Choosing the Best Pin Multiplexing Method

Gabe Moretti Senior Editor

S2C has recently published a white paper “Choosing the best pin multiplexing method for your multiple-FPGA partition”.  You can read the entire paper at www.s2cinc.com/resource-library/white-papers.  I think that a portion of the paper is interesting enough on its own merit to be published separately.

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/Os pins on an FPGA. The classic solution is to use a TDM (Time Domain Multiplexing) scheme that muxes two or more signals over a single wire or pin (Figure 1).

Figure 1   Signals Multiplexed with a Fast Clock

This solution is still widely employed, and coupled with the advances in FPGAs, the obstacles to constructing a multi-device prototype are greatly reduced. The latest FPGAs offer advantages such as a very high number of industry-standard I/O, integrated high-speed transceivers, and LVDS (Low Voltage Differential Signaling) signaling.

Single-ended Signals vs. LVDS

Single-ended TDM uses a single-ended signal which can transmit physical signals at a speed up to 290 MHz (Virtex UltraScale). This is determined by dividing the TDM ratio (or signal multiplexing ratio) and taking into account setup, synchronization and board delays.

With a TDM ratio of 4:1, the system clock speed will be around 17.8 MHz. If the TDM ratio is increased to 16:1, the system clock speed will drop to less than 10 MHz. From this we can see that as the TDM ratio increases, the performance drop linearly.

However, using the LVDS I/O standard supported by Xilinx FPGAs, the physical transmission data rate between FPGAs can achieve up to 1.6 Gbps. This offers tremendous advantages over single-ended transmission, even when considering that a single LVDS signal requires a pair of single-ended pins.

Figure 2    Single-Ended TDM and LVDS TDM performance with Asynchronous mode

Figure 2 shows a comparison between Single-Ended TDM and LVDS TDM using Xilinx UltraScale devices. (Note: performance for different FPGA families vary.) Performance of TDM implemented with LVDS is better than Single-Ended TDM, especially for higher TDM ratios.

Figure 3 shows another comparison of Single-ended TDM and LVDS TDM. It shows the number of physical I/O needed to accommodate a given number of virtual I/O, assuming a system speed of 11 MHz:

Figure 3 Number of physical interconnections needed for a system running at 11MHz

This shows that for a system with a clock speed of 11 MHz, if 12800 virtual connections are needed, single-ended TDM consumes 1600 physical I/O. With LVDS TDM, this number is cut in half to 800.

Given the physical I/O limitation of FPGAs, partitioning becomes easier if less physical interconnections are needed. LVDS TDM has clear advantages over traditional Single-Ended TDM.

Partitioning and Automatic TDM Insertion

Combining the technique of using asynchronous LVDS TDM with a single clock cycle design, it’s possible to create a tool that can partition a design and perform automatic TDM insertion. Ideally, such a tool would be able to:

  • Optimizes buses and match the LVDS resources in each bank considering such factors as trace lengths, matching impedances, and impedance continuity.
  • 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

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.

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