What I've been doing and progress on DMA

These last few weeks I've been working on a project for work which is basically putting the DAC-based MP3 player onto a PIC32MX250F256B chip, attaching some serial flash memory and having it attach to the PC via USB in vendor mode, which meant I had to write an app to do that too. Harmony works fine for this and you can still open up their examples and modify them to get them to work. I'm really not a fan of how Harmony splits up everything into its own little file and then needs the Harmony library too, it makes working on the project on multiple machines - which I do - a pain. Personal preference I guess.

Anyway, I'm back to working on trying to get SD card reads to work in DMA mode. With my previous code, the maximum speed I saw was about 3302kB/s, even with huge sustained block reads of 64kB. I'm happy to report that I'm currently getting a bit over 5000kB/s with 64kB sustained block reads. Please note: I am not currently working on DMA mode to get background reads working, I am doing this solely for the speed increase it provides.

I know the SD card spec is for 25MHz but everything seems to work fine at 50MHz. I'm not putting my code into hospital equipment, it's all hobbyist / personal stuff so I just want speed. There are some reliability issues that I'm working on but I hope to upload the code in a few days. Even if it doesn't work, I'll upload it and someone smarter than me can hopefully show me where I've gone wrong. Until then, good luck with the PIC32!

Update (2019-01-07): Here are some results from my tests, comparing the original SD SPI code that I got 8 years ago, my newer 32-bit enhanced buffers code and my new 8-bit enhanced buffers DMA code. The results are fantastic at bigger block sizes!

HERE BE DRAGONS

There may well still be issues with this code!

I'm uploading this code for anyone who cares to test but be warned. You have to make the following changes to your code:

• Add this line PRISS = 0x76543210; after you've set up TRIS registers, to enable the SRS registers I use in the DMA interrupt
• Set SCK as an input (in my case, SPI2 needs TRISG6 set to 1). I have no idea why this is but it refuses to work without it and caused untold hours of debugging!
• Change your read buffer declaration from unsigned char read_buffer[8192] to __attribute__((coherent)) unsigned char read_buffer[8192];
• Pray

This code is provided as is and you should fully expect it to have issues. I'm providing it because it works for me, on my own board and in my own programs and I thought someone might like to get their hands on it while I slowly write the post on DMA. Good luck! Now to write that post on DMA...

Here's the code