Re-configuring RISC-V Post-Silicon

How do you reconfigure system characteristics? The answer to that question is well established – through software. Make the underlying hardware general enough and use platform software to update behaviors and tweak hardware configuration registers. This simple fact drove the explosion of embedded processors everywhere and works very well in most cases, but not all. Software provides flexibility at the expense of performance and power, which can be a real problem in constrained IoT applications. Consider tight-loop encryption/decryption for example. Meeting performance and power goals requires acceleration for such functions, which seems to require custom silicon. But that option only makes sense for high-volume applications. Is there a better option, allowing for a high-volume platform which can offer ISA extensibility for acceleration post-silicon? This is what Menta and Codasip are offering.

Step 1: First build a RISC-V core for your design

This step looks like any RISC-V instantiation though here you use a Codasip core, for reasons you’ll understand shortly. Codasip provides a range of directly customizable RISC-V cores which a semi supplier might choose to optimize to a specific yet broad market objective. The tool suite (Codasip Studio) offers all the features you would expect to get in support of such a core, including generating an SDK and the ability to hardwire customize the ISA. (Here, by “hardwire” I mean extensions built directly into the silicon implementation.) Codasip Studio also provides tools to explore architecture options and generate a custom compiler.

The hardware implementation of custom instructions is through an HDL block in parallel to the regular datapath, as is common in these cases. The HDL for this block is defined by the integrator to implement custom instructions, a byte-swap for example. Codasip Studio takes care of vectoring execution to the HDL rather than the ALU as needed, also connecting appropriate register accesses.

Step 2: Add an eFPGA block in the datapth

So far, this is just regular RISC-V customization. Extending customization options to post-silicon requires reprogrammable logic, such as that offered by Menta. Their technology is standard cell based and is claimed portable to any process technology, making it readily embeddable in most SoC platforms. You can start to see how such a RISC-V core could host not only hardwired extensions but also programmable extensions.

This needs involvement from Codasip Studio (CS) at 2 stages. First, you as an SoC integrator must tell the system that you plan to add ISA customization after manufacture. This instructs CS to embed an unprogrammed eFPGA IP into the datapath.

Second, when silicon is available, you (or perhaps your customer?) will re-run CS to define added ISA instructions, along with RTL to implement those instructions. This will generate a revised compiler and SDK, plus a bitstream to program the eFPGA. Voilà – you have a post-silicon customized RISC-V core!

Post-silicon ISA customization

To recap, this partnership between Codasip and Menta offer the ability to not only customize RISC-V cores pre-silicon but also post-silicon, enabling an SoC vendor to deliver products which can be optimized to multiple applications with potential for high volume appeal. You can learn more in this white paper.

Codasip is based in Europe but has customers worldwide, including Rambus, Microsemi, Mythic, Mobileye and others. Menta is also based in Europe and have particular strengths in secure, defense and space applications. As a technologist with roots in the UK, it’s nice to see yet more successful growth in European IP 😊.

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