SPI interface cpol & cpha

[[email protected]]

Hi all,

In SPI interface if i see there are two signals cpol and cpha, which
are used for differnet data transfer formats.
What i wanted to know is why 4 different data transfer formats have
been defined???
If both the master and slave agreed upon one format that shoud be
fine right.

Any suggestions are appreciated.

Thanks in advance,
Praveen

 

[James Meyer]

Hi all,

In SPI interface if i see there are two signals cpol and cpha, which
are used for differnet data transfer formats.
What i wanted to know is why 4 different data transfer formats have
been defined???

There are four formats because that is the number of combinations
available for two variables each with two states.

If both the master and slave agreed upon one format that shoud be
fine right.

That would be fine, but boring.

The SPI "standard" was not always a standard. In the beginning, there
were two or more manufacturers making chips that used similar, but different,
serial connection schemes. In an effort to make the chips appear to be usable
between manufacturers, the SPI standard was defined. The four cpol/cpha
combinations were defined so that each manufacturer could claim conformance to
the "standard" without having to actually change their established chip
architecture.

If you thing the cpol/cpha thing is strange, take a look at the
different specifications for number of bits per "word" on the data lines.

Jim