In card mode, seven, nine or thirteen pins on the SD/MMC device are used, with the functions listed in the table below. All cards start up in “1 bit” mode (upon entering identification mode), which involves only a single data line. Once the host (the MCU/MPU) discovers the capabilities of the card, it may initiate 4- or 8-bit communication (the latter available only on new MMCs). Some card holders contain circuitry for card detect and write protect indicators, which the MCU/MPU may also monitor.
Pin | Name | Type | Description |
1 | CD/DAT3 | I/O | Card Detect/Data Line (Bit 3) |
2 | CMD | I/O | Command/Response |
3 | Vss1 | S | Supply voltage ground |
4 | VDD | S | Supply voltage |
5 | CLK | I | Clock |
6 | VSS2 | S | Supply voltage ground |
7 | DAT0 | I/O | Data Line (Bit 0) |
8 | DAT1 | I/O | Data Line (Bit 1) |
9 | DAT2 | I/O | Data Line (Bit 2) |
10 | DAT4 | I/O | Data Line (Bit 4)* |
11 | DAT5 | I/O | Data Line (Bit 5)* |
12 | DAT6 | I/O | Data Line (Bit 6)* |
13 | DAT7 | I/O | Data Line (Bit 7)* |
Table 14-2 SD/MMC pinout (Card mode)
*Only present in MMC cards.
Exchanges between the host and card begin with a command (sent by the host on the CMD line), often followed by a response from the card (also on the CMD line); finally, one or more blocks data may be sent in one direction (on the data line(s)), each appended with a CRC.
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