Hardware emulation
Encyclopedia
In integrated circuit design
, hardware emulation is the process of imitating the behavior of one or more pieces of hardware (typically a system under design) with another piece of hardware, typically a special purpose emulation system. The emulation model is usually based on RTL (e.g. Verilog
) source code, which is compiled into the format used by emulation system. The goal is normally debugging and Functional verification
of the system being designed. Often an emulator is fast enough to be plugged into a working target system in place of a yet-to-be-built chip, so the whole system can be debugged with live data. This is a specific case of in-circuit emulation
.
Sometimes hardware emulation can be confused with hardware devices such as expansion cards with hardware processors that assist functions of software emulation, such as older daughterboards with x86 chips to allow x86 OSes to run on motherboards of different processor families.
respins and steppings
are due to (at least in part) functional errors and bugs inadvertently introduced at RTL stage of the design process. Thus, comprehensive Functional verification
is key to reducing development costs and delivering a product on time. Functional verification of a design is most often performed using logic simulation
and/or prototyping. There are advantages and disadvantages to each and often both are used. Logic simulation is easy, accurate, flexible, and low cost. However, simulation is often not fast enough for large designs and almost always too slow to run application software against the hardware design. FPGA
-based prototypes are fast and inexpensive, but the time required to implement a large design into several FPGAs can be very long and is error-prone. Changes to fix design flaws also take a long time to implement and may require board wiring changes. Since FPGA prototypes have little debugging capability, probing signals inside the FPGAs in real time is very difficult, if not impossible, and recompiling FPGAs to move probes takes too long. The usual compromise is to use simulation early in the verification process when bugs and fixes are frequent, and prototyping at the end of the development cycle when the design is basically complete and speed is needed to get sufficient testing to uncover any remaining system-level bugs. Prototyping is also popular for testing software.
Simulation acceleration can address the performance shortcomings of simulation to an extent. Here the design is mapped into a hardware accelerator to run much faster and the testbench (and any behavioral design code) continues to run on the simulator on the workstation. A high-bandwidth, low latency channel connects the workstation to the accelerator to exchange signal data between testbench and design. By Amdahl's law
, the slowest device in the chain will determine the speed achievable. Normally, this is the testbench in the simulator. With a very efficient testbench (written in C or transaction-based), the channel may become the bottleneck. In some cases, a transaction-level testbench is able to feed as much data to the design being emulated as "live" stimulus.
In-circuit emulation improves greatly on FPGA prototyping’s long time to implement and change designs, and provides a comprehensive, efficient debugging capability. While it takes weeks or months to implement an FPGA prototype, it takes only days to implement emulation and design changes take but a few hours or less. Emulation does this at the expense of running speed and cost compared to FPGA prototypes. Looking at emulation from the other direction, it improves on acceleration’s performance by substituting "live" stimulus for the simulated testbench. This stimulus can come from a target system (the product being developed), or from test equipment. At 10,000 to 100,000 times the speed of simulation, emulation is often the only technique that can deliver the speed necessary to test application software while still providing a comprehensive hardware debug environment.
With a prototype:
Acceleration and emulation are more like prototyping and silicon in terms of RTL execution and debugging since the entire design executes simultaneously as it will in the silicon. Since the same hardware is often used to provide both simulation acceleration and in-circuit emulation, these systems provide a blend of these two very different debugging styles.
High end hardware emulators provide a debugging environment with many features that can be found in logic simulators, and in some cases even surpass their debugging capabilities:
These tasks are properly carried out during logic simulation
or with a static timing analysis
tool.
Integrated circuit design
Integrated circuit design, or IC design, is a subset of electrical engineering and computer engineering, encompassing the particular logic and circuit design techniques required to design integrated circuits, or ICs...
, hardware emulation is the process of imitating the behavior of one or more pieces of hardware (typically a system under design) with another piece of hardware, typically a special purpose emulation system. The emulation model is usually based on RTL (e.g. Verilog
Verilog
In the semiconductor and electronic design industry, Verilog is a hardware description language used to model electronic systems. Verilog HDL, not to be confused with VHDL , is most commonly used in the design, verification, and implementation of digital logic chips at the register-transfer level...
) source code, which is compiled into the format used by emulation system. The goal is normally debugging and Functional verification
Functional verification
Functional verification, in electronic design automation, is the task of verifying that the logic design conforms to specification. In everyday terms, functional verification attempts to answer the question "Does this proposed design do what is intended?" This is a complex task, and takes the...
of the system being designed. Often an emulator is fast enough to be plugged into a working target system in place of a yet-to-be-built chip, so the whole system can be debugged with live data. This is a specific case of in-circuit emulation
In-circuit emulator
An in-circuit emulator is a hardware device used to debug the software of an embedded system. It was historically in the form of bond-out processor which has many internal signals brought out for the purpose of debugging...
.
Sometimes hardware emulation can be confused with hardware devices such as expansion cards with hardware processors that assist functions of software emulation, such as older daughterboards with x86 chips to allow x86 OSes to run on motherboards of different processor families.
Introduction
The largest fraction of silicon integrated circuitIntegrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
respins and steppings
Stepping level
The term stepping level in the context of CPU architecture or integrated circuitry is a version number.Stepping level refers to the introduction or revision of the lithographic mask or masks within the set of plates that generate the pattern that produces the CPU or integrated circuit...
are due to (at least in part) functional errors and bugs inadvertently introduced at RTL stage of the design process. Thus, comprehensive Functional verification
Functional verification
Functional verification, in electronic design automation, is the task of verifying that the logic design conforms to specification. In everyday terms, functional verification attempts to answer the question "Does this proposed design do what is intended?" This is a complex task, and takes the...
is key to reducing development costs and delivering a product on time. Functional verification of a design is most often performed using logic simulation
Logic simulation
Logic simulation is the use of a computer program to simulate the operation of a digital circuit. Logic simulation is the primary tool used for verifying the logical correctness of a hardware design. In many cases logic simulation is the first activity performed in the process of taking a hardware...
and/or prototyping. There are advantages and disadvantages to each and often both are used. Logic simulation is easy, accurate, flexible, and low cost. However, simulation is often not fast enough for large designs and almost always too slow to run application software against the hardware design. FPGA
Field-programmable gate array
A field-programmable gate array is an integrated circuit designed to be configured by the customer or designer after manufacturing—hence "field-programmable"...
-based prototypes are fast and inexpensive, but the time required to implement a large design into several FPGAs can be very long and is error-prone. Changes to fix design flaws also take a long time to implement and may require board wiring changes. Since FPGA prototypes have little debugging capability, probing signals inside the FPGAs in real time is very difficult, if not impossible, and recompiling FPGAs to move probes takes too long. The usual compromise is to use simulation early in the verification process when bugs and fixes are frequent, and prototyping at the end of the development cycle when the design is basically complete and speed is needed to get sufficient testing to uncover any remaining system-level bugs. Prototyping is also popular for testing software.
Simulation acceleration can address the performance shortcomings of simulation to an extent. Here the design is mapped into a hardware accelerator to run much faster and the testbench (and any behavioral design code) continues to run on the simulator on the workstation. A high-bandwidth, low latency channel connects the workstation to the accelerator to exchange signal data between testbench and design. By Amdahl's law
Amdahl's law
Amdahl's law, also known as Amdahl's argument, is named after computer architect Gene Amdahl, and is used to find the maximum expected improvement to an overall system when only part of the system is improved...
, the slowest device in the chain will determine the speed achievable. Normally, this is the testbench in the simulator. With a very efficient testbench (written in C or transaction-based), the channel may become the bottleneck. In some cases, a transaction-level testbench is able to feed as much data to the design being emulated as "live" stimulus.
In-circuit emulation improves greatly on FPGA prototyping’s long time to implement and change designs, and provides a comprehensive, efficient debugging capability. While it takes weeks or months to implement an FPGA prototype, it takes only days to implement emulation and design changes take but a few hours or less. Emulation does this at the expense of running speed and cost compared to FPGA prototypes. Looking at emulation from the other direction, it improves on acceleration’s performance by substituting "live" stimulus for the simulated testbench. This stimulus can come from a target system (the product being developed), or from test equipment. At 10,000 to 100,000 times the speed of simulation, emulation is often the only technique that can deliver the speed necessary to test application software while still providing a comprehensive hardware debug environment.
Debugging simulations vs. emulations/prototyping
It is worth noting that simulation and prototyping involve two different styles of execution. Simulation executes the RTL code serially while a prototype executes fully in parallel. This leads to differences in debugging. In simulation:- The user can set a breakpoint and stop simulation to inspect the design state, interact with the design, and resume simulation.
- The user can stop execution “mid-cycle” as it were with only part of the code executed.
- The user can see any signal in the design and the contents of any memory location at any time.
- The user can even back up time (if they saved checkpoint(s)) and re-run.
With a prototype:
- The user employs a logic analyzer for visibility, and so can see only a limited number of signals which they determined ahead of time (by clipping on probes).
- The target does not stop when the logic analyzer triggers, so each time the user changes the probes or trigger condition, they have to reset the environment and start again from the beginning.
Acceleration and emulation are more like prototyping and silicon in terms of RTL execution and debugging since the entire design executes simultaneously as it will in the silicon. Since the same hardware is often used to provide both simulation acceleration and in-circuit emulation, these systems provide a blend of these two very different debugging styles.
High end hardware emulators provide a debugging environment with many features that can be found in logic simulators, and in some cases even surpass their debugging capabilities:
- The user can set a breakpoint and stop emulation to inspect the design state, interact with the design, and resume emulation. The emulator always stops on cycle boundaries.
- The user has visibility to any signal or memory contents in the design without the need to set up probes before the run. While visibility is provided also for past time, the amount of time that it can show in the past might be limited in some cases to the depth of the emulator's trace memory.
- The user can even back up time (if they saved checkpoint(s)) and re-run.
Emulation and 2-state logic
Another difference between simulation and acceleration and emulation is a consequence of accelerators using hardware for implementation – they have only two logic states – acting the way the silicon will when fabricated. This implies:- They are not useful for analyzing X-state initialization.
- They cannot analyze strength resolution, or at least this must be done statically at compile time.
- Emulators do not model precise circuit timing, and hence they will probably not find any race conditions or setup and hold time violations.
These tasks are properly carried out during logic simulation
Logic simulation
Logic simulation is the use of a computer program to simulate the operation of a digital circuit. Logic simulation is the primary tool used for verifying the logical correctness of a hardware design. In many cases logic simulation is the first activity performed in the process of taking a hardware...
or with a static timing analysis
Static timing analysis
Static Timing Analysis is a method of computing the expected timing of a digital circuit without requiring simulation.High-performance integrated circuits have traditionally been characterized by the clock frequency at which they operate...
tool.
Emulation versus prototyping
A key distinction between an emulator and an FPGA prototyping system is that the emulator provides a rich debug environment while a prototyping system has little to no debug capability and is primarily used after the design is debugged to create multiple copies for system analysis and software development.See also
- Hardware-assisted virtualization
- EmulatorEmulatorIn computing, an emulator is hardware or software or both that duplicates the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system...
- In-circuit emulatorIn-circuit emulatorAn in-circuit emulator is a hardware device used to debug the software of an embedded system. It was historically in the form of bond-out processor which has many internal signals brought out for the purpose of debugging...
- Background Debug Mode interfaceBackground Debug Mode interfaceBackground Debug Mode interface is an electronic interface that allows debugging of embedded systems. Specifically, it provides in-circuit debugging functionality in microcontrollers...
- Examples:
- Microprocessor Emulator HP 64000HP 64000The HP 64000, introduced circa 1980, is a tool for developing hardware and software for products based on commercial microcomputers. The earliest commercial 64000 development systems contain from one to six 64100A Development Stations sharing a hard drive and line printer...
(different CPUs)
- Microprocessor Emulator HP 64000