JTAG daisy chain over several boards

Go To Last Post
3 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi,

 

I want to daisy chain a JTAG signal over several boards. I have a problem thou, after the last board, the TDO has to be returned as TDI. Earlier I have done this using a special PCB I push into the flex connector. I would prefer if I did not need this PCB but instead had some other way of linking it back.

 

One obvious way would be a jumper. But when its forgotten to be placed or is placed on several boards it takes the software developers days to find the reason why the JTAG does not work.

 

Has anyone had any creative idea on solving this?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You could add 0R resistors on the TDO signal into the design of the boards that let you select whether that particular board is at the end of the chain or somewhere in the middle? That doesn't help you with any existing hardware I know, but if it were me I'd simply persevere with jumpers for now and then change the design for the next batch.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thanks for your input. We used your solition on a board now where we seldom have more tha one board.

 

On the boards where we often connect boards in a long chain we just solved this issue because we had 1.8V going back from the next board. We use this power to enable two buffers, one with inverted input and one with non-inverted and with high-Z when disabled. So when a board is connected the TDO from that is forwarded, when a board is not connected or not powered, the TDO from same board is forwarded.