Every sensor will vary and need calibration. Some sensors have a 'range' for their error output, that helps us know if the sensor is functioning - but we still need to calibrate every sensor/board. One of the challenges with per-device-calibration is how to store that calibration
We could put the calibrations at the top of the Arduino sketch but we would have to re-compile it for each device, and its also easy to forget or lose the calibration.
So, instead, we try to store the calibration in non-volatile memory - that means either EEPROM or external FLASH. We try to use external flash not internal flash, because internal flash can be erased and overwritten when we upload new code.
There's lots of different ways you could save calibrations - but to make things easiest we're going to stick to those two techniques. However, that means our board has to have some sort of NVM available! Here's what you can expect:
Some chips have EEPROM memory, this is a tiny amount of memory, that is separate from FLASH, where you can tuck maybe 256 bytes of data. There's two kinds of EEPROM, 'real' and 'simulated'. Real is, well, real! There's literally a chunk of memory inside the chip itself, and you'll see it mentioned in the datasheet as EEPROM.
The ATmega328 datasheet indicates that you can get up to 1KB of EEPROM (depends on which model of the chip you buy)
Its not terribly common to see EEPROM in chips as you get to the more expensive models. Partially that's because they tend to have SPI Flash chips, but also cause they aren't that useful with larger chips. Either way, simulated EEPROM is when some internal SPI flash or external memory storage is dual-used as EEPROM. For example, the ESP8266 uses the very end of the SPI flash chip that holds the program memory for EEPROM.
Some chips with real EEPROM:
- ATmega328 (Arduino Uno/Metro/compatibles)
- ATmega32u4 (Arduino Leonardo/compatibles)
- ATmega2560 (Arduino Mega/compatibles)
Some chips with simulated EEPROM:
This is a separate chip used to store files, code, audio, graphics, etc. A lot of Adafruit products have SPI/QSPI memory in order to support CircuitPython. For example:
- All Adafruit nRF52840 boards
- All Adafruit SAMD51 ("M4") boards
- Many SAMD21 ("M0") boards (they'll often have Express in the name)
The reason we prefer external memory is that you can edit the file using any text editor, when CircuitPython (or another mass-storage enabled sketch) is loaded. Also, many of the above do not have EEPROM, so there's often not a choice.
The only thing to watch out for is that the SPI FLASH must be formatted with a filesystem - much like an SD card (altho, these days, SD cards come pre-formatted). A blank SPI Flash chip doesn't have anything on it.