Protocol and Interface Agnostic Universal D2D Controller for HPC and Chiplets

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Demand for die-to-die and chip-to-chip interfaces has been growing steadily in the past few years due to new applications in cloud/data centers, AI (training and edge applications), and High-Performance Computing (HPC). The demand is driven by the requirements of high throughput, low latency and low power in these applications. Advances in packaging technology are further helping the adoption of heterogeneous systems with chiplets/known-good-die(KGD) assembly to create solutions that require the evolution of a new kind of interface popularly known as "Die-to-Die" interfaces.

openfive.com, Jan. 18, 2021 – 

Interface Types: Serial vs Parallel Approach

Driven by high-performance networking applications, SerDes has been the primary choice for connecting multiple chips and dies as switches and routers are at the forefront of the bandwidth requirements. For homogenous applications where multiple dies are on the same process nodes, USR/XSR SerDes are widely used to connect and scale the performance. However, this comes with a penalty of higher latency and power for each of the SerDes links as they are primarily PAM-4 based at 56G/112G speeds. The new trend is to connect dies using wider parallel IOs. These IOs are single-ended, similar to HBM and DDR memory technologies, and usually forward the clock to eliminate using power-hungry clock and data recovery circuits. With parallel IOs one can expect almost half the latency and power as compared to SerDes interfaces. There are many consortiums and standards working on parallel IOs such as OpenHBI, BoW, AIB, etc. OpenFive is a proponent of open interfaces and is actively involved in standardization efforts.

Packaging Choices: Interposer and Organic Substrate

Depending on the overall throughput requirements and the number of signals connecting the dies, one can use the interposer (various types) or organic substrate to stitch the dies together. Silicon interposers and other technologies allow more signal density, whereas organic substrates provide cost-effective packaging options. Organic substrate options are particularly attractive to chiplets with heterogenous dies (using different process nodes). There are many new advances in this area which we will cover in a future blog.

In this blog, we will talk about OpenFive's recently launched D2D Controller. It sits on top of the D2D PHY (XSR SerDes/Parallel PHYs). The D2D Controller provides seamless connectivity to both types of interfaces.

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