MOSI pins will be pulled down and the SS pin will be pulled up.
end()
Description: turn off SPI BUS communication
setBitOrder()
Description: Set transmission order
setBitOrder()
Description: Set communication clock. The clock signal is generated by the master, and the slave is not configured. But the SPI clock
frequency of the master should be within the processing speed range allowed by the slave.
9. Advanced Tutorials
9.1 How to use SD Library
SD Class
begin(cspin)
Description: Initializes the SD library and card. This begins use of the SPI bus (digital pins 11, 12, and 13 on most Arduino boards; 50, 51,
and 52 on the Mega) and the chip select pin, which defaults to the hardware SS pin (pin 10 on most Arduino boards, 53 on the Mega). Note
that even if you use a different chip select pin, the hardware SS pin must be kept as an output or the SD library functions will not work.
Parameter: cspin：the Arduino pin connected to the chip select line of the SD card.
Return: boolean type. True on success; false on failure
exists()
Description: Tests whether a file or directory exists on the SD card. Grammar: SD. exists( filename)
Parameter:
filename: the name of the file to test for existence, which can include directories (delimited by forward-slashes, /)
Return*: boolean type, true if the file or directory exists, false if not
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