what circuit should I use for the interface between 3.3v LVTTL and 2.5V LVTTL?

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daraemon_liu

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Recently in my project I need to design a JTAG chain which contains two diffenent voltage powered JTAG I/Os,one is 3.3V LVTTL,the other is 2.5V LVTTL.
The chain can be seen in the attachment,the JATG scanner itself is powered by 3.3V.So it surely can connect to the first chip in this chain.But the second chip needs to be considered carefully.Does the 3.3V LVTTL output make overshoot in the signal for the 2.5V LVTTL input ?Can it damage the second chip? If so, what circuit should I use to avoid it? Dose the 2.5V LVTTL output meet the VIH for the JATG scanner?
TKS for your advices
 

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  • JTAG_CHAIN.bmp
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Since we don't want to add chip any more in our board, can I try approaches as below:
For the 3.3v lvttl to 2.5v lvttl ,can I use the divider resistor to attenuate the amplitude of 3.3V output to avoid damaging?
For the 2.5v lvttl to 3.3v lvttl ,can I place a pull-up resistor to 3.3V on the 2.5V output to improve the margin above the 3.3V VIH?
 

For the 3.3V to 2.5V, a simple potential divider will do the trick. For the reverse path (2.5V to 3.3V), I'm not sure but, I think it should work.
 

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