max values can be used in the entire simulation run. It is specified at the start of the simulation and depends on the simulator used. To distinguish precisely, analyze the following example:

Here, the typical value is the default value. In the above example, if the designer chooses min values, then rise delay = 3, fall delay = 4, turn-off delay = 5.

This helps the designer to have a much better real-time experience of design simulation, as in real-time logic circuits, the delays are not constant.

Difference between gate-level modeling and switch level modeling

About the author

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can you pls explain the syntax of monitor statement?

The logic for the buf_not_gates is incorrect. It should just be buf (c,a); not (d,b);

I don’t understand why you have taken 2 inputs for not and buf in (single input gate primitives’) examples code. Compiler would through an error I guess as not and buf cannot accept 2 inputs…..Rectify that if it’s a mistake

Control signal

Isn’t it failing under Single input gate primitive criteria . if buf and not have 2 inputs,why is it in that tab??

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Verilog Assignments

Variable declaration assignment, net declaration assignment, assign deassign, force release.

  • Procedural continuous

Legal LHS values

An assignment has two parts - right-hand side (RHS) and left-hand side (LHS) with an equal symbol (=) or a less than-equal symbol (<=) in between.

The RHS can contain any expression that evaluates to a final value while the LHS indicates a net or a variable to which the value in RHS is being assigned.

Procedural Assignment

Procedural assignments occur within procedures such as always , initial , task and functions and are used to place values onto variables. The variable will hold the value until the next assignment to the same variable.

The value will be placed onto the variable when the simulation executes this statement at some point during simulation time. This can be controlled and modified the way we want by the use of control flow statements such as if-else-if , case statement and looping mechanisms.

An initial value can be placed onto a variable at the time of its declaration as shown next. The assignment does not have a duration and holds the value until the next assignment to the same variable happens. Note that variable declaration assignments to an array are not allowed.

If the variable is initialized during declaration and at time 0 in an initial block as shown below, the order of evaluation is not guaranteed, and hence can have either 8'h05 or 8'hee.

Procedural blocks and assignments will be covered in more detail in a later section.

Continuous Assignment

This is used to assign values onto scalar and vector nets and happens whenever there is a change in the RHS. It provides a way to model combinational logic without specifying an interconnection of gates and makes it easier to drive the net with logical expressions.

Whenever b or c changes its value, then the whole expression in RHS will be evaluated and a will be updated with the new value.

This allows us to place a continuous assignment on the same statement that declares the net. Note that because a net can be declared only once, only one declaration assignment is possible for a net.

Procedural Continuous Assignment

  • assign ... deassign
  • force ... release

This will override all procedural assignments to a variable and is deactivated by using the same signal with deassign . The value of the variable will remain same until the variable gets a new value through a procedural or procedural continuous assignment. The LHS of an assign statement cannot be a bit-select, part-select or an array reference but can be a variable or a concatenation of variables.

These are similar to the assign - deassign statements but can also be applied to nets and variables. The LHS can be a bit-select of a net, part-select of a net, variable or a net but cannot be the reference to an array and bit/part select of a variable. The force statment will override all other assignments made to the variable until it is released using the release keyword.

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Drive Strength Detection in SystemVerilog

From time to time I will run across small problems or challenges in my day job which seem as though they should be simple enough but get stuck in my mind until they are fully investigated. This is one such issue I ran into a while ago: How can a verilog model determine the drive strength of a net value in SystemVerilog?

When I initially ran into this problem, I thought it would require a quick search through the LRM and I’d be on my way. However, after some searching and not finding any useful constructs in the language that would directly provide visibility into a net’s drive strength, I ended up on google, and all the recommendations were similar to this post from stack overflow: How to check signal drive strength? . It seemed the recommended solution was to format the net value using the "%v" format string, and then do string comparisons on the formatted value.

Having run into my fair share of string and string formatting performance issues in SystemVerilog in the past, the suggested “solution” was instantly ringing some alarm bells. This is the problem that latched into my mind, and I needed to know if there was a better way… but first, a quick refresher on the drive strength modeling features in SystemVerilog.

Drive Strength Modeling

In SystemVerilog (and Verilog before it), a “net” is a special type of variable construct which is used to structurally model a wire in a circuit. The value of the “net” variable is a combination of a 4-state logical value ( 0 , 1 , z , x ), and a strength value ( highz , weak , pull , strong , or supply ). The value of a net is determined by “driver” elements connected to the net, which can be either continuous assignment expressions, or gate level modeling primitives. If semantically a SystemVerilog net represents a wire in a circuit, and the logical value represents the voltage on the wire, then the strength of the net represents how much current can that wire can source (or said another way, the impedance of the driver).

For every delta cycle of a simulation, the simulator will determine the logical and strength values of each net, by looking at the value and strength of all driver expressions on the net. If a net has no active drivers, then the value of the net is “high impedance” represented as 'z in SystemVerilog 4-state values. If a net has one driver expression or primitive, then the value and strength of the net are equal to that of the driver. However, if there are more than one driver connected to a single net, the logical value of the net is determined by comparing the strength of each driver on the net. If there is one driver on the net that has a strength lager than the rest, or there are multiple drivers that all have the same largest strength value, and the same logical value, then the logical value of the net is determined by the logical value of the driver with the greatest strength. If there are multiple drivers that all have the same greatest strength on the net, but have different values, then the value of the net is “unknown” or 'x with the strength of the strongest drivers.

Extracted figures from SystemVerilog LRM showing combinations of different strength drivers (left) and resulting net value (right)

If the strength value of a net is determined by the relative strength values of the drivers on the net, then there must be a way to define the strength of a driving expression. In SystemVerilog, by default all driver expressions have a strong strength value, which for a lot of use cases, is all you would ever need. However, the strength of logical 1 and 0 values can be individually set for both continuous assignment expressions and gate primitives by including a token of the form (<strength>1, <strength>0) after the assign keyword or primitive name. For example the expression assign (strong1, weak0) foo = ~bar; will drive the net foo with a logical value of 1 and strength of strong , when bar is a logical 0, and drive a logical value of 0 with strength value weak when bar is 1.

As an example, one might use drive strength modeling when modeling an I 2 C bus, which uses pull-up resistors to pull the bus high, and open drain drivers in each device to pull it low. In such a model, the pull up resistors on the bus can be simply modeled as a pull1 continuous assignments with a value of 1'b1 :

Additionally, the multiple I 2 C IO drivers can be modeled as driving the bus using a continuous assignment expressions with a (highz1, strong0) strength expression:

This is is not just nice because it’s a concise way of having the simulator figure out the interactions between devices on the bus, but it does so in a way that structurally mirrors how the circuits work. So if the simulator can do all the work of understanding the strengths of different drivers on a net, and resolving the net to the correct value, why would you need to be able to determine the strength of a net? There’s a few use cases I have actually run into, but continuing the I 2 C example, one use case would be if you wanted to write an assertion to check the bus is never driven high with a strong driver, since according to the specification, this should never happen.

Three ways of detecting drive strength

As part of this investigation I came up with two new ways for SystemVerilog code to detect or observe the strength of a net’s driver, in addition to the initial format string based solution from Stack Overflow that kicked all this off. For all three of these approaches I will be using them to implement a generic strength detection module, that accepts the target net to detect the strength of as an inout port, and outputs a value from 0-4 indicating the drive strength of the net.

The idea being if you wanted to write an assertion to check that an I 2 C bus was never driven high with a strong driver, you could use the module to get the drive strength, and then put a simple assertion on that value, as shown below:

The String Formatting Approach: Google me up some code

As already mentioned, if you start looking into how to detect the drive strength of a net in SystemVerilog, all the recommendations I could find on the first page of Google results suggested to use the $sformatf() function with the "%v" format string, which creates a string representation of the net value, including the drive strength. For example a net being driven to a value of 1 with a strong driver, when passed to the "%v" format string results in the string “St1”.

Generalizing this approach into a module might look something like the following code. Every time the inout signal sig changes, we format the value of sig to a string, and then do a bunch of string comparisons to figure out the drive strength of the net.

Now, this does work, but I had performance concerns. SystemVerilog is good at some things, but string manipulation is not one of it’s strong suits, and I have seen simulations crawl to a halt because of particularly large or complex string formatting and manipulation tasks. We will discuss the performance of this later, but suffice it to say, when I saw this as the recommended approach to detect the strength of a net’s driver, I couldn’t help but think: There has to be a better way!

“Sense Net” Approach: Using the simulator against itself

After re-reading most of the LRM a few times, I eventually convinced myself that within the confines of the SystemVerilog language, there is no syntax or API that returns an integer value with a net’s drive strength. However, while we can’t directly query the drive strength, we can observe the effect that the strength of a driver has on net value resolution! The basic idea is to connect drivers with various strengths and opposite value to the net which we are measuring the strength of, and observe if the net value changes. The simulator’s drive strength resolution will tell us if strength of the net is stronger, equal, or weaker than the applied driver based on if the value changes, goes to 'x , or remains the same.

The trick with this, of course, is we need to do this without actually ever changing the value of the net, because that would change the result of the simulation. It turns out we can get around this by using the cmos gate primitive, which is a unidirectional model element which (when enabled) will drive it’s output net with the same strength and logical value as it’s input net. This allows us to essentially create a “duplicate” net with the same driver values (logical and strength) as that of our target net, but which we can add additional drivers to without effect the real target net.

After some optimization, I ended up up with with the following module with the same ports as the str_strength_detect module above, but doesn’t use any strings:

Now, while this solution is almost perfect, it has one limitation: It cannot observe the difference between supply and strong strength values. The reason for this is that the cmos primitive has one special case where the strength value is not passed through, if the input drive strength is supply the output drive strength is only strong . There’s a lot of cases where this limitation doesn’t matter (including the problem that got this whole thing started), and in those cases I actually really liked this solution because it’s simple and pure SystemVerilog. However, I wanted to know if there was a way around this limitation…

DPI-C API Method: Going even deeper

Now, as already mentioned, I believe SystemVerilog has no way to directly query the strength of a driver, however, what it does have is a well defined C interface API specification. The “Direct Programming Interface” chapters of the SystemVerilog specification defines a C API that in theory allows C code to directly interact with any SystemVerilog code running in any (complaint) simulator. One thing this API provides is a method to query the value of a net, including strength value information, using the vpi_get_value() function. Since you cannot call the DPI C directly from SystemVerilog (even though you can call other linked C functions), this approach was initially less interesting to me because I would need to add yet another language to my code stack, but I was curious how this solution would look.

While the SystemVerilog specification may define the DPI, it’s far from clear how the API is intended to be used. After more experimentation than I would like to admit, I ended up with the following solution: Define two C helper methods that my SystemVerilog code can call, one to get a chandle (pointer) to the net of interest, and one to return an integer representation of the net’s drive strength from the DPI call:

Using these two helper C methods, we can make a simple module that calls the get_strength() C function every time the net value changes as shown below:

This solution works as intended, and can even differentiate between supply and strong drivers, but does have the complexity downside of needing to deal with and compile in C code. Most simulators provide relatively simple ways of compiling in DPI-C code, but it’s not as clean of a solution as I would have liked. If I could find a way of directly calling the DPI-C API functions from SystemVerilog code, this solution would have been more appealing.

Performance

I started digging into this issue because I had a hunch that the string formatting approach would have too much overhead, so does this really matter? Is there really any benefit to avoiding strings?

I built a very simple test bench which would cycle through all possible drive strengths with both 0 and 1 logical values 5 million times. Without a strength detection module in the model, only the multi-strength driver use to generate the signal, 5 million iterations took 14 seconds to simulate with the Xcellium Cadence simulator. The table below shows the simulation time when one strength detection module is added to the simulation for each approach I have discussed.

The data does indeed prove out my initial expectation that string manipulation is slow. However, I was quite surprised to see that the DPI-C method was slower than my initial SystemVerilog only approach. I would have thought that the additional nets and value resolution work the simulator would have to do for the sense net approach would have out-weighed any overhead in calling out to a C library.

Overall this was an interesting deep dive into a little explored part of the SystemVerilog language. If you can get away with not needing to differentiate strong from supply strength values, then I would definitely recommend the “sense net” detection approach as it’s not only the fastest, but also pure SystemVerilog. If you really need to detect supply strengths and the addition of two DPI-C functions isn’t problematic, then the DPI-C approach is a good choice. Either way, is better than the commonly recommended approach of using formatting the net value as a string.

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Can i force a weak 1 to an internal net in verilog?

  • Thread starter kaushikrvs
  • Start date Jun 24, 2017
  • Jun 24, 2017

Member level 5

Can i forcce a weak 1 to an internal net in verilog ? I have a top level , Which has an internal net say VRR . The VRR is usually generated by an analog block , but for some reason , the analog block is still not functional. I have to force a weak1 to the VRR signal. I am able to do force top.VRR = 1; but when I tried to do; force (weak1,weak0) top.VRR = 1; it throws an error.  

ThisIsNotSam

Advanced member level 5.

kaushikrvs said: I have a top level , Which has an internal net say VRR . The VRR is usually generated by an analog block , but for some reason , the analog block is still not functional. I have to force a weak1 to the VRR signal. I am able to do force top.VRR = 1; but when I tried to do; force (weak1,weak0) top.VRR = 1; it throws an error. Click to expand...

Advanced Member level 3

Re: Can i forcce a weak 1 to an internal net in verilog ? You need to use a continuous assignment, and then procedurally set the value when you would have done the force. reg fval = 1'bz; assign (weak1,weak0) top.VRR = fval; // in your procedural code ... fval = 1; // instead of force ... fval = 1'bz; // instead of release  

Re: Can i forcce a weak 1 to an internal net in verilog ? Hello Dave, Will the above act like a force on a net? because the wire is being driven by some other analog block?  

Re: Can i forcce a weak 1 to an internal net in verilog ? A continuous assignment does no override any other stronger assignments. You may need to disconnect the signal manually, or insert a tranif primitive.  

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Wednesday 22 June 2016

Strength in verilog.

verilog assign weak

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【Verilog-41】Verilog中强度strength的用法

verilog assign weak

除了逻辑值外,net类型的变量还可以定义强度,因而可以更精确的建模。net的强度来自于动态net驱动器的强度。在开关级仿真时,当net由多个驱动器驱动且其值互相矛盾时,可常用强度的概念来描述这种逻辑行为。 (strength0, strength1) (strength1, strength0) (strength0)------------pulldown primitives only (strength1)------------pullup primitives only (chargestrength)------trireg nets only strength0 = {supply0/strong0/pull0/weak0/highz0}强度由左至右依次减弱 strength1 = {supply1/strong1/pull1/weak1/highz1}强度由左至右依次减弱 chargestrength = {large/medium/small}

用法: 1.关键词strength0和strength1用于定义net的驱动强度。其中,strength表示强度,与紧跟着的0和1连起来分别表示输出逻辑值为0和1时的强度。 2.在强度声明中可选择不同的强度关键词来代替strength,但(highz0, highz1)和(highz1, highz0)这两种强度定义是不允许的,在pullup和pulldown门的强度声明中highz0和highz1是不允许的。 3.默认的强度定义为strength0和strength1,但是有下列例外: 对于pullup和pulldown门,默认的强度为pull1和pull0; 对于trireg的net,默认的强度为medium。 强度定义为supply0和supply1的net,总是能够提供强度。 4.在仿真期间,net的强度来自于net上的主驱动强度(即具有最大强度值得实例或连续赋值语句)。如果net未被驱动,它会呈现高阻值,但是以下情况例外: tri0和tri1类型的net分别具有逻辑值0和1,并为pull强度; trireg类型的net保持它们最后的驱动值。 强度为supply0和supply1的nets分别具有逻辑值0和1,并能提供驱动能力。 5.强度值有强弱顺序,可从supply(最强)依次减弱并排列到highz(最弱)。当需要确定net的实际逻辑值和强度时,或者当net由多个驱动器驱动而且驱动相互间出现冲突时,出现冲突的两个强度值在强弱顺序表中的相对位置就会对该net的真实逻辑值起作用。 6.强度时不可综合的。 7.可以在$display和$monitor等中用特定的格式控制符%V显示其强度值。 例子:

verilog assign weak

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verilog assign weak

Modeling Using Weak0 Weak1

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COMMENTS

  1. pullup

    To print the strength of a bit in a display message, use %v instead of %b. (§ 21.2.1.5) Verilog mostly works in the digital logic space. Verilog strength only comes into play when two or more assignments continuously drive the same net as a from of conflict resolution. The strength value does not propagate through assignment statements.

  2. Assigning weak1 and weak0 to an input pin to verify pulldown

    The first assign statement conditionally drive your vip out. When pulldown_verif is true, it drives a 'z on P. Driving a 'z on a signal is essentially removing the drive from the assign statement. The second assign statement is given two driving strengths, when driving a 0 it has weak strength, and when driving a 1 it has a highz strength.

  3. Verilog

    The drive strengths can be used for nets (except trireg net), gates, and UDPs. The charge strengths can be used only for trireg nets. The drive strength types are supply , strong, pull , weak, and highz strengths. The charge strength types are large , medium and small strengths. All strengths can be ordered by their value.

  4. Strength in Verilog

    Strength in Verilog. The strength is used to have more accurate modeling that specifies a value on a net. The strength of a net is derived based on the strength of multiple drivers and the final strength will get the strength of the strongest driver. Verilog provides three types of strength. Driving strength. Capacitive strength. High impedance.

  5. Strengths

    1. Small capacitive. small. 0. High impedance. highz0 , highz1. The default strength is strong drive. For pullup and pulldown gates, the default strength is pull drive; for trireg the default strength is medium capacitive; and for supply nets, the default strength is supply drive. A net that is not being driven has a high impedance strength ...

  6. Verilog assign statement

    Verilog assign statement. Signals of type wire or a similar wire like data type requires the continuous assignment of a value. For example, consider an electrical wire used to connect pieces on a breadboard. As long as the +5V battery is applied to one end of the wire, the component connected to the other end of the wire will get the required ...

  7. Difference between pullup(scl) and assign (weak0, weak1) scl = 1'b1;

    In reply to Sagar Wakle:. The only differences are syntax and default driving strength. A pullup is considered a gate-level primitive, whereas an assign statement is considered RTL.. A SystemVerilog interface does not allow gate-level primitives. So if that is what you are using to model your I2C, you would have to use an assign statement.. Some tools, like static timing analysis, expect ...

  8. Gate level modeling in Verilog

    These strength values assign a relative value to logic values. Strengths are used to resolve which value should appear on a net or gate output. ... The types of strengths are mentioned below: supply, strong, pull, weak, highz strengths, large, medium, and small strengths. ... In Verilog, the gate delays may be defined by a designer in a ...

  9. verilog

    Sorted by: 3. Yes, it does matter. assign a = b; assigns a to get whatever value b has, whenever b changes. In simulation, this means if you also have somewhere. assign a = c | d & e; Then. You'd have a conflict because a is assigned in two places. This could be resolved by declaring the two assignments with different strength settings, for ...

  10. How do I specify weak feedback inverter in Verilog

    In the meantime, there is a weak feedback inverter IVW(.in(b), .out(a)). The value of node a is driven by IV0 when ctrl is 1. When ctrl is 0, node a is driven by weak feedback inverter. When IV0 drives node a, weak feedback inverter IVW is driving node a as well. If they drive node a to different values, verilog will produce X.

  11. Pullup in sv

    Just declare the signal as a 'tri1' nettype, or use a continuous assignment with a pull strength. tri1 *signal*; or. wire *signal*; assign (pull1,pull0) *signal* = '1; Then treat the signal as you would any other bi-directional and drive it with a 'z when you want the pullup to have an effect. KaitooKid September 17, 2015, 9:40am 3.

  12. Verilog Assignments

    This is used to assign values onto scalar and vector nets and happens whenever there is a change in the RHS. It provides a way to model combinational logic without specifying an interconnection of gates and makes it easier to drive the net with logical expressions. // Example model of an AND gate wire a, b, c; assign a = b & c; Whenever b or c ...

  13. Drive Strength Detection in SystemVerilog

    In SystemVerilog (and Verilog before it), a "net" is a special type of variable construct which is used to structurally model a wire in a circuit. The value of the "net" variable is a combination of a 4-state logical value (0,1,z,x), and a strength value (highz, weak, pull, strong, or supply). The value of a net is determined by ...

  14. Can i force a weak 1 to an internal net in verilog?

    Re: Can i forcce a weak 1 to an internal net in verilog ? You need to use a continuous assignment, and then procedurally set the value when you would have done the force. reg fval = 1'bz; assign (weak1,weak0) top.VRR = fval; // in your procedural code... fval = 1; // instead of force... fval = 1'bz; // instead of release

  15. verilog, weak signals

    assigns a weak pulldown to the signal y. Likely this is sufficient, and you can get rid of the assign statement. If you only want the weak driver there when en is not high, you could do it in the assignment as well: assign (weak1,weak0) y = en ? 1'bz : 1'b0; See page 51 of the Verilog 1364-1995 specification. If you don't have

  16. Verification Engineer's Blog: Strength in Verilog

    Strength in Verilog. Strengths can be used to resolve which value should appear on a net or gate output. The combinations (highz0, highz1) and (highz1, highz0) shall be considered illegal. Signals with driving strengths shall propagate from gate outputs and continuous assignment outputs. Signals with the charge storage strengths shall originate ...

  17. 【Verilog-41】Verilog中强度strength的用法

    在Verilog中经常会遇到pullup、pulldown和assign (strength0, strength1),那么它们究竟有什么区别?它们之间的区别主要是语法和驱动强度。pullup、pulldown看做是门级原语,assign语句看做是RTL。 SystemVerilog接口不允许门级原语。如果是用来写ip,应该使用assign语句。

  18. Weak 'H', Pullup on inout bidirectional signal in simulation

    For VHDL, it should be possible to simply add an extra driver to the signal (which has to be of std_logic type), with the constant value 'H'. In Verilog one would use a simple '1' driver and the net type wand for wired and. 'H' specifically means a weak high driver, so it will be overridden by the low drivers. Yes, in VHDL simply add a ...

  19. verilog, Modeling Using Weak0 Weak1

    the best way to do this is in an assign statement (think of it as an attribute. of a driver of a wire, not the wire itself) for example: assign (weak0, weak1) w = value; you can use this when declaring a wire but ONLY if you are doing. an implicit assign in the declaration: wire (weak0, weak1) w = value; Paul Campbell.

  20. verilog

    Here, in blocking assignment, y is evaluated after x is assigned a value. While using non-blocking assignment, x and y are evaluated and pushed into simulators internal queue and assigned at 10ns and 5ns respectively. A blocking assignment effects immediately. A nonblocking assignment takes place at the end of processing the current "time delta".