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General
SD Cards
- SD cards, or Secure Digital Memory Cards, are small solid-state cards that store data, which means they have no moving parts, so they are durable, since there are no moving parts to break
- They work using flash memory, which is another way of describing electronically stored memory which is stored separate from the main computer
- When the card is given power, the electrons in the circuits of the card are moved to the flash memory chip, where the digital patterns stored correspond to the data stored
- A transistor works to either switch electrical power on or off, or amplify it. They are found in the circuitry of SD cards
- Transistors have three pieces: a source, a drain and a gate. Electricity flows through the source, goes through the drain and then out the gate. When the gate is closed, no electricity goes through it, and it stores a value of 0. When electricity is let through, the transistor stores a value of 1. When the card is turned off, i.e. no electricity flows through it, all the transistors are turned off, and when it is turned on again, there is no way of knowing which ones previously stored values of 1 or 0, so the data is lost. The transistors in an SD card go around this problem by having two gates. When electricity flows through a transistor, some of it leaks in between the first and second gates, recording a value of one. Even when the power is turned off, the transistor maintains its value of one.
- To erase the data on an SD card, a higher voltage must be applied, which allows for re-writing the contents of the SD card
- SD cards can erase entire sections, or blocks, which makes them more efficient than other ways of storing data which may erase data one byte at a time
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SPI and SD Cards: http://nerdclub-uk.blogspot.ca/2012/11/how-spi-works-with-sd-card.html
Our implementation:
Initialisation
- We're only going to use SDHC cards (high capacity) so we will follow this chart for initialisation: http://elm-chan.org/docs/mmc/ima/sdinit.png [following the path that leads to SD Ver2+(Block address)]
