SD cards can be accessed with two modes. The first is the SDIO mode, which is what cameras, laptops, and other card readers use. The second mode is SPI mode. SPI is slower, but much, much simpler.

A maker developer has squeezed an Arduino onto an SD card. He was inspired by Intel’s Edison compute module and he tried to go one better. The result is the SDuino, an ATMega328P processor-based ...

When the clone starts up, it searches the SD card for a file called ‘program.hex.’ This file is sent over to the target Arduino and the new firmware is installed.