Editing Serial Peripheral Interface (section)

=== Flow control ===
Some devices require an additional [[Flow control (data)|flow control]] signal from slave to master, indicating when data is ready. This leads to a 5-wire protocol instead of the usual 4. Such a ''ready'' or ''enable'' signal is often active-low, and needs to be enabled at key points such as after commands or between words. Without such a signal, data transfer rates may need to be slowed down significantly, or protocols may need to have dummy bytes inserted, to accommodate the worst case for the slave response time. Examples include initiating an ADC conversion, addressing the right page of flash memory, and processing enough of a command that device firmware can load the first word of the response. (Many SPI masters do not support that signal directly, and instead rely on fixed delays.)