SD/MMC Cards

SD and MMC cards are exposed through TinyCLR's File System library, with the FAT16/FAT32 file system handled by the built-in driver. SD cards over the built-in SDIO interface use one StorageController name; SPI-attached SD cards go through the ManagedFileSystem software utility driver.

NuGet packages: GHIElectronics.TinyCLR.IO, GHIElectronics.TinyCLR.Devices.Storage, and GHIElectronics.TinyCLR.Pins.

Mounting an SD card

using GHIElectronics.TinyCLR.Devices.Storage;
using GHIElectronics.TinyCLR.IO;
using GHIElectronics.TinyCLR.Pins;

var sd = StorageController.FromName(FEZBit.StorageController.SdCard);
var drive = FileSystem.Mount(sd.Hdc);

Once mounted, drive.Name gives you the root path (something like A:\) — pass it to DirectoryInfo, FileStream, and the rest of System.IO to read and write files.

Reading and writing files

See the File System page for a complete read/write example. The short version:

using System.IO;
using System.Text;
using GHIElectronics.TinyCLR.IO;

var file = new FileStream(drive.Name + "log.txt", FileMode.OpenOrCreate);
file.Write(Encoding.UTF8.GetBytes("hello"), 0, 5);
file.Flush();

FileSystem.Flush(sd.Hdc);       // Flush the file-system buffers to the card.

tip

Always call FileSystem.Flush(sd.Hdc) before removing power or pulling the card — otherwise pending writes can be lost.

SPI-attached SD cards

When the SD card is wired over SPI instead of the native SDIO interface, use the ManagedFileSystem software utility driver to bridge the SPI bus to the file-system layer.

The same FAT16/FAT32 file-system code from the File System page works on top of it — only the controller setup differs. Check the driver's source in the TinyCLR-Drivers repo for the current API.

Advanced: tuning the SD clock speed

The default SDMMC clock runs at 25 MHz. Some board designs or low-cost SD cards can't keep up cleanly at that speed and need a slower clock for reliable operation. The clock divider is exposed through a Marshal write to a TinyCLR-internal configuration address:

using System;
using System.Runtime.InteropServices;

const int SD_CLOCK_ADDRESS_REG = 0x00000004;

// Divider: 0,1,2,3 → 25 MHz; 4 → 12 MHz; 5 → 10 MHz; 6 → 8 MHz; ...
var clockDivider = 4;

Marshal.WriteInt32((IntPtr)SD_CLOCK_ADDRESS_REG, clockDivider);

Use this only if you're seeing read/write errors that go away at lower clock speeds. Most boards run reliably at the default and don't need tuning.

API reference

Namespace	Description
GHIElectronics.TinyCLR.Devices.Storage	Storage controller for mounting SD cards
GHIElectronics.TinyCLR.IO	File system I/O classes