Flexibility Vs. Portability In Emulation

Transactors are vendor-specific. While that allows for customization, it also causes interoperability problems.


Complete and exhaustive verification of low-power designs requires a substantial effort and part of this includes running real applications on the hardware. Simulators fall short as designers realize that the so-called testbenches they create are artificial and don’t necessarily represent typical applications. As such, this is the sweet spot for emulators, also known as hardware accelerators, in the overall verification environment.

While it sounds like a straightforward approach, this industry segment has been anything but.

Hardware emulation was invented as a means to connect a yet-to-be-built chip—the design under test or DUT—to the target system (the testbench) where the silicon chip will eventually play. “This configuration supports testing with live data to some extent,” said Lauro Rizzatti, vice president of marketing and general manager of EVE-USA. “Since the emulation system runs at a lower speed than the real silicon, you have to buffer the two speed domains to avoid losing data via a speed-rate adapter. When an emulation system is deployed this way, the setup is called in-circuit emulation (ICE).”

Connected to this, speed-rate adapters were developed by each emulation provider and typically they were not compatible. Then, around 2000, Ikos Systems introduced the notion of synthesizable transactors used for emulation. The company was acquired by Mentor Graphics in 2002.

The Standard Co-Emulation Modeling Interface (SCE-MI) was first introduced at that time as a way to standardize the communication between the hardware portion running in the emulator and the software portion running in the PC of a transactor.

SCE-MI1 is the equivalent of the “data link layer” of the International Standard Organization’s Open System Interconnect (ISO/OSI) model for networking protocols. SCE-MI2, at best, is the equivalent of the “transport layer,” although all the complexity of a transactor resides in the equivalent of the top three layers, Rizzatti said.

“SCE-MI2.0 raised the abstraction level of the communication scheme between the hardware and software (using mainly DPI calls between C and SV). But it still did not address the way hardware and software were developed to create the fully synthesizable transactors, and ultimately failed to address the way transactors were simply used in a C or a SV testbench. SCE-MI2.0 does not guarantee any compatibility between platforms. No other layers are defined by SCE-MI. This is probably the biggest challenge, since the communication layer is actually defined in the SV standard with DPI,” he pointed out.

Rizzatti compared transactors with verification IP (VIP) for complex software and hardware. There is no standard to create VIP, whether it is synthesizable (transactors) or not. For example, VIP from Cadence is unlikely to be compatible with VIP from Synopsys, despite complaints from large customers.

“There are two levels of transaction issues here,” explained Erich Marschner, product manager for Questa Power Aware Verification at Mentor Graphics. “One is the transactor that maps from records representing transactions to pin wiggles on the bus. This generally involves code that is synthesizable and therefore can be loaded onto an emulator. There shouldn’t really be any reason why you can’t share that kind of code between different emulators from different sources because it is basically the same as what you would run in simulation. As far as I know most of the VIP that we provide and that others provide, at least if it’s open source, can run on any simulator.”

The problem is sending a collection of transactions from the host to the emulator to be executed on the DUT side. That requires a connection between the host and emulator, and it may differ from one vendor to another. “There is a hardware connection there that is being negotiated and that is almost certainly dependent upon the kind of emulator you are dealing with,” Marschner said.

It’s important to understand that it takes no less than few weeks all the way to years to develop synthesizable transactors, depending on the complexity of the protocol. The UART is a simple transactor while the PCIe is a very complex transactor. Each and every company cannot afford such a large investment , which is the reason why the emulation providers need to offer a catalog of transactors, Rizzatti said.

“While it’s not realistic to ask competitors to work together to make the transactors fully compatible, one possibility would be at least to ask them to normalize the API level of each transactor (for each protocol) so that the customers could re-use the same C or SV test bench for different emulation platforms,” he added.

Different companies have different mindsets of what the API could be. Michael Young, director of product marketing for the Palladium product group at Cadence, related the situation to a software giant. “If you look at Microsoft and the API that they offer, in the beginning maybe they have 100 API calls. Now we are talking about thousands. So, can we do that? We can probably do that, but over time each API becomes a customization of a particular environment of all customers. The minimum thing that we want to give the customer is the flexibility and the knobs that they need to create the environments for the best productivity that they can achieve. What we overlay on top of that is what we call the accelerated VIP that would be a pre-cooked, off-the-shelf kind of solution for a particular protocol, so whether it is AXI, AHB or PCI Express, those things will be off the shelf. Then we would give them knobs within that.”

He added that everyone is trying to standardize this, but that’s not so easy.

“The difficulty is really more of a personality than style. Specific preferences become very difficult to resolve when you have multiple companies trying to optimize it from their perspective. Some of the customers that we talk to are looking for some transportability. I think over time the industry will mature and as we participate in the SCE-MI committee those items will try to cover that in the near term as well as the long term,” he concluded.