A microSD card module exposes the SPI interface (MOSI, MISO, SCK, CS) to interface with standard SD/SDHC cards. FAT32 formatting allows cross-platform file access. Typical use is data logging — writing sensor readings to timestamped CSV files.
| Interface | SPI (Mode 0) |
| Voltage | 3.3V (with 5V adapter) |
| Max speed | ~2 MB/s (SPI) |
| Format | FAT32 |
| Card support | SD / SDHC / SDXC |
| CS pin | Active-low chip select |
Initialisation negotiates SPI clock speed and card capacity. Writes use FAT32 cluster chains — each write finds a free cluster, updates the FAT, and writes the data sector. Buffering writes (flush every N records) dramatically improves throughput and reduces wear.
FAT32 has 4GB file size limit. SPI mode is slower than SDIO. Frequent small writes cause file system fragmentation. Sudden power loss can corrupt the FAT — always unmount before removing. SD cards have finite write endurance (~100k P/E cycles per cell).