IRIS

SdFat Library

Authored by Andrew Y

We will be switching to the SdFat library over the standard SD library for performance reasons. The standard SD library is a wrapper for SdFat, which introduces a lot of performance overhead.

This is not an exhaustive guide. These are simply notes that can help you avoid the troubleshooting, experimentation, and profiling that led to all this info.

General Notes

  • M0’s max SPI clock frequency is 12MHz
  • M4’s max SPI clock frequency is 50MHz
    • they are pretty adamant that it cannot go higher even if you overclock the SAMD51
  • M4’s chip select pin is 10
  • SD cards are addressed in 512 byte sectors
    • keep our writes and files in multiples of 512 for best performance and least fragmentation

Files

  • Try to align file size to allocation unit size for fastest operations
  • Preallocation is Key!
    • Preallocation dedicates a contiguous block of flash to your file. This makes writes and reads much, much faster.
    • file.preAllocate(u64)
      • takes number of bytes as argument

Configuration

General Config

  1. Define your desired SPI configuration. Use SdSpiConfig(CS pin, DEDICATED_SPI, SD_SCK_MHZ(ulong)). This includes:
    • SD chip select pin
    • SPI mode
      • Dedicated (preferred)
        • SD card is only device on SPI bus
        • DEDICATED_SPI
      • Shared
        • SPI bus is shared with other devices
        • SHARED_SPI
    • SPI Clock frequency
      • M4 should be 50Mhz
      • M0 should be 12Mhz (found through experimentation)

Example

const uint8_t SD_CS_PIN = 10;
#define SPI_CLOCK SD_SCK_MHZ(50)
#define SD_CONFIG SdSpiConfig(SD_CS_PIN, DEDICATED_SPI, SPI_CLOCK)
 
sd.begin(SD_CONFIG);

When Using in Conjunction with DMAC Code

Calling sd.begin() automatically configures the DMAC’s descriptor section, so when initializing DMA descriptors and whatnot, add the following to point your descriptor and writeback sections to those created by SdFat:

descriptor_section =  reinterpret_cast<dmadesc(*)[DMAC_CH_NUM]>(DMAC->BASEADDR.reg); //point array pointer to BASEADDR defined by SD.begin
 
wrb =  reinterpret_cast<dmadesc(*)[DMAC_CH_NUM]>(DMAC->WRBADDR.reg);                 //point array pointer to WRBADDR defined by SD.begin
 
DMAC->CTRL.reg = DMAC_CTRL_DMAENABLE | DMAC_CTRL_LVLEN(0xf);

Graph View

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