Device-locked designs for Nengo FPGA on the PYNQ-Z1

Apr 25, 2019, Ben Morcos

ABR’s Nengo FPGA project recently matured to the point of being a commercial product for sale online. As part of commercialization, we had to address the issue of how to protect our IP from unauthorized distribution. Many standard methods to protect FPGA designs, known as bitstreams, rely on control of the physical device. They work by preloading secret encryption keys or locking out access to read or configure certain parts of a device. In our case, however, our distribution model is more of the bring-your-own-board flavour, which makes such standard protection methods infeasible.

Instead, we lock our bitstreams to a specific device using a read-only, unique identifier that Xilinx calls the Device DNA. Before Nengo FPGA begins execution on the FPGA we check this Device DNA and compare it to a reference value compiled into the bitstream itself. With this DNA-lock, if we find a valid DNA then it’s business as usual, but a mismatch raises an error. So even if our bitstreams are replicated and distributed, without an authorized DNA they won’t be of much use.

In order to use this DNA-locked system we need access to the Device DNA so we can program the reference value into the bitstream. The PYNQ-Z1 board that we use with Nengo FPGA is a system on a chip (SoC) device, which means it has both an ARM processing system (PS) and programmable logic (PL) on the same chip. Having the PS available is very convenient for integration since it runs a Linux operating system and can provide an easy interface to both Nengo and the PL but, for better or for worse, the Device DNA is only accessible from PL and not from the PS. This means we have a few hoops to jump through in order to read this Device DNA value. One of the main goals of Nengo FPGA is to make the FPGAs easy to use for the end user so, to make this as simple as possible, we’ve added a script in the Nengo FPGA frontend. This dna_extractor script makes reading the Device DNA a simple one-line command from your PC!

Simple for the user usually implies a little more work on the development side though, so let’s take a look at what’s really going on here. When you run the script from your PC it will open up a connection with the PS on the PYNQ device. Using the PYNQ API from Xilinx, the PS now loads the dna_extractor bitstream onto the PL, which in turn reads the Device DNA registers and puts that value into PS accessible memory. The PS can now read the Device DNA and send it back to your PC – easy! We want to reuse the same mechanism to read the Device DNA for both the DNA-lock implementation of our design and the dna_extractor script that reads the DNA for the user so we decided to tack on an AXI interface and package up a DNA extractor as an independent IP block that can be easily dropped into any design. I was surprised this function didn’t exist in a library somewhere (the HDL primitive exists, but nothing plug-and-play) so we decided to make this block public for anybody that is looking at using Device DNA with Zynq-7000 devices!

The Zynq AXI DNA IP block, along with source code and an example implementation, is publicly available on GitHub.

For more information about Nengo FPGA check out the Brain Board section of our website or get in touch!