SD Card use with Core-Compute Module

The Meadow Core-Compute Development Kit includes an SD card add-on, and one can be connected to a Core-Compute module designs. You can use the SD card system for many tasks such as saving data to a removable card or providing configuration updates.

There is an event subscription system to monitor for external storage events in your Meadow app. And, from there, file operations can be done with the System.IO API as described in the File System docs.

Enable SD card hardware support

Before writing or reading from an SD card, we need to tell the Meadow OS that we have SD card hardware available on our project build. To do this, you will need to set the Device > SdStorageSupported configuration value to true in the meadow.config.yaml file for your project.

Device:
    # Enable if your Meadow build is using SD card hardware.
    SdStorageSupported: true

For more information about this setting, read the details in the OS & Device Configuration documentation.

Check for external storage devices

If your Meadow device might have external storage at some point while your app is running, you can find it by iterating over your device's external storage items.

var sdCardStorage = Device.PlatformOS.ExternalStorage.FirstOrDefault();

On the Core-Compute Development Kit, the SD card will be the only external storage location, which means any non-null value that comes back will be the SD card. (On platforms like Meadow.Linux, you may have several external storage devices.)

With the SD card storage identified, you can then write a file to the card using any approach you prefer. For example, you can write strings to a simple text file.

if (sdCardStorage != null)
{
    using (var file = File.CreateText(Path.Combine(sdCardStorage.Directory.FullName, "hello_meadow.txt")))
    {
        file.Write("Hello, Meadow!");
    }
}

Because the SD card can be inserted or removed, you'll want to have error handling in place to recover from errors. If your SD card is removed between finding it through ExternalStorage and writing to it, for example, your file writes may result in a DirectoryNotFoundException being thrown.

Monitor for external storage events

In addition to iterating over external storage, you can also listen for storage to be inserted or ejected. To have your app notified of external storage events, subscribe to the ExternalStorageEvent for your device.

public async override Task Initialize()
{
    // Watch for new external storage events...
    Device.PlatformOS.ExternalStorageEvent += PlatformOS_ExternalStorageEvent;

    await base.Initialize();
}

void PlatformOS_ExternalStorageEvent(IExternalStorage storage, ExternalStorageState state)
{
    // The affected storage and what happened to it are available from the event arguments.
    Resolver.Log.Info($"Storage Event: {storage.Directory.FullName} is {state}");
}

For example, when external storage such as an SD card is inserted, you can react by writing to it.

void PlatformOS_ExternalStorageEvent(IExternalStorage storage, ExternalStorageState state)
{
    if (state == ExternalStorageState.Inserted)
    {
        using (var file = File.AppendText(Path.Combine(sdCardStorage.Directory.FullName, "hello_meadow.txt")))
        {
            file.Write($"Hello, Meadow! {DateTime.Now.ToString("yyyy-MM-dd")}");
        }
    }
}

If a Meadow device were running in a remote location, local log files could be written to one of the Meadow named directories. Then, when external storage is inserted, it could copy those log files to the new storage location for analysis.

Or, if your app wants to allow configuration changes via SD card import, your Meadow app could scan for new config files and alter local configuration accordingly.