sdcard.py fixes for SDXC cards

[multicoder9]

This is my first post to GitHub, but I've been working with micropython for about 10 years.

Recently, I have been writing some micropython based sdcard dataloggers for a CANbus project, mostly with rp2040 and rp2350 devices. I have been using the sdcard.py source file as a driver in my projects.

Today I noticed that switching from my 32GB SanDisk Ultra SDHC which worked just fine to a 32GB SanDisk Ultra SDXC caused it to fail on initialization. Others have noticed the same thing caused by missing CRC7 such as issue #12687. But none of them resulted in the fix getting merged back upstream. Here is a trivial diff for sdcard.py which adds the CRC7 values needed to get it working with my SDXC card, while still appearing to work on the older cards I've tested it on. Just for completeness, I've added a commented out minimalist Python function to compute these CRC7 values. (A quick internet search didn't pull up any Python functions for this.)

Here is a simple diff against today's git master branch:
sdcard.py.diff.zip

I figured I'd try to get the minimal changes merged upstream while more significant rewrites like the one in the linked thread above work their way through the process. Is this something I should do a pull request for?

[multicoder9]

I tried creating my first pull request for this here. Please let me know if I need to change anything.

[robert-hh]

See also the discussion about SDXC SD cards here: https://github.com/orgs/micropython/discussions/12687
It has a nice fast but memory consuming CRC7 implementation.

[scruss]

The last PR about this got an initial review, but it's languished with no activity for over a year:
SDcard sdcard.py rewrite for efficiency and function by mendenm · Pull Request #765 · micropython/micropython-lib

[multicoder9]

Yes, that was the reason I hoped to get this one merged in the meantime. It is a "light touch" pull request. I did nothing more than change the incorrect 0 valued CRC7 parameters to the correct constant value. Doing just that got sdcard.py working for my SDXC card. The CRC7 implementation I wrote up (and commented out) is definitely not fast. But I ran it on my desktop to compute the constant values and hard coded them. No need to run CRC's at all at runtime at all based on my tests. Thus, I haven't significantly changed the current code structure at all.

The only dynamic messages that can't be computed at runtime were the read and write blocks commands (since the block number is specified at runtime). But based on a hint on a forum somewhere, I tried hardcoding 0x7F for these CRC's and the card was happy with it.

This pull request doesn't intend to improve the sdcard.py library structurally. Just make the minimal changes to get it working with modern SD cards. I literally just changed 10 CRC parameters from 0 to a working value and one from 0xFF to a working value, and now it solves the "it doesn't initialize" issue.

By the way, I'm not sure I correctly linked the pull request to this issue #. Any suggestions would appreciated.

[multicoder9]

I will close this to move the conversation to the PR request: Add CRC7 values for SDCard.cmd() function calls #992