Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
12 years ago

USB-Blaster JTAG max cable length

Hi All,

May not have been specific enough with my search, but did not find an answer to my question. I am using the USB-Blaster in JTAG mode. I know that the max cable length on the USB side is 5 meters. Can someone tell me what the max recommended cable length is on the JTAG side?

TIA,

Charlie

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    12 years ago

    I don't think it's specified anywhere, but shorter is better.

    These are all realativily slow signals, but they are also all signle ended signals. The JTAG clock can be slowed down, so you could make a longer cable work. But at some point you will need to modifiy the USB Blaster source impediatence to minimize clocking transmission line issues.

    I've gotten away with a couple of foot extension, using standard ribbon cable to access the JTAG from outside of an industrial box, but unless you do good impedence matching, I would not recommend more on a ribbon cable.

    If you use a better cable type, slow down the clock, and match impeadence well, you can do much better, but far I don't know.

    There are eithernet to jtag solutions that would get you much better distances. I would recommend them first.

    Pete
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    12 years ago

    Usually the JTAG side should be kept as short as possible. Same for routing jtag signal on the pcb, unless you carefully control trace impedance and add proper termination.

    AFAIK the recommended cable length is below 20cm / 8"
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    12 years ago

    The acceptable JTAG cable length depends on what you are trying to do with the JTAG. If you are downloading images to the FPGA with a human in attendance then the cost of an error is that you have to do it again - in this case a 99% success rate might be acceptable. If you have an automated system or are debugging a processor then any error will cause problems (JTAG does not have error correction or retransmits)

    The chance of getting hit by a bit error also depends on how noisy your environment is - equipment like motors can generate spikes on the JTAG which will cause more problems.

    If you are not happy with bit errors then I would recommend no more than 5cm of standard ribbon cable, or no more than 20cm of the flexible PCB which comes with the USB-Blaster.

    You can extend the USB cable quite a lot more (USB has error detection and retransmits). I think you can chain upto 6 powered hubs if you need a longer cable. I would recommend extending the USB side of the connection and keeping the JTAG side as short as possible.