Step 2: Storage with MicroSD Card
SD and MMC cards are easy to use because they provide a SPI (serial peripheral interface) interface that can be used to read and write data to and from the card. Please read the following resources to understand SPI and the SD card:
AT90USB1286 Datasheet section 17 http://www.atmel.com/dyn/products/product_card.asp?part_id=3874
If you don't read the above three links, you will not know what I'm talking about next.
In short, the SPI bus is a bus where you place data onto the data lines (MISO and MOSI) one bit at a time, and the bit is sampled on the edge of a clock signal.
Our microcontroller has a dedicated SPI peripheral. By examining the above links I provided, we know the following facts:
Our microcontroller is the "master" and the SD card is the "slave"
The SD card uses SPI mode 0 (CPHA=0, CPOL=0), this means the clock signal starts low and the data input samples data when the clock transition to high
The maximum clock speed of the SPI bus
From the above information, we are able to initialize the dedicated SPI peripheral within the microcontroller. Refer to section 17 of the AT90USB1286 datasheet.
If you didn't work out the obvious electrical connections you will need, here's an explaination:
MOSI (Master Out Slave In)
The master refers to the device that generates the clock (the microcontroller), the SD card is the slave. Data on this pin travels from the microcontroller to the SD card. Also known as "DI".
Connect the DI pin on the SD card to the microcontroller's MOSI pin
MISO (Master In Slave Out)
Data on this pin travels from the SD card to the microcontroller. Also known as "DO".
Connect the DO pin on the SD card to the microcontroller's MISO pin
Chip select, the SD card pays attention to the data traveling on the SPI bus when this pin is low, and ignores the data on the bus when this pin is high. This is also known as "SS".
The CS pin on the SD card can be connected to any free pin on the microcontroller
SCK or SCLK or CLK
This is the serial clock pin,
Connect this pin on the SD card and microcontroller's SPI clock in (called SCK in the datasheet)
The next step will take you through a step-by-step that shows you the basics of communicating to a SD card. As preparation, if you do not already have a good MicroSD card holder, then take some male pin headers with 0.1" spacing, and solder it to a MicroSD card adapter, as shown in the pictures below. The steps to make this makeshift card holder is in my appendix.