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Observer
Observer
8,150 Views
Registered: ‎02-27-2014

JTAG scan show " unknown bypass"

Hello everyone,

i have met a strange problem when i using a spartan6 FPGA 

when i scan the JTAG using ISE , the message shows "unknown bypass" and the FPGA could not be programmed.

the Vref is 3.3V , and i measure the voltage of JTAG and results are as belows:

TMS:3.3V

TCK:3.3V

TDI:3.3V 

TDO:3.3V

 

and i change the Vref to 2.5V, the FPGA could be scanned sometimes, but not always, 

sometimes the error meassage shows" a problem may exist in the hardware configuration"

and i measure the voltage of JTAG, the results are as belows:

TMS:2.89V

TDI:2.89V

TDO:3.25V

TDI:2.89V

the voltages are somehow strange.

the error message is as belows:

3V3.png

 

p.s: the schematic is as belows:

JTAG.jpg

 

what may be the problem? could someone give some advice?

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4 Replies
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Xilinx Employee
Xilinx Employee
8,141 Views
Registered: ‎07-31-2012

Re: JTAG scan show " unknown bypass"

Hi,

 

Please note that you have to give proper voltages Vref at the JTAG header based on the voltage VCCAUX which you are giving to the FPGA. Check the screenshot below.

 

What is the VCCAUX power on your FPGA?

 

I also observed that a pullup was not given in the user guide on these lines. TDI, TMS by default have internal pullup's which you can check from Pg 69 of UG 380 - http://www.xilinx.com/support/documentation/user_guides/ug380.pdf

 

 

jtag.PNG

 

Also Read through the below references which should help you indenfity the problem.

 

1)      Make sure you have followed all the guidelines in the JTAG ISP checklist - http://www.xilinx.com/support/documentation/application_notes/xapp104.pdf

2)      Also check the JTAG programmer Troubleshooting from Pg 55 of the link - http://www.xilinx.com/support/sw_manuals/2_1i/download/jtag.pdf

Thanks,
Anirudh

PS: Please MARK this as an answer in case it helped resolve your query.Give kudos in case the post guided you to a solution.
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Observer
Observer
8,124 Views
Registered: ‎02-27-2014

Re: JTAG scan show " unknown bypass"

Hello,

the situation has some change

1.the VCCAUX is 3.3V ;

2.when i give the Vref 2.5V-3.3V, especially the Vref is 2.5V-3.0V, the JTAG chain could be scanned on the ISE , and the chip is normal, but  when the Vref  is 3.1-3.3V, the boundary scan sometimes works, sometimes fails to work.

That is to say, sometimes the ISE could find the chip(150T);

sometimes the ISE could not find the chip, ISE shows "unknown bypass", the boundary scan is not steady.

and the schematic stays unchanged.

and what could be the problem?

 

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Xilinx Employee
Xilinx Employee
8,115 Views
Registered: ‎07-31-2012

Re: JTAG scan show " unknown bypass"

Hi,

 

In that case, please probe the TCK, TMS, TDI, TDO pins when the operation fails and check what is the difference. 

 

This should help you narrow down to your problem. Check when the device is detected and when it is not detected and then compare. however the voltage of Vref should be same as that of the Vccaux. 

 

Check if the JTAG connector is fine. Probe the JTAG lines on the board also if possible.

Thanks,
Anirudh

PS: Please MARK this as an answer in case it helped resolve your query.Give kudos in case the post guided you to a solution.
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Xilinx Employee
Xilinx Employee
8,074 Views
Registered: ‎08-01-2012

Re: JTAG scan show " unknown bypass"

The external pull-up for JTAG pins are not required for Spartan-6 because the FPGA JTAG internal circuitry has required.internal pull-up resisters. The additional external pull-up (4k7) may cause sink current (drive strength) problems. Please remove the additinal pull-up resisters and let us know the status

 

 

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