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Altera_Forum's avatar
Altera_Forum
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16 years ago

Vccio level for LVDS IOs

Hi,

AN479 says that LVDS ip & op must be powered with a 2.5V Vccio except: all LVDS inputs for cyclone device are powered by Vccint.

The PCB with a cyclone was designed for other purpose, whose Vccio = 3.3V and Vccint = 1.5V. I am trying to build duplex LVDS comm. bet. the PCB and Cyclone III starter kit. I have some concern on apply those Cyclone's differential pins directly. as the LVDS i/p is powered at 1.5V, while o/p 3.3V. Will that LVDS comm. still be feasible without modifying its power supply? Thanks.

Y.Z.

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    AN479 also tells:

    --- Quote Start ---

    Supplying higher than supported voltage to VCCIO for LVDS would violate the rated specifications and may possibly damage the buffer.

    --- Quote End ---

    Apart from this, operating Cyclone at VCCIO of 3.3V with standard 3R network for LVDS output results in an incorrect common mode level, although it possibly works at lower rates. A 5R network with additional pull-down resistors could achieve correct LVDS levels.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    Thanks. There is another requirement says that the resistor network should placed within 1 inch from the device. However, in my case, it has to be 5 inch, what should I do with the resistor network?