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Going FPGA

Since our latest redesign, I have been trying to figure out which mechanical interface we should use for our submodule interconnect. Most importantly, I have been trying to find pre-built systems that could be used as submodules, which would remove the need for custom PCB design.

After some research, I stumbled upon the Enclustra Mars Zx3 System-on-Chip module. This highly-integrated system-on-module (SoM) uses a 200 pins SO-DIMM form-factor, which is really easy to install on a backplane. It is powered by a Xilinx Zynq-7020, which provides an ARM dual-core Cortex-A9 and a Xilinx Artix-7 28 nm FPGA fabric with 85K logic cells and 108 user I/Os. In other words, you have a powerful CPU tightly integrated with a powerful FPGA, and mounted on a tiny SO-DIMM module.

The benefit of using this SoM would be to replace both the Raspberry Pi 2 and the XMOS micro-controller with a single device. It would remove the bottleneck between the Pi and the XMOS, while providing a fully integrated development environment. Prototyping would be supported by using the Mars PM3 p-ITX base board.

Of course, developing with an FPGA would be more difficult than using the XMOS. By the same token, it would simplify the hardware side of the story, while allowing for faster iterations.

Once I started down that path, I quickly realized that we should use the SO-DIMM form factor for our sub-modules as well, because it would provide support for boards of variable width. For example, the Critical Link MityDSP-L138 development kit is compatible with 7 different modules that can include an ARM CPU, a TI DSP, a Spartan FPGA, or any combination of the three.

In other words, if we were to adopt the Enclustra Mars pinout for our core fabric and the Critical Link pinout for our submodule interconnect, the hardware side of our synthesizer could be simplified to a single PCB used to mount all faceplate components and connect 9 SO-DIMM modules. All we would need to add are the ADC and DAC converters, the pair of AD75019 crosspoint switches, and our analog mixing section. And that would give us a truly amazing collection of 7 digital submodules that we could play with right away, without having to design any hardware.

Let’s go for it!

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