Electronic – Card Detect Pin of SDHC Interface

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I am working on a design with a processor which has I2C SCL line & SDHC_CD(Card Detect) pins multiplexed. I need to use I2C line. I am giving SD Card for boot code of processor. I have below doubts :-

  1. Is it mandatory to connect Card Detect pin to the Host?
  2. If I dont use SDHC_CD, Will the Processor be able to read from SD Card?
  3. If I need to use CD pin, Is there any way to use both functionalities?

Can anyone please share the link for SDHC protocol?

Best Answer

  1. Is it mandatory to connect Card Detect pin to the Host?

    • No. If you use MMC protocol (or 1 bit SD protocol), the bare minimum pins that need to be connected are just these three: CMD, CLK and DAT0. And power and ground, of course. If you use SPI protocol, then you need the Chip Select pin also.

  2. If I dont use SDHC_CD, Will the Processor be able to read from SD Card?

    • Yes. The Card Detect pin isn't actually connected to the card at all, it's a mechanical switch at the slot.

  3. If I need to use CD pin, Is there any way to use both functionalities?

    • Maybe. It depends on how the Card Detect pin is connected in your system. If it's a hard pull-down to ground when the SDHC card is connected, then it's stuck low and you can't do anything about it (except desolder it from the connector). If not, then there is some hope.

  4. Can anyone please share the link for SDHC protocol?

    • This is the public, official, simplified specification, which is freely available: https://www.sdcard.org/downloads/pls/simplified_specs/. But you might benefit more from some software examples, as the specification is tedious reading and the real life is in some parts simpler than the spec. But there are a lot of pitfalls also...
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