When I use Boundary Scan tools like EXTEST to verify MPC8308 connectivity ,I can't get valid data from the TDO pin of the MPC8308.Why ?

[wenshank]

I can use the IDCODE instruction get the right IDCODE number and the BYPASS instruction also work ,but when I use the EXTEST intruction test the MPC8308 connectivity ,I can't get right data from TDO. The BOUNDARY_LENGTH of MPC8308 is 491.The data first out  is 314 bit which I got from TDO ,and the rest are all zero,when I use the EXTEST instruction test the MPC8308.

[Pavel · Accepted solution]

Check please connection of the MPC8308 Y23 pin. This pin cannot be floated. If this pin is floated, the boundary scan error is possible.

Have a great day,
Pavel Chubakov

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[Pavel]

1. 1. Monitor the /TRST pin with an oscilloscope and verify that this pin is not accidently reset when the "EXTEST" command occurs. Search for transitions anywhere within a TCK cycle, not just on a TCK edge. JTAG is slow enough that /TRST can be accidently asserted in the middle of a TCK cycle. If this happens, the device next in the chain could be latching incorect data.
   2. Check connection of the TST_MODE pin.
   3. Verify that there is no contention between the MPC8308 and the next device on the chain. (Check for pull-ups and pull-downs on the pins.)
2. 4. Verify that the correct version of the BSDL file is being used for your device and the next device in the chain.
   Generally, if HIGH_Z works but EXTEST does not, it suggests that the I/O drivers on our device may not be getting enough power. If the BSDL tester can scan data in and out of our device to get the correct chain length, it then suggests that the JTAG logic has valid OVDD. However, the power for the I/O drivers still needs to be verified.

Have a great day,
Pavel Chubakov

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Note: If this post answers your question, please click the Correct Answer button. Thank you!
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