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Altera_Forum's avatar
Altera_Forum
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15 years ago

isolation for Altera programmer

I have a situation where the ground of my FPGA circuit is at a potential a few volts below the earth ground. Normally, if my FPGA and PC grounds are the same, I use a USB Blaster to program either the FPGA directly via JTAG or the EPCS config device via the AS protocol.

There are commercial USB isolation boxes available. These typically work only for specific USB speeds. Some work for Full Speed only, some Full or Low, none that I have seen yet work for High Speed. The USB Blaster does not specify what speeds it uses. Does anyone have experience using an isolator with the USB Blaster?

Another possible solution is to use an Ethernet-Blaster instead, with no isolator. I believe that ethernet has DC isolation built-in as part of the specification. Should this work?

Thanks in advance for any input.

4 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    USB Blaster uses standard full speed. Commercially available isolators should generally work.

    On the JTAG side, the total round-trip delay (the critical path is TCK + TDO) must not exceed 80 ns, including two isolator instances and all driver and target chip delays. Thus rather fast couplers are needed. NVE IL71x are e.g. good for this application. A DC-DC converter is required to feed the target power supply to the USB Blaster's JTAG drivers - or a modification of the USB Blaster to supply it internally with fixed 3.3 V.

    I append a circuit of a 3.3V JTAG isolator that has served it's purpose in power electronics applications.
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    Thank you for the responses. Full speed seems to be the one that most of the USB isolators work with, so that is welcome news.

    If I were only using the port for JTAG purposes, I would be more inclined to build in isolators. The ones you suggest look like good choices. However, since I will also be using the port with a USB Blaster to load data into the EPCS device, this gets a bit more complicated. During that process, the USB Blaster apparently tri-states some of the signals and there is no access to the tri-state control signals. With more knowledge of the EPCS writing protocol, isolation of it might be possible too but isolating at the USB level seems to be easier. (especially now that I know the USB Blaster uses Full speed)
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    A follow-up in case anybody is interested:

    The Keterex USB-150 isolator works with the USB-Blaster.

    That device is spec'd to work only for USB "Full Speed" so this confirms the above responses.