Editing Serial Peripheral Interface (section)

===Bit-banging the protocol===
The [[pseudocode]] below outlines a software implementation ("[[bit-banging]]") of SPI's protocol as a master with simultaneous output and input. This pseudocode is for CPHA=0 and CPOL=0, thus SCLK is pulled low before {{Overline|SS}} is activated and bits are inputted on SCLK's rising edge while bits are outputted on SCLK's falling edge.

* Initialize SCLK as low and {{Overline|SS}} as high
* Pull {{Overline|SS}} low to select the slave
* Loop for however many number of bytes to transfer:{{NoteTag|Peripherals may allow or require a particular number (or any number) of transfer bytes while selected, as specified in their datasheet.}}
** Initialize{{Code|byte_out|C}}with the next output byte to transmit
** Loop 8 times:
*** [[Left-shift operator|Left-Shift]]{{NoteTag|Left-shifts are used because SPI normally transmits the most-significant bit first. Right-shifts could instead be used to transfer least-significant bit first.}} the next output bit from{{Code|byte_out|C}}to MOSI
*** [[NOP (code)|NOP]] for the slave's [[setup time]]
*** Pull SCLK high
*** Left-Shift the next input bit from MISO into{{Code|byte_in|C}}
*** NOP for the slave's hold time
*** Pull SCLK low
** {{Code|byte_in|C}}now contains that recently-received byte and can be used as desired
* Pull {{Overline|SS}} high to unselect the slave
Bit-banging a slave's protocol is similar but different from above. An implementation might involve [[busy waiting]] for {{Overline|SS}} to fall or triggering an [[Interrupt handler|interrupt routine]] when {{Overline|SS}} falls, and then shifting in and out bits when the received SCLK changes appropriately for however long the transfer size is.