What is it?

This is a small micro SD Card reader that is designed to mount directly onto the Butterfly STM32L433 Development Board and uses SPI to read and write to a micro SD Card. Because the STM32L4 Development Boards expose a USB/MSC (Mass Storage Device) utility to the user, the SD Card appears as a disk drive on your laptop as long as the USB cable is connected to your Butterfly. This makes downloading files to and uploading files from the Butterfly as easy as dragging and dropping icons on your pc!

The micro SD Card reader add-on is designed to mount either on the top or bottom, non-USB end of the Butterfly. You could mount it on headers so it can be used on multiple Butterfly boards or you can solder it to the top or bottom for more permanent use. If you solder to the bottom be sure to leave enough room so the micro SD card can be removed and replaced, since the SD card reader will be between the add-on board and the Butterfly. This is actually a very convenient way to mount it and, since the SD card sticks out a couple of mm from the add-on edge, it is not hard to remove it and replace it if there is some clearance between the add-on and the Butterfly.

Why did you make it?

Sometimes you just need a lot of data storage, and while SPI NOR flash is fast and easy to use it gets expensive to add more than 256 MBytes. I usually buy the 8 GByte micro SD Cards because they are cheap but these days you can find up to 64 GByte micro SD cards so it is getting pretty hard to run out of data storage memory for small MCU applications!

For long term data logging this add-on board is ideal, especially in applications where data rates are high.

What kind of data rates can you expect?

For Butterfly/Dragonfly using SPI and this SD Card Reader Add-on:

• 8 GB Kingston 2.4 MB/s Read 0.55 MB/s Write
• 32 GB Samsung EVO 2.4 MB/s Read 2.3 MB/s Write
• 32 GB Sandisk Ultra 2.4 MB/s Read 2.3 MB/s Write

Compared to using the 4-lane SDIO Card Reader for Dragonfly at Default (20 MHz) Speed:

• 8 GB Kingston 11.3 MB/s Read 5.2 MB/s Write
• 32 GB Samsung EVO 11.4 MB/s Read 10.2 MB/s Write
• 32 GB Sandisk Ultra 11.4 MB/s Read 10.3 MB/s Write

or the 4-lane SDIO Card Reader for Dragonfly at Fast (48 MHz) Speed:

• 8 GB Kingston 21 MB/s Read 5.2 MB/s Write
• 32 GB Samsung EVO 21 MB/s Read 18.3 MB/s Write
• 32 GB Sandisk Ultra 21 MB/s Read 17.6 MB/s Write

So for the fastest reads and writes, the 4-lane SDIO Card Reader for Dragonfly is recommended.

What makes it special?

This micro SD Card Reader add-on board is super small and designed to mount right onto the Butterfly Development Board resulting in a compact device with high perfromance and lots of data storage perfect for all of your portable and wearable applications.

The cool thing about Ladybug, Butterfly, and Dragonfly (and the STM32L4 in general) is that we have implemented the USB/MSC protocol in the Arduino core meaning that the SD card reader add-on can appear as a mass storage device to your laptop; select USB+mass storage in the Arduino IDE and any memory device attached to the development board will show up as a disk drive on your computer. This means you can drag and drop files onto the SD card (or SPI flash) from the laptop and vice versa and transfer data via the USB cable. What could be easier?!

Order the pcbs from OSH Park and assemble some of your own or buy the fully assembled and tested micro SD Card Reader Add-ons from me and see what having too much memory on your Butterfly can do for your applications!