I’ve finally had time to play around a bit more with the SD sniffer I designed a few weeks ago. I started placing all the active components on the board and also got to curse myself for choosing the tiny, tiny, SC70 packages for the comparators. The SC70 package is tiny and the components were quite tricky to solder, mostly because I don’t have access to a stereo microscope any more. But it all worked out in the end.

Since I have used the OpalKelly FPGA board before and have a lot of existing code for it, it didn’t take long to hack together an FPGA image that could control the D/A converter on the board to set the reference voltage for the comparators. With that done it was possible to probe the outputs from the comparators with a scope and see that the signals matched the signals on the SD bus. One of the purposes with this board was to not put too much load on the SD data lines, and this actually seems to work. I have a high speed SD card which runs at 48MHz which gets a lot of errors if I probe the CLK line with a normal 10x scope probe. With the sniffer board it works fine at 48MHz without any errors.

A little while later I could actually get the signals into the FPGA and then stream them onto my PC. The current FPGA image is rather simple, on every SD CLK edge it samples the SD CMD and SD DATA lines and stores them in a FIFO buffer inside the FPGA. The Cypress FX2 USB controller on the OpalKelly board then streams the data from the FIFO to the PC. This is basically what the USBee does, so this has the same limitation of 30MHz clock speed on the bus. So the only advantage so far is that my SD sniffer doesn’t affect the SD bus signals as much as the USBee does.

But since I have access to the FPGA on the OpalKelly board there’s a lot more that can be done. The USBee wastes a fair bit of the USB bandwidth since it always transfers 8 bits to the host for each sample. The SD bus only uses 5 bits per sample, the CMD line and four DATA lines, so 3 out of every 8 bits going over the USB bus are unused. With the FPGA it’s quite simple to pack eight 5 bit samples into five 8 bit words. So to be able to handle a 50MHz bus it needs 50*5/8 = 31.25MBit/s throughput on the USB bus which is just about what’s possible.

But first I’ll clean up the existing code a bit and then start working on some way of processing and presenting the raw SD bus data.