asicsystem-verilogverilogvlsi
I have always read that delays declared in RTL code can never be synthesized. They are meant only for simulation purpose and modern synthesis tools will just ignore delays declarations in the code.
For example: x = #10 y; will be considered as x = y; by the synthesis tool.
What are the reasons delay declarations in any hardware description language (for example, VHDL, Verilog or Sytem-Verilog) cannot be synthesized?
Typically ASIC design is a team endeavor due to the complexity and quantity of work. I'll give a rough order of steps, though some steps can be completed in parallel or out of order. I will list tools that I have used for each task, but it will not be encyclopedic.
Build a cell library. (Alternatively, most processes have gate libraries that are commercially available. I would recommend this unless you know you need something that is not available.) This involves designing multiple drive strength gates for as many logic functions as needed, designing pad drivers/receivers, and any macros such as an array multiplier or memory. Once the schematic for each cell is designed and verified, the physical layout must be designed. I have used Cadence Virtuoso for this process, along with analog circuit simulators such as Spectre and HSPICE.
Characterize the cell library. (If you have a third party gate library, this is usually done for you.) Each cell in your library must be simulated to generate timing tables for Static Timing Analysis (STA). This involves taking the finished cell, extracting the layout parasitics using Assura, Diva, or Calibre, and simulating the circuit under varying input conditions and output loads. This builds a timing model for each gate that is compatible with your STA package. The timing models are usually in the Liberty file format. I have used Silicon Smart and Liberty-NCX to simulate all needed conditions. Keep in mind that you will probably need timing models at "worst case", "nominal", and "best case" for most software to work properly.
Synthesize your design. I don't have experience with high level compilers, but at the end of the day the compiler or compiler chain must take your high level design and generate a gate-level netlist. The synthesis result is the first peek you get at theoretical system performance, and where drive strength issues are first addressed. I have used Design Compiler for RTL code.
Place and Route your design. This takes the gate-level netlist from the synthesizer and turns it into a physical design. Ideally this generates a pad-to-pad layout that is ready for fabrication. It is really easy to set your P&R software to automatically make thousands of DRC errors, so not all fun and games in this step either. Most software will manage drive strength issues and generate clock trees as directed. Some software packages include Astro, IC Compiler, Silicon Encounter, and Silicon Ensemble. The end result from place and route is the final netlist, the final layout, and the extracted layout parasitics.
Post-Layout Static Timing Analysis. The goal here is to verify that your design meets your timing specification, and doesn't have any setup, hold, or gating issues. If your design requirements are tight, you may end up spending a lot of time here fixing errors and updating the fixes in your P&R tool. The final STA tool we used was PrimeTime.
Physical verification of the Layout. Once a layout has been generated by the P&R tool, you need to verify that the design meets the process design rules (Design Rule Check / DRC) and that the layout matches the schematic (Layout versus Schematic / LVS). These steps should be followed to ensure that the layout is wired correctly and is manufacturable. Again, some physical verification tools are Assura, Diva, or Calibre.
Simulation of the final design. Depending on complexity, you may be able to do a transistor-level simulation using Spectre or HSPICE, a "fast spice" simulation using HSIM, or a completely digital simulation using ModelSim or VCS. You should be able to generate a simulation with realistic delays with the help of your STA or P&R tool.
Starting with an existing gate library is a huge time saver, as well as using any macros that benefit your design, such as memory, a microcontroller, or alternative processing blocks. Managing design complexity is a big part as well - a single clock design will be easier to verify than circuit with multiple clock domains.
Add (and use) a reset signal.
Xilinx FPGAs have a global set/reset (GSR) signal that puts all registers in the their default state or as specified in the register declaration (this is documented in the XST User's Guide at the beginning of chapter 5). AFAIK, the @initial block is ignored.
However, things are chaotic when the FPGA starts up, because:
So the initial Flip-Flop values after the GSR are not enough.
Create a module that generates a reset signal for each clock domain. You can create it by by AND'ing relevant asynchronous reset signals, such as an external reset pin, PLL/DCM locked signals, and using it with a synchronizer, as follows:
(Source: How do I reset my FPGA?)
Best Answer
Synthesizing means somehow converting what you have described (in Verilog here) into real hardware.
Now in your Verilog you say that you have a 50ns delay. Ok, but now, in term of hardware, how would you convert this into actual hardware?
If you are using an FPGA, how would you actually build your 50ns delay using the available FPGA resources (LUT, Registers, Ram element, ...)? By adding additional routing delays? imagine that you specify 1s delay! Impossible without using ALL the routing capability of your chip (maybe not enough). Your design can't be fitted. Same for an ASIC. You would use 80% of the silicon surface to add a delay to ONE line.
The way is it supposed to work is that you use synchronous design and you implement the delay by yourself using counters or other techniques. But delays have to be multiples of the clock of that element.
Usually you find things such as "after 10 ns" theses are propagation delays. When doing an ideal simulation on a Verilog simulator, outputs happen exactly when the inputs change. This is not realistic and does not describe the way real hardware work. To account for that you can specify after how much time your output will be changed: using the delay declaration.