The figure displays SPI
interface analysis window, beneath it there is an image with work
screen area containing the timing charts for the clock channel (SCK),
select slave channel (~SS), receiving channels (MISO) and transmitting
channels (MOSI). Marker 1 is set to start byte 0xAA and marker 2 is set
to start of the following byte 0x55 of the channel MOSI.
The
upper part of SPI windows contains an image of typical timing chart of
one byte of SPI interface. Beneath it there is the panel of the main
settings incident to the analyzed interface. Before you start decoding
it is recommended to set the names of the interface channels on the
panel "Channels" of logical analyzer. If you set the names SCK, MISO
and MOSI then they automatically appear in the fields of the clock
channel, receiving channel and transmitting channel.
As is known from SPI interface specification,
there are not any special signs to determine start byte, if the data
are transmitted without pause and select slave signal is not used. The
only way to determine start byte is to start measuring before
communicating with "correct" triggering in SCK channel. As is known
from SPI interface specification, SCK channel level in the standby mode
(if no communication) may be both high and low. That's why to set
"correct" triggering it is necessary to determine SCK level in the
standby mode. To this effect it is necessary to perform the
measurements several times before a long pulse/pause appears in SCK
channel, which length has to differ sufficiently from adjacent
pulses/pause. If a long pulse is found then SCK channel level in the
standby mode is high, i.e. triggering has to go as to the falling edge
if a long pause is found then SCK channel level in the standby mode is
low and triggering has to go as to the rising edge. Before you start
measuring it may be well to turn off the analyzed device and then press
the Run button and turn on the device. If triggering is set correctly
then the start of timing charts has to concur with start of
communication. For the normal timing charts decoding it is necessary
that the sampling frequency is 3-4 times higher than the triggering
frequency.
Upon taking the timing charts it is necessary to set
the values for all interface settings correctly as without it you
cannot start the waveform decoding. The most common settings of SPI
interface are set as default. Upon setting all the parameters it is
necessary to set the marker 1 to start communication (byte).
Upon
completing all such procedures you can start decoding when pressing the
Decode button. Decoding results will be displayed in the table in
binary and hexadecimal formats. When double clicking on the selected
cell with the decoding results the marker 1 goes to start of the
corresponding byte.
It is necessary to note that in addition to
the timing charts decoding SPI window also allows generating the timing
chart on the basis of interface settings and table data (column MOSI)
for the channels SCK and MOSI, which are to be one of the logical
generator channels.
It makes sense to use the flag Auto (table field autofill) only if signal generation is required.