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JTAG chain incompatibility

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shaiko

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Has anyone ever encountared devices that "don't like" sitting together on the same JTAG chain ?
Not because of poor PCB design and signal integrity issues - but because of true system conflict/collision between them...?
 

Yes, this is quite common, particularly if the jtag chain runs from a device which does use TRSTN to a device which doesn't (or the other way around). This is because the TDO timings are different for the different devices. I found that putting a 10k pull-up resistor on each TDO cured this problem. Also not all Devices (particularly Xilinx FPGAs are properly 1149 compliant. This is a major issue and can result in the need to bypass these devices altogether.
 
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