From Python to silicon
 

What MyHDL is not

There are many good reasons to consider MyHDL. However, to use it effectively, it is important to have realistic expectations. The easiest way to achieve this is perhaps to describe what MyHDL is not.

MyHDL is not a way to turn arbitrary Python into silicon

Arbitrary Python into silicon, that sounds like a dream, right? And it is :-) Don't expect this from MyHDL.

To convert MyHDL code into hardware, you will have to learn about synthesis, and especially about the modeling constraints that it imposes. These constraints are significant, and therefore synthesizable code may be suprisingly low-level to newcomers.

MyHDL is not a radically new approach

The mainstream Hardware Description Languages are Verilog and VHDL. MyHDL was not created because these languages do everything wrong. Quite the contrary.

According to the analysis of MyHDL's creator, the promise of HDL-based design has not been fulfilled despite the fact that Verilog and VHDL do many things right. MyHDL tries to lower the barrier to entry and improve on certain language features that make the task more difficult than necessary.

The most important MyHDL design choice is to implement it as a Python library instead of as a separate language. This makes it available to a much broader public, and opens the way to modern software development techniques such as test-driven design (TDD).

However, fundamentally MyHDL is based on the same event-driven paradigm as Verilog and VHDL. This has proven to be the winning solution for HDL design.

MyHDL is not a synthesis tool

MyHDL alone is not sufficient to convert code into a hardware implementation. In other words, it is not a synthesis tool.

However, MyHDL contains a convertor tool that you can use to convert synthesizable MyHDL code into equivalent Verilog or VHDL code. From there on, you can use standard synthesis tools to get to an implementation. Synthesis tools are available from EDA vendors and FPGA vendors. Some synthesis tools from FPGA vendors are cheap or even free.

Although the convertor in MyHDL is not a synthesis tool, it automates a number of tasks which are harder to do in Verilog or VHDL directly.

MyHDL is not an IP block library

The term IP (Intellectual Property) is used in the context of hardware design to refer to hardware blocks that are designed to be easily reusable in other designs.

MyHDL does not contain IP blocks. What it provides is the raw material to design them. Actually, MyHDL is the ideal platform for IP block development, because of its powerful Python foundation, and because it provides a path into both Verilog and VHDL design flows with a single development effort.

MyHDL is not only for implementation

MyHDL is very effective to describe a hardware implementation. The corresponding abstraction level is called RTL (Register Transfer Level). Typically, RTL code is converted into hardware implementation by a synthesis tool. Another name for RTL is therefore synthesizable code.

However, MyHDL is not an RTL-only language. This point is frequently misunderstood, even though it is also valid for mainstream HDLs such as Verilog and VHDL.

MyHDL is a library for general event-driven modeling and simulation of hardware systems. There are many reasons why it can be useful to model at a higher level of abstraction than RTL. For example, you can use MyHDL to verify architectural features, such as system throughput, latency and buffer sizes. You can also write high level models for specialized technology-dependent cores that are not going through synthesis. Last but not least, you can use MyHDL to write test benches that verify a system model or a synthesizable description.

RTL is just a specific and restrictive way to use MyHDL's modeling capabilities with the goal to create synthesizable code.

MyHDL is not well suited for accurate timing simulations

MyHDL's delay model is simple, which is fine for RTL and higher levels of abstraction. These are the levels where MyHDL is strong.

However, MyHDL's timing model is not suited for accurate timing simulations, which are typically useful at the gate level. For such simulations, it is advisable to use industry standard Verilog or VHDL simulators, which are optimized for the task. Moreover, gate level simulation libraries are typically available in those formats from silicon vendors. In a MyHDL-based design flow, this methodology is logical, because all tasks below RTL are part of the “back-end”. The synthesis tool will create a gate level net list in Verilog or VHDL format.

MyHDL can still play a significant role in gate-level simulations, thanks to its co-simulation capabilities. This means that you can reuse all MyHDL test benches that were developed for RTL verification to control gate level simulations.

whatitisnot.txt · Last modified: 2012/05/07 11:30 by cfelton
 
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