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20. ANALYSIS OF STANDARD INTERFACES
When testing digital devices you may want to analyze a data communication channel. At first glance this trivial
task such as to take the timing charts from the necessary communication channel lines, determine the interface
type and manually decode the timing charts, does not appear very complex. But upon making some attempts to
decode the timing chart you will obviously not want to perform this mundane procedure too often. To set about
decoding it is necessary at least to become familiar with the specifications of the analyzed interface and master
its timing diagram formats. It will take long time, especially when you do not have the interface description to
hand. Upon familiarizing with the interface you will know the most interesting thing that is decoding. Generally,
for most interfaces there are some signs of byte transmission starting and ending, except for SPI, when the data
is transmitted without any delay. That's why decoding starts with the searching of signs. Upon finding, e.g. the
start transmission sign, one starts decoding the data and a lot of questions arise about particularities of the
analyzed protocol that require examining the specification of the analyzed interface, comparing the timing with
their waveforms in the specification. This, probably simple routine, can eat much of your precious time and keep
you away from more important tasks.. We can testify from our own experience that a lot of time can be lost for
interpretation of timing and decoding os an I2C (used in the video capture board). It take us three and half hours
to manually decode 48 bytes. All this was done on paper of coure subject to any error. When decoding the same
interface with a program on PC (prototype model of the logic analyzer) the whole communication cycle of 1026
bytes was analyzed in 8 minutes but it took one and a half days to write the analysis program for I2C.
It does not mean that without any preliminary preparation one can decode the data of any interface by pressing a
couple of buttons for several minutes. In any case before analyzing the selected interface it is strictly
recommended to read its specification as no program can help you without comprehension of interface operation
principles.
20.1. UART interface analysis
Picture 32: UART interface analysis window
Picture 33: Working waveform
The upper figure displays the UART interface window. Underlying you can see the work screen area containing
the timing charts for the receiving channel (RxD) and transmitting channel (TxD). Marker 1 is set to the start
bytes 0x0F and marker 2 is set to the following start byte 0xAA of the channels RxD.
The upper part of the UART window contains an image of a typical timing chart of one byte of UART interface.
Beneath it there is the main settings panel relevant to the analyzed interface. Before you start decoding it is
recommended to set the names of the interface channels on the panel “Channels” of the logic analyzer. If you set
the names to RxD and TxD they automatically appear in the fields of the receiving and transmitting channels.
As is known from the UART interface specification, the start bit that is always logical zero goes before the data
bit transmission, then the data bits are transmitted, the lower bits come first, followed by even, odd or no parity.
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