How to determine the AGP voltage used

Discussion in 'Asus' started by Michael S., Jul 21, 2004.

  1. Michael S.

    Michael S. Guest

    As I understand it, an 8X AGP video card operates at 0.8 volts. How can I
    determine the voltage being used by the AGP 8X video card by my P4C800E-D

    I have looked everywhere in the BIOS, in AIDA32 and in Probe data, but
    either have missed the AGP voltage or just do not know where to look.
    Aida32 does show an 8X speed. Suggestions?

    Michael S., Jul 21, 2004
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  2. Michael S.

    Paul Guest

    AGP video chips use many voltages, but the voltage in question is
    used to power the I/O pins on the AGP video card and also the
    I/O pins on the Northbridge AGP interface. Matching the capabilities
    of those two chips, so they can share the same voltage, is
    what all this nonsense is about. That is why AGP slots have plastic
    keys, and AGP video cards have slots cut in them, to prevent an
    inappropriate mix of technology.

    There are, in fact, only two I/O supply voltages. They are
    3.3V and 1.5V. The 1.5V is necessary, as modern small geometry
    chips no longer like the 3.3V voltage. They can only handle lower
    voltages. Your P4C800E-D can only handle the lower of the two
    voltages. There is a setting in the BIOS with options like this -
    basically 1.5V, but with room to adjust if needed. Sometimes
    a boost is needed for stability.

    "AGP VDDQ Voltage" [1.5, 1.6, 1.7]

    So, where does the 0.8V come from ? There are two ways to
    terminate the AGP bus. Termination is any means used to
    suppress signal reflection on an electrical interconnect.

    There are two cases. Both electrical cases use VDD = 1.5V for the
    silicon. One case doesn't use a parallel termination resistor
    (it is AGP 4X). The VDDQ fed to the I/O pad results in a full
    ampliude signal and we refer to that as 1.5V I/O.

    VDD=1.5V Typical AGP 2.0 case VDD=1.5V
    | Full 1.5V signal swing |
    | |
    Driver ------resistance-----the_bus-----+------Receiver
    | |
    | |
    | |

    In the second case, a parallel termination resistor is used,
    and as near as I can tell, this resistor is actually inside the
    chip. The I/O pads at either end of the bus are still powered
    by 1.5V, but there is a voltage divider action, due to the
    use of the termination at the receiver.

    AGP 3.0 case 0.8V swing
    VDD=1.5V Parallel terminator | VDD=1.5V
    | | |
    | v |
    Driver ------resistance-----the_bus-----+------Receiver
    | | |
    | resistance |
    | | |

    Please note - I haven't build any AGP interfaces, and the
    above figure is my interpretation of what I've been reading.
    Start at PDF page 63 to do your own interpretation.

    In conclusion, for an AGP 3.0 card, you cannot measure this
    0.8V swing, because it only exists on a signal pin, and they change
    too rapidly for any hardware to measure or make sense of them.
    The driver and receiver pads are still powered by 1.5V.

    The only thing you would be able to see, is the value of VDD.
    The P4C800E-D doesn't connect the hardware monitor chip to the
    VDD of the AGP. You can set the voltage in the BIOS, but the
    hardware monitoring page in the BIOS doesn't show the result of
    your change. Using a voltmeter on one of the AGP slot pins would
    be one way to verify the voltage.

    Paul, Jul 22, 2004
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  3. Michael S.

    Michael S. Guest

    WOW Paul--what a nicely-written, detailed answer--THANKS!. I vaguely
    remember running some program about a month ago that reported a number of
    parameters of the computer and came up with 1.5 volts on the AGP--which is
    why I was asking about the 0.8 volts. The general public is told that AGP
    4x = 1.5 volts and 8x = 0.8 volts, so I was concerned that my Radeon x800
    Pro was being fed too many volts because I overlooked some setting in BIOS
    in this computer build, my first since 486's. From your detailed
    explanation, I understand WHY the program I was running came up with 1.5
    volts on the AGP.

    Michael S., Jul 22, 2004
  4. Michael S.

    Paul Guest

    BTW - Apologies for the AGP doc link. It appears Intel has erased
    history, by removing their AGP info. This is the first site I
    could find that still has copies:

    Docs like that don't get backed up on, and I also
    notice that a number of large companies have had archiving activities
    stopped, making it easier for them to bury their past.

    Paul, Jul 22, 2004
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