Sunday, December 10, 2006
FPGA design from scratch. Part 10
It took longer than expected to finalize the verification phase but I think we can regain this lost time when we start debugging the real design. Hopefully we will not have that many bugs left in the design. We are now ready for the final synthesis runs.
Synthesis with timing constraints
Logic synthesis is a process by which an abstract form of desired circuit behavior (typically register transfer level (RTL) or behavioral) is turned into a design implementation in terms of logic gates. Common examples of this process include synthesis of HDLs, including VHDL and Verilog. Some tools can generate bitstreams for programmable logic devices such as PALs or FPGAs, while others target the creation of ASICs. Logic synthesis is one aspect of electronic design automation.
The synthesis is part of the design closure process by which a VLSI design is modified from its initial description to meet a growing list of design constraints and objectives. A constraint is a design target that must be met in order for the design to be considered successful. For example, a chip may be required to run at a specific frequency in order to interface with other components in a system. Other constraints can be the power consumption and the chip size.
Designing an embedded system design in an FPGA that meets all timing requirements, starts already in the planing phase and continues throughout the whole project. Here are some important steps in achieving design closure.
Here is an article from Altera describing the differences between ASIC and FPGA design. From Altera you can also download the ASIC to FPGA Design Methodology & Guidelines.
- Understanding the differences between ASIC and FPGA
- System analysis and design partitioning
- RTL design for FPGA
- Synthesizing using the "right" timing constraints.
- Floor planing critical parts of the design
- Place and route and pin placement
In our design the only constraints we have are timing constraints for input and output signals. The design will operate at a maximum clock frequence of 54 MHz .This will be easily met in the Virtex-4 FPGA family.
We will use the Xilinx synthesis tool XST because it is part of the Integrated Software Environment (ISE) and it is free. Here is an interesting article form FPGA Journal about "free" synthesis tools, how good are they. Other synthesis tools we could have used can be found in this table:
Before we start the synthesis run, let's take a look in the XST User Guide and the FAQ
XST is a Xilinx tool that synthesizes HDL designs to create Xilinx specific netlist files called NGC files. The NGC file is a netlist that contains both logical design data and constraints that takes the place of both EDIF and NCF (Netlist Constraints File) files. This manual describes XST support for Xilinx devices, HDL languages and design constraints. The manual also explains how to use various design optimization and coding techniques when creating designs for use with XST. We can choose to run XST from the command line or from inside the Project Navigator. Let's start the Project Navigator and follow these instructions.
=> ise &
After the synthesis the ISE project directory looks like this:
The synthesis report file
The Synthesis Report file is called <ETC.syr>, let's take a look at it. We can see that the synthesis was successful, that the timing looks good and that the device utilization is about 10%. We could have used a smaller FPGA.
Timing Summary:What else can we find out from the synthesis report file
Speed Grade: -12
Minimum period: 5.839ns (Maximum Frequency: 171.250MHz)
Minimum input arrival time before clock: 5.433ns
Maximum output required time after clock: 6.491ns
Maximum combinational path delay: 8.409ns
Device utilization summary:
Selected Device : 4vfx12ff668-12
Number of Slices: 601 out of 5472 10%
Number of Slice Flip Flops: 570 out of 10944 5%
Number of 4 input LUTs: 1113 out of 10944 10%
Number of IOs: 129
Number of bonded IOBs: 109 out of 320 34%
Number of GCLKs: 2 out of 32 6%
The memory blocks are black boxed:
WARNING:Xst:2211 - "../../design/ETC_DUAL_PORT_1024x32.v" line 327:
Instantiating black box module <ETC_DUAL_PORT_1024x32>.
WARNING:Xst:2211 - "../../design/ETC_DUAL_PORT_1024x32.v" line 339:
Instantiating black box module <ETC_DUAL_PORT_1024x32>.
Black box instantiation
A black box is any instantiated component that is not represented by HDL code, but rather by another netlist format. Synthesis tools will generally report some kind of warning when a black box (this is, an instantiated component with no associated VHDL code) is detected.
Examples of black boxes include:
- CORE Generator modules (in our case)
- Instantiated EDIF files
- Instantiated primitives
If you are instantiating a component that is represented by something other than VHDL code, no response to the warning message is needed. If your intent was not to instantiate a black box, check your component declaration and instantiation to ensure that the component is properly represented by VHDL code.
To avoid "black box" warning messages, add the following lines to your HDL code:
attribute box_type : string;
attribute box_type of <component_name> : component is "black_box";
//synthesis attribute box_type <module_name> "black_box"
Unused register bits
Some registers have unused bits:
WARNING:Xst:646 - Signal <tdi_data_reg1<31:30>> is assigned but never used.One-hot encoding not safe
WARNING:Xst:646 - Signal <tdi_data_reg2<31:30>> is assigned but never used.
WARNING:Xst:646 - Signal <tdi_data_reg3<31:30>> is assigned but never used.
WARNING:Xst:646 - Signal <tdi_data_reg4<31:30>> is assigned but never used.
INFO:Xst:2117 - HDL ADVISOR -Mux Selector <jtc_tck_source> of Case statement line 313
was re-encoded using one-hot encoding.The case statement will be optimized
(default statement optimization),but this optimization may lead to design initialization problems.
To ensure the design works safely, you can:
- add an 'INIT' attribute on signal <jtc_tck_source> (optimization is then done without any risk)
- use the attribute 'signal_encoding user' to avoid onehot optimization
- use the attribute 'safe_implementation yes' to force XST to perform a safe (but less efficient) optimization
We will take a closer look at these warnings and info messages but before we do, we will generate a Post-Synthesis Simulation Model.
Post-synthesis simulation model
The Post-Synthesis Simulation Model will contain all the building blocks used in the FPGA, like LUTs, muxes and I/O buffers. It is functionally correct but it has no timing information and it should be simulated using the unisim library.
Double-click the <Generate Post-Synthesis Simulation Model> entry in the Processes window in the Project Navigator to start the generation.
The flat netlist file ETC_synthesis.v will be found in the <ETC/netgen/synthesis> directory.
Before we can use it in our simulation we have to add the following instantiation <glbl glbl();> inside the module ETC.
We replace the ETC RTL design files with the ETC netlist file in our simulation setup and rerun our testcases and they all pass. The synthesis tool did its job.
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Posted at 08:57 by svenand
March 15, 2011 03:01 PM PDT
i have a query regarding black box instantiation...when i integrate ise and edk, the instance for edk for the hdl is generated as below...
// Instantiate the module
(* BOX_TYPE = "user_black_box" *)
system instance_name (
but when i instantiate this in the top mosule the black box instantiation is giving some errors how do i instantiate this black box can u provide me the syntax????