SD 카드에 CSV 쓰기

An SD card gives your device cheap, roomy storage that survives power loss. It is perfect for keeping a long history of logged readings. 💾

CSV means comma-separated values: plain text where each line is a record and commas split the fields. Spreadsheets open it instantly.

One log entry per line keeps things tidy. A row might pair a timestamp with a reading so each value has clear meaning later.

An SD card module connects over the SPI bus. You wire its MOSI, MISO, SCK, and CS pins to the matching pins on your board.

Before reading or writing you must initialize the card and tell the library which pin is chip select. If it fails, the card or wiring is off.

You open a file by name, choosing a mode. Use FILE_WRITE to add data; it creates the file if it does not exist yet.

FILE_WRITE seeks to the end, so each new line is appended. Your old entries stay safe and the history keeps growing.

You print fields and commas just like Serial output. End the row with println so the next entry starts on a fresh line.

Calling close() flushes your data and frees the file. Skip it and your last writes may never reach the card.

Write a header line once when the file is new, naming each column. Spreadsheets then label your charts automatically.

Each write costs power and time. For battery nodes, log every few seconds or minutes rather than every loop to keep the card happy.

What happens if you forget to close the file after writing?

Wire an SD module over SPI, open a CSV file in append mode, print comma-separated rows, and always close it. Now your readings are saved for good. ✅