Teledyne LeCroy USB2-HSIC Decoder - Manual (Page 6)

USB2/USB2-HSIC Decode and Trigger
USB Decoding
Serial Decode Technical Overview
The algorithms described here at a high level are used by all Teledyne LeCroy serial decoders sold for
oscilloscopes. They differ slightly for serial data signals that have a clock embedded in data or a clock
separate from data.
Bit-level Decoding
The first software algorithm examines the embedded clock for each message based on a default or user-
specified vertical level. Once the clock signal is extracted or known, the algorithm examines the
corresponding data signal at the predetermined vertical level to determine whether a data bit is high or low.
The default vertical level is set to 50% and is determined from a measurement of peak amplitude of the
signals acquired by the oscilloscope. It can also be set to an absolute voltage level, if desired. The algorithm
intelligently applies a hysteresis to the rising and falling edge of the serial data signal to minimize the
chance of perturbations or ringing on the edge affecting the data bit decoding.
NOTE: Although the decoding algorithm is based on a clock extraction software algorithm using a vertical
level, the results returned are the same as those from a traditional protocol analyzer using sampling point-
based decode.
Logical Decoding
After determining individual data bit values, another algorithm performs a decoding of the serial data
message after separation of the underlying data bits into logical groups specific to the protocol (Header/ID,
Data Length Codes, Data, CRC, Start Bits, Stop Bits, etc.).
Message Decoding
Finally, another algorithm applies a color overlay with annotations to the decoded waveform to mark the
transitions in the signal. Decoded message data is displayed in tabular form below the grid. Various
compaction schemes are utilized to show the data during a long acquisition (many hundreds or thousands of
serial data messages) or a short acquisition (one serial data message acquisition). In the case of the
longest acquisition, only the most important information is highlighted, whereas in the case of the shortest
acquisition, all information is displayed with additional highlighting of the complete message frame.
User Interaction
The order of your interaction with the decoder software in many ways mirrors the order of the algorithms.
You will:
l
Assign a protocol/encoding scheme, an input source, and a clock source (if necessary) to one of the
four decoder panels using the Serial Data and Decode Setup dialogs.
l
Complete the remaining dialogs required by the protocol/encoding scheme to decode Transitions, Bits
and Words.
l
Work with the decoded waveform, result table, and measurements to analyze the decode.
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