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MMC and SD what's the diff

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Member level 1
Jul 30, 2005
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I was just wondering whether or not a piece of hardware setup to perform MMC read/writes would do the same for SD cards. I actually have a hardware setup to perform read/writes on MMC but when I plug in my SD nothing seems to happen, after a couple of card pulls and pushs I manage to get one sector to be written, according. According to the Sanddisk product manual a card operaing in SPI, the pins are pretty different, SD allocates the first 7 pins for SPI mode, these do not correspond to the 7 pins of a MMC card if the same socket is used.

sorry after reading the specs again, I see that the pin assignments do match. But i'm still unable to perform a proper read/write cycle with my SD card.

MMC (SPI) card has 1-bit interface and SD has 4-bit, so SD is faster than MMC. With SPI interface setting, SD is compatible with MMC. I guess you didn't set the SD card mode correctly. There is a control to select between SD or SPI. Check the enclosed SD spec.

I guess you're right, I'm using an SD card I got from a Canon camera , there seems to be a switch at the side of the card I didn't notice before... but even chaging the switch position doesn't get the card to work. I'll have to read through the specs before attempting this again. Thanx.

i've read through the specs (briefly) and noticed even in SPI mode, the SD card has an extra interrupt pin, a interrupt is triggered by asserting this pin to high, so I'm guessing normal operation would be to assert this pin to LOW... can anyone confirm this? If I have to assert pin 8 to LOW, then could I just tie it to GND together with pins 3 and 6, if I'm only gonna use MMC/SPI mode.

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