I'm getting bad overshoot and undershoot spikes on an SD card interface. The spikes are so bad that 1/300 large (2MB) file writes fails, and it's damaging the SD cards so that a card that has been subjected to these spikes will continue to fail after it is moved to a good board.
I can knock down the spikes by adding a 33-51 ohm series resistors to the data, clk and cmd lines. With 51 ohms I have done over 50,000 large file writes without errors as long as I use a virgin SD card that hasn't been subjected to the spikes.
The boards have traces equalized for length, and one version uses 8 mil wide traces, the other uses 12 mils. Both have overshoots. One of my boards has long 4.5" traces to the card. But I'm also seeing spikes on a shorter version with only 3.2" traces to the card. There are lots of example SDIO schematics online but only 1/10 show series resistors. Application note AN5200 on SDMMC does not mention resistors.
When I watch the data lines on a scope, an initial burst of traffic has perfectly rounded corners with no overshoots, but after the init phase then the overshoots start. I'm assuming the driver increases the rise/fall times to allow higher frequencies, and that these faster transitions cause the overshoots. Is there a way to force the driver to stick with slower transitions?
The clk line has the overshoots throughout the init and file write functions.
