What is it?

This is an Micro SD Card reader add-on board for the Dragonfly STM32L476 Development Board to take advantage of the 4-bit (quad) read/write capability of the STM32L476 MCU.

Since all STM32L4 have embedded USB controllers we are using the USB-MSC protocol to allow memory devices like this SDIO Card Reader to appear on the laptop as a disk drive. This means easy-to-use drag and drop transfer of files to and from the pc to the SD card.

Why did you make it?

It can be very useful to have lots of data storage available. The Dragonfly has an on-board QSPI 16 MByte low-power SPI NOR flash, which also can appear as a disk drive on the pc. But when 16 MBytes is not enough, then an SD card reader is the next best thing. The two downsides of SD Cards are the high power usage and slow speed. Here the speed is addressed by taking advantage of the four data pipes embedded in the STM32L476 for quad SDIO. This significantly speeds up reads and writes compared to (1-bit) data transfers with conventional SPI-based SD Card readers. The power usage can still be high, so there are some options to choose speed and data width (1- or 4-bit) in the SDIO API. Some SD cards require more power than others; some brands require up to 200 mA at the fastest speeds and cannot be supported by the 150 mA LDO on this board.

The SDIO API takes advantage of CRC error checking as well as transmission error detection and correction to improve robustness and stability. The code is smart enough to test current requirements and drop to the next level of power if 100 mA might be exceeded. We've tested a host of SD card brands and haven't found any that don't work at some power level.

What kind of data rates can you expect?

For Butterfly/Dragonfly using a standard SPI 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 this 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 this 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, this 4-lane SDIO Card Reader for Dragonfly is recommended.

What makes it special?

The Micro SDIO Card Reader add-on is specifically designed for Dragonfly and as far as we know will not work for any other development board out there. If you would rather have a more general-purpose Micro SD card reader that just uses SPI then this add-on board is a better choice since it will work on the Butterfly STM32L433 Development Board as well as Teensy 3.2, and others.

Order some pcbs from OSH Park and assemble some of your own, or buy the fully assembled and tested Micro SD Card reader from me and see how fast and easy using SD Cards in your project can be!