An Interesting White Paper from S2C | System Design Engineering

Gabe Moretti Senior Editor

S2C has published a white paper on Chip Design with the title: "Choosing the best pin multiplexing method for your Multiple-FPGA partition".  It is of particular interest to designers that use FPGA based prototyping in their development of SoC designs.

Using multiple FPGAs to prototype a large design requires solving a classic problem: the number of signals that must pass between devices is greater than the number of I/O’s pins on an FPGA. The classic solution is to use a TDM (Time Domain Multiplexing) scheme that multiplexes two or more signals over a single wire or pin.

There are two distinct types of TDM implementations: synchronous and asynchronous. In synchronous TDM the multiplexing circuitry is driven by a fast clock that is synchronous with the (user's) design clock.

In asynchronous mode, the TDM fast clock runs completely independent of the design clocks. Although asynchronous mode is slower, it supports multiple clocks and the timing constraints are easier to meet.

The paper shows that S2C's Prodigy Play Pro is a tool that provides design partitioning across multiple FPGAs, and offers automatic TDM insertion based on an asynchronous TDM using LVDS.   Prodigy Play Pro Combines the technique of using asynchronous LVDS TDM with a single clock cycle design, and can partition a design and perform automatic TDM insertion. The result is that the tool is able to:

1)   Optimize buses and match the LVDS resources in each bank considering such factors as trace lengths, matching impedances, and impedance continuity, and

2)   Avoid consuming FPGA design resources for the TDM circuity by taking advantage of built-in reference clocks (e.g.: IODELAY) to drive TDM clocks and resets.