Permanent data storage uses storage media that can keep their content even when powered off. The most commonly used non-volatile memory is flash memory.
Flash Memory
Almost all microcontrollers come with flash memory, either internal (integrated into the main chip) or external (as a separate chip). Flash memory is the place where your firmware is stored. That’s why the microcontroller can remember your code even when you powered it off.
Built-In
Most modern microcontrollers come with at least 4MB of flash memory. When you compile a firmware, you can partition this memory area into designated parts.
This way, you can utilize part of the memory for storing your firmware, repurpose other parts as a file system similar to a disk drive for data storage, and use yet other sections to implement features like OTA (over-the-air wireless updates where you need a storage area to hold the new firmware while the old firmware is still executing).
To store information, you have these options:
- File System: store information on a virtual disk drive that lives in your built-in flash memory. This is useful for storing large amounts of data. There are a number of different libraries that can emulate file systems in flash memory.
Whether or not you have access to a file system depends on the partitioning of your flash memory.
- EEPROM: Some microcontrollers like Arduinos come with dedicated EPROM memory which is another type of non-volatile memory. ESP32 microcontrollers - like many others - do not invest in dedicated EPROM. Instead, they utilize a small part of their flash memory as virtual EPROM. With the *EEPROM library, you can use up to 512 bytes to permanently store data. That’s useful to “remember” user options and other information that doesn’t require much space.
SD Card
The built-in flash memory has two disadvantages: its size is limited, and it cannot be removed.
If you need to export data, i.e. sensor data gathered by a device, then you can use SD Cards. They are basically just flash memory chips that expose their pins through the SD Card contacts.
Typically, you would use a cheap SD Card reader breakout board, but keep in mind that these “readers” really are just a mount for SD cards without any other logic. You could as well directly solder wires to the contacts of a SD card (which would make it hard to remove the card, though).
SD cards must be formatted in the FAT file system to be recognized by most libraries. There is no size limit, and you can safely use SD cards that have gigabytes of capacity. However, for non-technical reasons, most formatting software limits the FAT filesystem to media up to 32MB. SD cards with larger capacity can only be formatted using FAT32 or other file systems which are all incompatible.
Caveats
Flash memory is very affordable, but it is relatively slow, and it wears with each read-write-process.
That’s why flash memory is not ideal for storing high-frequency data: it may pose a bottle neck due to its slow speed, and if you store values in high frequency, your flash memory may wear out and become unusable after a few months.
The role of external SD cards as declined as most modern microcontrollers today support wireless communication and can conveniently transfer sensor data immediately to a server without the need for local storage.
FRAM (Ferroelectric RAM)
Another relatively unknown memory type is FRAM. This memory does not wear, so you can read and write data as often as you wish.
| Feature | FRAM | Flash |
|---|---|---|
| Speed | Very fast read/write speeds, no erase-before-write required | Slower, requires erase-before-write for updates |
| Write Endurance | Extremely high (billions to trillions of cycles) | Limited (10,000 to 1,000,000 cycles) |
| Data Retention | 10-100 years depending on technology | 10-20 years depending on usage and quality |
| Power Consumption | Very low power for both read and write | Higher power, especially for write and erase |
| Capacity | Lower capacity, typically in kilobytes to megabytes | Higher capacity, typically in megabytes to terabytes |
| Cost | More expensive per bit | Cheaper per bit |
| Durability | Resistant to radiation and magnetic fields | More susceptible to radiation and magnetic fields |
| Technology | Uses ferroelectric materials for data storage | Uses floating-gate transistors for data storage |
| Applications | Real-time logging, high-write-cycle applications, embedded systems | Long-term data storage, firmware, and consumer devices |
There are external FRAM breakout boards available. However, FRAM memory is much more expensive than flash memory. An external FRAM board with 8KB of storage capacity costs around €5.00. A SD Card with 8GB of storage (that’s a million times more) costs less.
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(content created Jan 07, 2025)