MultiMedia Card - 7 pin connector
The MultiMediaCard (MMC) is a removable solid-state flash memory card introduced in 1997 by SanDisk and Siemens/Infineon, governed by the MultiMediaCard Association (MMCA) System Specification and later ratified by JEDEC as the JESD84 series. It uses a 7-pin flat-contact serial bus and supports both a native MMC command/response protocol and an SPI compatibility mode, operating at a supply voltage of 2.7–3.6 V and a clock of 0–20 MHz in the standard specification. The card measures 32 mm × 24 mm × 1.4 mm and was widely used in early portable electronics; later revisions (MMC 4.x / MMCplus) extended the interface to 13 pins with 4-bit and 8-bit bus widths and higher clock speeds up to 52 MHz. MMC shares the same physical slot geometry as SD, making MMC cards backward-compatible in SD hosts at reduced speed.
| Pin | Signal | Type | Description |
|---|---|---|---|
| 1 | RSV | NC | No Connection |
| 2 | CMD | In/Out: OD/PP | Command/Response |
| 3 | VSS1 | Power Supply | Ground |
| 4 | VDD | Power Supply | Power Supply |
| 5 | CLK | Input | Clock |
| 6 | VSS2 | Power Supply | Ground |
| 7 | DAT[0] | In/Out: PP | Data |
Signals
- RSV — Pin 1, labelled reserved/no-connect in legacy 1-bit and SPI modes; corresponds to DAT3 in 4-bit native MMC mode and doubles as CS# (chip select) in SPI mode.
- CMD — Bidirectional command/response line; open-drain during card identification phase, push-pull during data transfer.
- VSS1 — Ground reference, supply return (pin 3).
- VDD — Card power supply; 2.7–3.6 V (standard high-voltage range) or 1.65–1.95 V (low-voltage variant defined in MMC 4.x).
- CLK — Host-generated clock; 0–20 MHz for standard MMC; up to 52 MHz in MMC 4.x high-speed mode.
- VSS2 — Second ground reference, supply return (pin 6).
- DAT[0] — Serial data line; bidirectional, push-pull; the sole data wire in 1-bit native and SPI modes.
Notes
- In SPI mode the host drives CLK, DAT[0] (MOSI/MISO), and uses DAT3/pin 1 as CS#; CMD is not used in SPI mode.
- CMD and DAT lines are open-drain during the card-identification phase and switch to push-pull for data transfer.
- The 7-pin MMC connector is a subset of the SD card’s 9-pin connector; an MMC card fits and operates in an SD slot at standard (not high-speed SD) rates.
- MMCplus and MMCmobile cards extend to 13 contacts to support 4-bit and 8-bit bus widths; these cards are not mechanically interchangeable with standard 7-pin MMC slots without an adapter.
- The low-voltage supply variant (1.65–1.95 V) requires host negotiation during initialisation; it is not backward-compatible with 3.3 V-only hosts.
References
- MultiMediaCard System Specification Version 3.31 — MultiMediaCard Association (MMCA) Technical Committee
- JEDEC Standard JESD84-B42: MultiMediaCard (MMC) Electrical Standard, High Capacity — JEDEC Solid State Technology Association / MMCA