A clocking block is a set of signals synchronised on a particular clock. It basically separates the time related details from the structural, functional and procedural elements of a testbench. It helps the designer develop testbenches in terms of transactions and cycles.
Why is default clocking block required?
Simply put, a clocking block encapsulates a bunch of signals that share a common clock. Hence declaring a clocking block inside an interface can help save the amount of code required to connect to the testbench and may help save time during development.
How do clocking blocks avoid race conditions?
1. The same signal is driven and sampled at the same time. => To avoid this race condition, a clocking block in interface is used as it provides an input and output skews to sample and drive, respectively. 2.
What is input skew and output skew in clocking block?
If an input skew is mentioned for a clocking block, then all input signals within that block will be sampled at skew time units before the clock event. If an output skew is mentioned for a clocking block, then all output signals in that block will be driven skew time units after the corresponding clock event.
What is the difference between the clocking block and Modport?
Clocking block is used to introduce input/output sampling/driving delays. Modport defines directions of signals and can be used to represent set of signals.
What is the use of Modports?
Modports in SystemVerilog are used to restrict interface access within a interface. The keyword modport indicates that the directions are declared as if inside the module.
What is skew in Verilog?
Clocking skew specifies the moment (w.r.t clock edge) at which input and output clocking signals are to be sampled or driven respectively. A skew must be a constant expression and can be specified as a parameter.
What is the difference between clocking block and Modport?
Why always block is not used in program block?
If you had an always block, it would never stop, so you would have to explicitly call $exit to signal that the program block completed. This is the reason why we can not have always block inside program.
What is clocking block in SV?
SystemVerilog adds the clocking block that identifies clock signals and captures the timing and synchronization requirements of the blocks being modeled. A clocking block assembles signals that are synchronous to a particular clock and makes their timing explicit.
Why do we need Modports?
Modports are used to specify the direction of signal with respect to a specific module/component. They are also used to restrict access to certain signals from some modules/classes. Usually testbench modport contains a bunch of stimulus-driving signals as output while the same signals are taken as input to the RTL.
What are the guidelines for clocking block events?
However, they do come with several additional guidelines that you need to follow All samples and drives to clocking block inputs and outputs respectively must be synchronized to the clocking block event (cb). Do not use any other events or waits other than @cb or @ (cb iff signal==1)
Is clocking blocks in Verilog A good idea?
The answer really depends on your knowledge of the Verilog scheduling mechanism – do you know how to code to prevent race conditions, and do you know how to deal with bidirectional buses from the testbench. If you don’t understand this, then clocking blocks simplify the problem.
Can I use a program to trigger a clocking block?
The clock event used to trigger the clocking block must not come from a program. (we do not recommend using program blocks anyways) Once you start using clocking blocks in an interface, do not reference those signals from anything other than the clocking block in your testbench.
Can signal appear in multiple clocking blocks at the same time?
In case you use a different tool vendor, I would find it surprising that they complain about any conflict as the standard clearly states that a signal can appear in multiple clocking blocks (regardless of direction) – 14.6 Signals in multiple clocking blocks.
