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My Protocol Analyzer Reports Many Errors and None of the Data Readings are Correct – What Could Be the Root Cause for this Many Problems?
Rena Ayeras

Beagle at laptop trying to figure out protocol errors

Illustration from Mohamed Hassan

Question from the Customer:

We are using the Beagle I2C/SPI Protocol Analyzer and Data Center Software to decode data from the I2C EEPROM of a display monitor system that stores HDMI EDID data. We are not receiving any valid information, it is all “FF”, and errors are displayed for each captured line: MTP4, M, and P.

We have checked the following in our setup:

  • We have pull-up resistors on SCL and SDA, and power is ready.
  • We used the master to read EEPROM data and confirmed data is transferred.
  • We used the Data Center Software to open the pull-up resistor and the results remained the same.

What do all those errors indicate, and what more can we analyze to resolve them?

Response from Technical Support:

Thanks for your questions! We will go over the error messages and their causes, and recommend additional analyses for your system.

The Indication and Cause of Error Messages

TMP4 errors indicate Time out (T) errors, Middle of packet (M) errors and Partial last byte (P) errors.

Partial Errors

The errors P1, P2, P3 .... P7, displayed in the Error column in the Data Center Transaction Log, indicate Partial last byte (P) errors. Since the analyzer operates at a byte level, the P error means that the analyzer was not able to capture an entire byte. The number following the P represents how many bits of the last byte were captured. Essentially, partial last byte errors occur when the last byte in the buffer is incomplete.

In this case, the number of bits of data captured did not align to the expected data size. For example, the Beagle I2C/SPI Protocol Analyzer uses the SPI slave select line to frame each transaction. If the Beagle analyzer is seeing 1 bit and then seeing the slave select line goes inactive, then it pads the rest of the byte with 0s.

Time Out Errors

The time out (T) errors indicate that the capture for transaction timed out while waiting for additional data. The timeout used by the Data Center Software is 250ms.

The Timeout error occurs when no data was seen before the timeout interval occurred. This may indicate that no data was seen on the bus or there was a pause in the transmission of data longer than the timeout interval.

Middle of Packet Errors

The Middle of packet (M) errors indicate that data collection started in the middle of a packet.

The Middle of packet error is displayed when data collection started in the middle of a packet. This indicates that a transaction was already being transmitted across the bus when the read function was called.

For more information about transactions, go to the Transaction Window section of the Data Center Software User Manual.

Additional Causes to Analyze

Because of the extensive list of errors, we recommend analyzing your system test and design to locate. the root causes of the T, M and P issues. Corrupted signals are a likely cause when that many errors and failures occur. Here are additional items to look at:

  • The USB cable could be defective. Try using another cable and see if that resolves the problems.
  • Also, make sure you are using the shortest cable possible. A long cable length affects signal integrity, as well as the operational speed. The I2C/SPI bus does not have any inherent distance capability, and excessive cable lengths can cause corruption of the signal, as well as significant delays.
  • If electrical inconsistencies are present and distorting the data signals, using a self-powered USB hub between the system and the Beagle analyzer could solve that problem.

We hope this answers your questions. Additional resources that you may find helpful include the following:

If you want more information, feel free to contact us with your questions, or request a demo that applies to your application.