JTAG supply voltage - error in data sheets?

Technical discussions around xCORE processors (e.g. General Purpose (L/G), xCORE-USB, xCORE-Analog, xCORE-XA).
CousinItt
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Joined: Wed May 31, 2017 6:55 pm

JTAG supply voltage - error in data sheets?

Postby CousinItt » Wed Jan 03, 2018 3:11 pm

Hi all,

I've just been checking the voltages on the xcore-200 explorerkit JTAG pins with nothing connected. The TDI and TMS inputs are pulled up to about 2.5V, but the XE216 data sheet says they should be pulled to VDDIOL, which is 3V3 in the explorer kit.

However VDDIOT is 2V5, so I'm wondering if the data sheet is in error.

Do you have any other ideas that might explain the discrepancy? I was measuring with a high impedance instrument, so it's not that.

Thanks.
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mon2
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Postby mon2 » Wed Jan 03, 2018 5:22 pm

VDDIOT believe is for the Ethernet PHY interface.

It is not uncommon to have port pins which are working @ 2v5 but are 3v3 tolerant. If it is working for the XMOS kit, consider it to be safe to use. If really concerned, apply a 0 ohm resistor to allow you to steer this power rail to a 2v5 or 3v3 source.
CousinItt
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Posts: 137
Joined: Wed May 31, 2017 6:55 pm

Postby CousinItt » Wed Jan 03, 2018 8:18 pm

Hi Mon2,

and thanks. Yes I also thought that VDDIOT is used for the PHY interface, that's why I'm surprised the JTAG appears to be using the same supply.

Ignoring the absolute maximum ratings, the maximum input specified in the data sheet is 3V6 for a 3V3 supply. There's no mention of any tolerance for these voltage levels when using a 2V5 supply for VDDIOT, so I'd like to be cautious. I need to use a voltage regulator for the JTAG interface, so it's just a question of what to use. I think I'm going to stick with 2V5 for now.

I'd still like an answer to my original question. Failing that I'll have to look at the JTAG levels when using 3V3 for VDDIOT when I produce an xcore-200 board in the next month or so.
CousinItt
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Joined: Wed May 31, 2017 6:55 pm

Postby CousinItt » Wed Jan 10, 2018 4:49 pm

All JTAG signals have roughly 3V3 high levels when the XTAG is in use, so not sure why the 2V5 levels appear when it's not.

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