STM32 writing to SD card using FATFS sometimes takes longer than normally

Question

I am using nucleo-f401re board for my project and I want to write sensor data to SD card. I am writing data when sensor interrupts occur, and that happens at 408 Hz, so I have around 2 ms to write data. I am using SPI at max frequency to communicate with SD card reader, I am using micro SD card reader module for mounting SD card. I am only using 1 interrupt which reads sensor data and sets flag, writing is occurring in while(1). Everything seems to work fine and writing finishes before next interrupt arrives, but every 8-9 writing cycle, writing function takes way longer than normally and I lose 1 data sample:

As you can see, yellow signal is my while cycle and that short pulse is writing time, except when 1 writing cycle takes 2.34 ms. Blue is my interrupt, I must write during low state of blue and finish it before new high level arrives. Why this keep happening? As I said, this longer writing occurs every 8, sometimes 9 writing cycle. Is there a way to fix it, or this is FATFS problem? I know that this occurs during FATFS write because I debugged it, I keep my file open all the time when I am gathering data.

Answer 1

SD card are not a realtime medium. You need to buffer the writes to it.

Internally the SD card prefers full blocks. It then applies wear leveling. You have no control over this. So sometimes the writes will take longer.
If it has no free blocks it has to erase one, this takes several milliseconds when brand new. Or longer when worn out.

The standard allows a latency in hundreds of milliseconds.

Answer 2

In addition to Jeroen3's answer, here are a few things that I found helped make SD card latency more predictable:

• Buffer the data so that you always write whole blocks (usually 512 bytes, though it might be more), never partial blocks. While I initially thought that writing one block at a time would minimise latency at the cost of throughput, it turned out that writing 32 blocks at once minimized the peak latency in my application, so you might want to experiment with that value.
• Ensure the card is properly formatted. Not all tools generate a volume that aligns properly with physical sectors. Use the official SD assocation tool, or f_mkfs() from FatFs.
• Don't delete files to make space (to avoid fragmenting the filesystem), format the card instead.
• If you know the maximum file size, you can try preallocating the file (the drawback is you then need to truncate the end when you close it).

To be honest, 2 ms sounds very tight, I'm not sure if that is achievable without a multi-core microcontroller or a real-time OS.