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TMS320F280045: External pullup at TMS pin using XDS110 with cJTAG mode

BO
Genius 12760 points
Part Number: TMS320F280045

Customer was facing connection problems using XDS110 USB interface in cJTAG mode while working fine in normal JTAG( 4 wire) mode.
Finally he found out that the pullups (2k2 resp.4K7) at TMS pin were to low recommended in the datasheet :

Now he is just having the C2000 internal pullup for software debugging but feels uncomfortable to use this configuration for his final production design.
What is for cJTAG mode the optimal ext. pullup value for TMS pin to have reliable signal stability?
Is this an XDS110 issue and changes when moving e.g. to XDS200?

Regards, Bernd

  • 0
    Guru 30330 points
    Bernd,

    Either resistor value is fine. This would be for the unconnected JTAG case to ensure that the TMS pin is not toggling unexpectedly and messing up the MCU out in the field. We will work on clarifying the value, but a resistor in that range is generally recommended.

    As for the issue the customer is seeing on the XDS110 with the resistors populated, is the connection just unstable? We have seen some similar behavior before. I will contact you offline and we can continue the conversation.

    I will update this post with our resolution.

    -Mark
  • 0 in reply to Mark Labbato
    Guru 30330 points
    Just following up here.

    There is an issue with the XDS110 driver while using cJTAG mode and made worse with a smaller pullup value. A fix has been made to the XDS110 firmware and will be released in the May emulation pack updates.

    Regarding the value of the pullup on TMS: The pullup is placed there to keep the on-chip JTAG interface held in reset while a JTAG probe is not connected. If the value of the pullup is too large, meaning the pullup is too weak, noise could couple in and over time JTAG will come out of reset. A pullup value in a range of 2.2 k to 4.7 k should generally be sufficient.

    Thanks,
    Mark