High Speed Printed Circuit Boards For ASIC Prototyping

Editor's note: Lars-Eric Lundgren, General Manager, Synplicity Hardware Platforms Group is guest writing this article for Ken McElvain.

Providing the highest performance and easiest to use ASIC prototyping system requires bringing together the best hardware with the best software. So when Synplicity decided to complement their software for ASIC prototyping with hardware products, the decision was made to acquire our company HARDI Electronics AB. HARDI has now become Synplicity Hardware Platforms Group (HPG). Our product is HAPS™ for short and that means High-performance ASIC Prototyping System™. At HPG we are specialized in designing high speed printed circuit boards (PCB). On these boards we always have the largest FPGAs one can get hold of. Today that is Xilinx Virtex-5 LX330. One LX330 device compares to about 2 million ASIC gates so it is quite large. Still most of our customers need a good number of the LX330 to fit their big ASIC or ASSP. Today it is not unusual for an ASIC size to be 20 million gates.

HAPS is extremely flexible and fits virtually any ASIC design, no matter how it is structured. We get this to work by having a large number of high quality, high speed connectors that are used for either FPGA interconnects or I/O. The connectors form a pattern we call HapsTrak. All special functions are put on HapsTrak compatible daughter boards. HapsTrak is also used to connect several HAPS boards together to make a prototype where only the sky is the limit concerning design size. The system with daughter boards for interconnects give a much higher flexibility than other techniques like switch matrices, etc.

After designing boards for more than 7 years, we have become quite advanced in designing PCBs. It takes a lot of experience but also very good PCB tools to get it right. This work is very time consuming and there is no way to rush the work since it would impact the quality. On the latest motherboard, HAPS-54, we have 4 LX330 and about 5400 nets with 18000 connections. Every wire is manually routed since the auto routers simply don’t make it good enough. All boards in the HAPS-50 family have 26 layers in the PCB. There are several very important issues that need to be addressed when making these high-speed boards:

1. Impedance matching of all wires
2. Length matching of all data and clock wires
3. Generation and distribution of high-speed, low skew clocks
4. Robust power supply

1. All wires on the HAPS boards are impedance matched at 50 ohms. This is ensures that there are no reflections which makes it possible to run data at very high speeds. Impedance matching is done by carefully adjusting the width of the wires to the electrical characteristics of the material used in the PCB. In addition, wires on one layer are always perpendicular to wires on the next. This is to avoid cross coupling.

2. Length matching is done on all wires to the connectors, wires between FPGAs, and also on all clock lines. This is needed when running a high speed data bus over a distance on the board. All signals should arrive at the same time at the destination with minimum skew. To minimize this skew, we make all wires to a connector the same length as the longest. This can be done to a fraction of a mm and it results in a skew less than 10ps.

3. Having high speed clocks on the board is very important. We have a large number of clocks on each board that can be distributed to all the FPGAs with very little skew. This is important since we are imitating the clock tree inside an ASIC which is very fast. On the new HAPS-50 family we also added PLLs (Phase Locked Loop) to be able to control the clocks even better than before. Customers can now connect many HAPS boards together and it is still very easy to get the same high speed clock distributed to all the FPGAs.

4. One problem we have seen with many customers’ in house built boards is the lack of a proper distribution of power. These big FPGAs consume a lot of power and have so many I/O-signals that can switch at the same time. The board has to be designed so that no matter how many signals change state, it should not impact the power supplied to anything on the board. The HAPS-54 has 15 on board voltage regulators so the only external supply needed is 5V. But you might need well over 20 amps of current.

Building a high-performance, reliable and easy to use ASIC prototyping system requires more than just mounting a number of big FPGAs on a printed circuit board. To be able to perform its task - verifying an ASIC, ASSP, or SoC design in a reliable, deterministic, and accurate way – the ASIC prototyping system must be designed and built even more thoroughly than the IC that will be verified with it. Or would you put $25 tires on your Ferrari and then try to drive 150m/h with it?

More information on HAPS can be found at http://www.synplicity.com/products/haps.

Good luck with your ASIC Prototyping!

The HAPS systems are the execution engine for the Confirma at-speed ASIC/ASSP verification platform.  More information on The Confirma platform can be found at www.synplicity.com/products/confirma.