If your next OEM design has a requirement to protect data, you may be tempted to use an SD card’s write protect switch to serve this purpose. It seems like an obvious choice considering that the SD card is a storage device with a write protect switch on its left side. In fact, the switch is part of the SD Association specification, and all compliant SD cards are required to support this feature.
Unfortunately, the write protect switch is just a piece of plastic with no electrical connection inside the card. This even includes the more expensive industrial SD cards based on the highly reliable SLC NAND.
The purpose of this switch is to indicate to host systems whether the card’s data should or shouldn’t be overwritten. The locked position is typically indicated by a closed padlock or a down facing arrow and “Lock” label. Note that the majority of host systems with SD cards ignore the switch and will write to the card as if there was no switch at all.
To implement this switch, you’ll need to use a special SD card connector that recognizes whether the switch is enabled or disabled, and operates accordingly. If the card in write protect mode is removed from your host system and put into another host system or card reader slot which doesn’t recognize the switch, then the data is subject to be overwritten.
miniSD and microSD cards don’t have this switch as part of the specification since it wasn’t widely implemented on its full-size SD card predecessor.
The bottom line is, use caution with this feature in your system level design. The last thing you want is to inadvertently lose valuable data.
Steve Larrivee is the VP of Sales and Marketing for Cactus Technologies and has over 30 years’ experience in the data storage market, including ten years with SanDisk and five at Seagate Technology.