Low SD write speed with Arduino Nano 33 BLE and mbed os

[Xraydylan]

The setup

I am using the Arduino Nano 33 BLE with the nrf52 and it is running on mbed os. The nrf52 is connected to an micro SD card adapter.

I have written a simple Arduino sketch with the standard SD.h library. (I did a separate test with the sdfat.h library with the same result) The sketch is writing 512 byte buffers to the SD card and logging the speed. The used SD card is a class 10 SD card.

I have repeated the same setup with the same sketch with a different board the Arduino Nano 33 IoT which does not run on mbed os with the SAMD21 processor.

The problem

When logging the speed I get drastically different results.

With the nrf52 and mbed os I am getting a write speed of approx 32-35 kBit/s speed. While the SAMD21 is able to produce a speed of over 100 kBit/s.

So far I couldn't find the problem with the nrf52 and mbed os. The arduino sketch is the same as well as the SPI clock.

I have also seen other micro-controller performances being much higher in the ballpark of 200 kBit/s as well.

So far I don't know where the problem exactly resides. I can only assume it has to do with maybe mbed os. But any help with that issue would be greatly appreciated since I need to reach write speeds of above 150kBit/s.