Electronics > Projects, Designs, and Technical Stuff
JTAG only works when scope is attached
ebastler:
--- Quote from: twam on February 01, 2020, 08:34:04 pm ---
--- Quote from: ebastler on February 01, 2020, 07:58:06 pm ---You should be able to quickly distinguish which effect is in play by connecting only a ground clip from the scope, leaving the actual probes disconnected. If that makes things work, it's due to a grounding issue which the scope works around.
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Ground of scope is/was connected all the time.
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You mean the scope ground was also connected when you had actual the scope probes disconnected? And under that condition the JTAG transmission did not work?
(I am not quite sure whether you got my point from the previous post. My proposal was to connect only the scope ground clip, and not the probes, for a test. This can be used to diagnose whether it is the added ground connection via the scope that makes JTAG work, or the capacitive loading of the signal lines.)
iMo:
Jtag likes pullups resistors on all signals (ie 4k7-10k) except TCLK which should be a pulldown.
Niklas:
Looking at the scope pictures without the probe attached to TCK shows an increase of noise on the other signals. Zooming in would probably reveal that the noise is synchronized with the edges of TCK.
The issue with both the JTAG connector and the probing setup is the return current path for the signals. There is no 0V/GND on the vertical breakout board and both the oscilloscope and the FTDI are referenced to other points on the PCBA. You would not probe on a switchmode power supply with the ground clip lead attached to your probe, but this is exactly what you have here. Essentially, the probing setup has created an invisible transformer between TCK and the other JTAG signals and the edges are coupled. By attaching the probe, the coupled signal is changed by the impedance in the probe.
There are pulldown resistors at the bottom of the PCBA next to the JTAG connector, but the values are unknown. The FTDI harness also only has one wire for the return current, so that will also make the setup more sensitive for fast edges by adding another transformer coupling between the signals.
twam:
--- Quote from: ebastler on February 01, 2020, 08:55:24 pm ---(I am not quite sure whether you got my point from the previous post. My proposal was to connect only the scope ground clip, and not the probes, for a test. This can be used to diagnose whether it is the added ground connection via the scope that makes JTAG work, or the capacitive loading of the signal lines.)
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I had the ground connect while the passive probe was not connected to TCK. I didn't do a test without any probes and only a GND connected.
--- Quote from: Niklas on February 02, 2020, 08:01:47 am ---There are pulldown resistors at the bottom of the PCBA next to the JTAG connector, but the values are unknown. The FTDI harness also only has one wire for the return current, so that will also make the setup more sensitive for fast edges by adding another transformer coupling between the signals.
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Do you have the same board? Did you notice any issues?
Also tried replacing the FT232H with a J-Link and this also works.
Niklas:
No, I don't have the same board but have seen similar symptoms in the EMC lab. In that case we saw a coupling of transients on the power supply over to a CAN bus. The fix was to twist the wires together in a 5 cm section where they were arranged similarly to your setup. The twisting reduced the loop area, hence less coupling.
The 20 pin JTAG on the JLink has 0V wires spread out between the signals in the flat cable. It also seems to have 22R series termination resistors.
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