Forum Discussion

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

Selectable LVTTL Inputs - Cyclone II

Hello All,

I recently discovered this forum and very excited about the wealth of information it contains.

Anyway, I have what might be considered a novice question. I'm hoping to find the best way to allow my Cyclone II to accept different input voltages. I have three serial interface lines (~60-80MHz) coming into my Cyclone II, that (depending on the device I connect to it) can be either 3.3V, 2.5V or 1.8V. It seems like the only way to work with this is to compile 3 different bitstreams with the 3 different I/O Standards set (3.3V LVTTL, 2.5V LVTTL or 1.8V LVTTL).

My question.. is there an easier more dynamic way?

Also, I think the VCCIO pins might be tied to 3.3V on the custom board I'm using, will this make it incompatable to read 1.8V LVTTL?

Thanks in advance for any help!

-- Chris

5 Replies

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

    --- Quote Start ---

    Also, I think the VCCIO pins might be tied to 3.3V on the custom board I'm using, will this make it incompatable to read 1.8V LVTTL?

    --- Quote End ---

    The device handbook will answer that question. Compare the VIH min that you think you will actually have to the requirement for the I/O standards that use VCCIO set to 3.3V.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    17 years ago

    Hi Chris:

    Since they are inputs, with VCCIO connected to 3.3 V, The inputs VIH is usually compatible with LVTTL 3.3 and LVTTL 2.5. LVTTL 1.8 is typically not compatible with LVTTL 3.3 since the standard VIH is 2.0 V for LVTTL 3.3.

    That being said however. It is possible for the Cyclone III to accept LVTTL 1.8 using a LVTTL 3.3 input, because they specify their VIH as 1.7 V instead of 2.0 V in the handbook for this IO standard.

    So As long as your LVTTL 1.8 device can at least drive 1.7 V VOH (Plus some margin) you should be OK.

    The IO spec are in the Cyclone III Device Handbook Volume 2 section 1-9 (Table 1-12)

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

    According to Cyclone II Device Handbook, Table 2-21 cyclone ii multivolt i/o support, an VCCIO of 1.8V would allow to connect 1.8 to 3.3 V I/O standard on input.

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

    Thanks for all the help everyone!! I've been researching this stuff all day. It seems my board has an I2C connection allowing me to dynamically program the VCCIO voltage. If were to say change the VCCIO voltage from 3.3V down to 1.8V in order to use the 1.8V to 3.3V range that FvM mentions, what would I need to set the I/O Standard in Altera's Pin Planner to use this range?

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

    Another way to approach this...

    On Cyclone II, you can send a 3.3V LVTTL signal into an I/O bank that's connected to 1.8V (or 2.5V). Make sure the PCI clamping diode is turned off, and be aware that the leakage current will be higher than spec (you're overdriving the input buffer, after all), but it should all work fine.