SD card pin arrangement and bus read and write methods

SD card pin definition: SD card pin function details:
  





 

Pin Number	SD Mode			SPI Mode
	name	type	describe	name	type	describe
1	CD/DAT3	IO or PP	Card detection/ data line 3	#CS	I	Chip Select
2	CMD	PP	Command/ Response	DI	I	data input
3	VSS1	S	Power Ground	VSS	S	Power Ground
4	VDD	S	power supply	VDD	S	power supply
5	CLK	I	clock	SCLK	I	clock
6	VSS2	S	Power Ground	VSS2	S	Power Ground
7	DAT0	IO or PP	Data Line 0	DO	O or PP	Data Output
8	DAT1	IO or PP	Data line 1	RSV
9	DAT2	IO or PP	Data line 2	RSV

 

                  Note: S: power supply  I: input O: output with push-pull drive
                      PP: input and output with push-pull drive
SD card supports two bus modes:
SD mode and SPI mode. SD mode is the standard read and write mode of SD card, but when using SD mode, it is often necessary to select an MCU with an SD card controller interface, or an additional SD card control unit must be added to support the reading and writing of SD card. However, many 51 microcontrollers do not have an integrated SD card controller interface. If SD mode communication is selected, the hardware cost of the product will be increased invisibly. In the case where the SD card data reading and writing time requirements are not very strict, the selection of SPI mode can be said to be the best solution. I used software to simulate the SPI bus timing to read and write SD card.
Among them, the SD mode uses a 6-wire system, using CLK, CMD, DAT0~DAT3 for data communication.
The SPI mode uses a 4-wire system, using CS, CLK, DataIn, and DataOut for data communication. The data transmission speed in SD mode is faster than that in SPI mode. When using a microcontroller to read and write SD card, SPI mode is generally used. Different initialization methods can make the SD card work in SD mode or SPI mode. Here we only introduce its SPI mode.
The connection diagram between the SD card and the microcontroller in SPI mode: The operating voltage range of the SD card is 2.0-3.6V . The logic level of the IO of the SD card is 3.3V. The above diagram is only suitable for the IO logic level of the microcontroller is 3.3V. If we use a 5V MCU, we have to perform level conversion. (The following method is a safer approach, and some people use resistor voltage division) There are two main principles to solve the level compatibility problem of the logic device interface: First, the minimum voltage value of the output level device outputting a high level should be greater than the minimum voltage value recognized as a high level by the receiving level device; second, the maximum voltage value of the output level device outputting a low level should be less than the maximum voltage value recognized as a low level by the receiving level device. Considering that the SD card is in the SPI protocol working mode, the communication is unidirectional, so when the microcontroller transmits data to the SD card, the transistor plus pull-up resistor method is used, and when the SD card transmits data to the microcontroller, it can be directly connected, because the level between them just meets the above-mentioned level compatibility principle, which is both economical and practical. In this way, the hardware connection is completed, pay attention to the working voltage of the SD card