I am working with a PIC18F67K22 and am seeing some bizarre things in the SPI data stream. I am trying to interact with a FAT32 partition using FatFs and a custom built diskio.c file. I am able to recognize the FAT32 partition, but when attempting to create a file, the disk_read operation suddenly gives weird errors.

I have attached a screenshot of the logic between the PIC18 and the SD card to show the unusual data.

enter image description here

In the screenshot, you can see that I am sending CMD17 (READ_SINGLE_BLOCK) along with a 4 byte address of 0x00007D10 and a CRC7 checksum for that command.

The command response that I receive is 0xC1, followed by 0x3F, neither of which make any sense. According to the How to Use MMC/SDC section on the FatFs website, the command response's most significant bit should always be a 0, but 0xC1 has the most significant bit set to 1. In addition, why is the 0x3F coming after? That should be 0xFF.

And even if I assumed there was a bit shift here and the actual value should be 0xC13F >> 6 (0x04), that would be an illegal command. What is illegal about CMD17 with that address? Is the data address of 0x00007D10 invalid? The partition should span the entire media, except for the MBR and boot record.


Here are the sectors returned during initialization, with some annotations on the data:

Edit 2:

I've discovered this only happens if I call f_mount well before f_open. I am doing an f_mount upon insertion of the SD card to verify that the card is a proper FAT16/FAT32 partition, and then only calling f_open when the user begins to record data. If I call f_mount and f_open in rapid succession (or call f_mount with delayed mounting), then the call works fine and the command returns 0x00 and then 512 bytes of data.

  • 1
    \$\begingroup\$ You should probably post the whole init sequence up to that point. Tell us your clk rate. Look at block size and see if your cross a block boundary with that address. \$\endgroup\$
    – FRob
    Commented Jun 15, 2015 at 23:44
  • \$\begingroup\$ My clock rate is 2.66mhz the entire time. I added the dumps of the MBR and partition 1's boot record. While annotating it, I did notice some pretty bizarre things. The partition 1 boot record says there are 0 sectors per FAT, which seems odd. And the volume label is all non printable ASCII. Am I reading the wrong area? I think my endianness might have gone weird in some places. Windows recognizes the SD card just fine, so I think the card is formatted properly. \$\endgroup\$
    – kingcoyote
    Commented Jun 16, 2015 at 0:16
  • \$\begingroup\$ Is there a chip select high - chip select low transition before the screen capture? Maybe the card is still outputting disk data at the time of capture? \$\endgroup\$
    – PkP
    Commented Jul 21, 2015 at 21:50

1 Answer 1


A year has passed since the question was asked, but I decided to add to it because I was having similar issues embedding SD-card driver into FPGA. I agree with commenters to your question that you need to give information about what was happening before the failed SPI cycle, and what happens after.

Generally, while there's SD specification for the SD-cards, it is slightly puzzled and made me spend not one hour understanding what is wrong with my design.

So, let's start...

  • Initialization: there's prerequisite to have spare clock cycles with CS deactivated to make card initialize itself and understand that there's host out there. It will also get an idea which type of SPI is used.
  • Command cycle start: you will need spare clock cycles before you assert CS low to start the command, and ideally several clock cycles after you assert CS low and start putting command onto MOSI line. Several here means n*8. Those spare cycles are required to ensure card recovers from previous I/O cycle if there was a problem it in (e.g you application did not transfer whole data/status, and card still expects that host will read/send this data or status). Sparely clocking with CS high will convince card that previous operation should be cancelled.

When I made these two above properly, I did not get invalid response codes from SD-card any more. There're some other peculiarities handling SD-cards through SPI, but they should be dealt when needed, because command cycle itself, then response and data phase are relatively well explained in specification, and you will probably not have issue implementing it.


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