Forum Discussion

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

Cyclone III JTAG on 3v3

I have an EP3C16 with both an JTAG and AS connector connected as shown in figure 10-29 in the Cyclone III handbook (http://www.altera.com/literature/hb/cyc3/cyc3_ciii51010.pdf) (http://www.altera.com/literature/hb/cyc3/cyc3_ciii51010.pdf%29).

But it is unclear to me why the JTAG connector needs pull-ups to 2v5 (Vcca). Is it possible connect these to Vccio (all my banks are powered by Vccio +3v3). The same for pin 4 of the JTAG connector and the MSEL pins, is it possible to power it with 3v3 (Vccio) instead of 2v5 (Vcca).

Kind regards,

Paul

1 Reply

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

    By specification, Cyclone III JTAG and configuration can work with 3.3V. All customer designs I have seen up to now have been using 3.3V, for obvious reasons.

    But there is some danger of damaging the chip by overshoots exceeding the absolute maximum voltage, thus Altera recommends 2.5V generally. I would read this as a recommendation to consider 2.5V VCCIO with new designs. One reason to continue using 3.3V is the unavailabability of AS configuration devices from Altera with 2.5V supply.

    When using 3.3V, schottky clamps with series protection resistors at the external JTAG interface can guarantee to keep the maximum voltage specification.

    There has been a longer discussion on this topic in the forum, I tried to summarize the key points in my subjective view.