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Back of Multi Media Card. By Andrzej Barabasz.
If you look up in the MMC card specification, you'll find out that these are what the 7 pins are for. (SD/high speed cards may have extra pads, but that standard was added later because the existing standard was considered too slow for big megapixel cameras)
Pin # | Pin Name | Pin Function |
---|---|---|
1 | #CS | Chip Select (Activate) |
2 | SI | Serial In |
3 | GND | Ground |
4 | VCC | Power |
5 | SCK | |
6 | Not connected | Not connected |
7 | SO | Serial Out |
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The pinout is reproduced here
The chip has 20 pins. Look carefully and you'll notice 6 of these pins are equivalent to the SD/MMC card pins. There is a power pin (VCC) and a ground pin (GND), there is also an SCK pin (its called UCSK for some reason) a Serial Out pin (MISO - Master In Serial Out ) and a Serial In pin (MOSI - Master Out Serial In). The only thing is doesnt have is a /ChipSelect pin. Instead it has a /RESET pin, which acts in a similar way. When the /RESET pin is at a postive voltage (the same voltage as VCC) the chip runs the program, when the /RESET pin is at ground (the same voltage as GND) then the chip stops and listens for programming instructions on the programming pins.
All SD cards have the same fingers in the same place so that when you slide it into the card reader, the right pads are connected. AVR programming works in a similar way. Instead of pads, the chip is often placed in a circuit board which has header pins that the progammer plugs into, the header pinout is standardized so that any programmer can be used once the header is wired up correctly.
There are two standards for AVR in-system programming:
The left is the 6-pin header standard, the right is the 10 pin standard, the headers look like this:
Here is an example photo showing what it looks like when the chip is wired up with a 6-pin header
Photo courtesy EvilMadScientistLabs