I’m using a Panda v1.2 with the SD card expansion. I am using this to prototype and wanted the SD card to be on the side like on the Domino just for physical packaging reasons, so I put down some Kapton tape and stuck the expansion board on the back (see picture.) I wired it up with some 30awg wire across the back of the board and found that it no longer booted. I did a lot of checking but couldn’t find anything until I removed the wires to the expansion board at the expansion board end and it would boot again. I thought maybe I had a bad expansion board so I switched it out but same issue. Doing more experimenting the only consistent factor I have found is that when the lines to the expansion card are low impedance the board will not boot. It doesn’t seem to go into the user code at all as I changed it to turn on the LED first thing and it fails to do that.
Also the problem is worse when I use the com port for debugging (which I will in my application.) However just removing the mode jumper doesn’t fix the issue every time. I found that I could get the system to boot by just raising the wires off the back of the board, though this sometimes proved intermittent. My only thought on why this would work is that it changes the capacitance thus raising the impedance. To examine that further I took some sheet ferrite material and placed it under the lines to raise the inductance, this makes it stable and with that setup it will boot every time.
I’m getting the board this is going into laid out over the next week or so and am hoping to have some more understanding of what’s going on so I don’t end up making boards with the same problem. I’ll be using the USBizi chip directly in the layout and not the Panda board, but am basically following the board as a reference design. I checked the lpc23xx user guide and don’t find any layout notes for the SD bus. I’m wondering why changing the debug interface would have any effect on the situation. I’m considering just removing the ground plane under the SD lines in the final layout as a precaution. Any insight would be greatly appreciated.
Thanks for your help.