Electronic – Sandisk SDHC Card Responding with Illegal Command to CMD8 by SPI

Analysis with a cheap USB logic analyzer sampling at 16MHz showed that occasionally there appeared to be only 7 clock cycles grouped together. From the hardware VHDL, I know this to be impossible - once a byte transfer is started, it will exchange bits on the following 8 clocks.

I theorized that there was noise on the SCLK signal, causing a clock that was too fast to see by the analyzer, and too fast for the computer to pick up on, but that the SD Card did actually exchange a bit on.

So I added deglitching logic to the source of the SCLK signal, and the problem has gone away.

Side note: The host computer is a retro computer running at 4MHz. a certain amount of noise on the clock probably isn't visible to most of the components in that computer, but to my modern FPGA based add-on (which can easily run designs with fMax > 50MHz) such things suddenly become an issue.

Thanks to those who have responded.

I was designing FPGA configuration and writing driver for SD-card a year or so ago, and caught very similar set of issues. My solutions to wrong responses / misbehave of SD-card was simple:

• when you end command, you do not stop clock just after command completion, you keep supplying clock for 8 or 16 spare cycles - with CS being active or inactive. I think I supply 8 when CS is still active, and another 8 after it deactivates;
• when you start command, you do the same as above - before activating CS you supply 8 spare clocks, after you activate CS you also supply 8 spare clocks and only then start the command, which is then sensed by octet's first bit being 0 (command start bit).

The clock signal is required for card to complete some its internal operations (the most obvious example is recovering from the data/command error). If your design would supply clock without interruptions you would not catch this issue.

Update: looking to your waveforms supplied I think you should have MOSI being high when CS is not activated and when command is not in progress. Look here, you may find it useful (however not sufficient) to design the SD-card interface.