19. Multimedia Card Interface (MCI)

The Stamp9G20 features a onboard Micro-SD-Card slot, which is connected to the MCI-A interface of the microcontroller. Please note that this is the same slot used on the Evaluation Board, so only one interface can be used at a time. Also operating systems like Linux do not necessarily support all features of the hardware unit.

The MultiMedia Card Interface (MCI) supports the MultiMedia Card (MMC) Specification V3.11, the SDIO Specification V1.1 and the SD Memory Card Specification V1.0.

The MCI includes a command register, response registers, data registers, timeout counters and error detection logic that automatically handle the transmission of commands and, when required, the reception of the associated responses and data with a limited processor overhead. The MCI supports stream, block and multi-block data read and write, and is compatible with the Peripheral DMA Controller (PDC) channels, minimizing processor intervention for large buffer transfers.

The MCI operates at a rate of up to Master Clock divided by 2 and supports the interfacing of 2 slot(s). Each slot may be used to interface with a MultiMediaCard bus (up to 30 Cards) or with a SD Memory Card. Only one slot can be selected at a time (slots are multiplexed). A bit field in the SD Card Register performs this selection.

The SD Memory Card communication is based on a 9-pin interface (clock, command, four data and three power lines) and the MultiMedia Card on a 7-pin interface (clock, command, one data, three power lines and one reserved for future use). The SD Memory Card interface also supports MultiMedia Card operations. The main differences between SD and MultiMedia Cards are the initialization process and the bus topology.