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MicroSD Card Module with SPI Connection


A MicroSD card can be directly connected to a 3.3v microcontroller without the need of any additional module. However, a cheap module such as shown in the picture above can be handy in terms of wiring and card replacement. Yet, if you get the module from China then you are on your own, you have to figure it out yourself how the module works.

The module above needs 5v VCC to operate. The 5v is needed by the 3.3v LM1117 regulator to power the MicroSD card and the 74125 bus buffer (the 14-pin IC in the module above). Hence, the bus buffer is operating at 3.3v. The VCC is connected directly to the input of the regulator and no where else. Thus, the VCC is solely meant for the regulator. The right-side SPI bus (CS, SCK, MOSI & MISO) is connected directly to the 3.3v bus buffer. No line in the SPI bus that is connected to the VCC. Thus, the SPI bus does not follow VCC. The buffer is 5v tolerant so the right-side SPI connection can be driven at either 3.3v or 5v. Hence, the SPI can be connected directly to a 3.3v or 5v microcontroller.

However, if your design has only 3.3v power line then you can just connect the input and the output of the regulator together as shown by the green line in the picture above and apply 3.3v to the VCC. That is bypassing the regulator. You may want to remove the regulator all together but not necessary.

The module was designed for 5v operation. The bus buffer is used as a level shifter between a 5v microcontroller and the 3.3v MicroSD card.

You may want to read Connecting a Micro SD Card Adapter to the ESP8266 Development Board.

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