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Altera_Forum's avatar
Altera_Forum
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15 years ago

PCIe ALTGX reconfig_clk frequency?

The documentation for the stratix IV is somewhat confusing on this matter. It says:

"Offset cancellation logic requires a separate clock. In PCIe mode, you must connect

the clock input to the fixedclk port provided by the ALTGX MegaWizard Plug-In

Manager. The frequency of this clock input must be 125 MHz. For all other functional

modes, connect the clock input to the reconfig_clk port provided by the ALTGX

MegaWizard Plug-In Manager. The frequency of the clock connected to the

reconfig_clk port must be within the range of 37.5 to 50 MHz. Figure 1–46 shows the

interface of the offset cancellation control logic (ALTGX_RECONFIG instance) and the

ALTGX instance."

The ALTGX instance has a "fixedclk" input and a "reconfig_clk" input, and the ALTGX_RECONFIG instance has a "reconfig_clk" input. The ALTGX fixedclk input should definitely be 125 MHz, but what should the reconfig_clk inputs be? It sounds like it should be connected to the same 125 MHz fixedclk input on the ALTGX instance, but it says numerous times that fMax for reconfig_clk is 50 MHz. Can anyone clarify?

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    While waiting for a better answer, here is my experience. I have not used the PCIe mode, but did use other modes that require the reconfig_clk. the reconfig_clk must be between 37.5 and 50 MHz. The doc also says that the reconfig_clk must come from an input pin. However, there is an app note (forgot which) that shows you how to use a pll for that - in case you do not have a separate 37.5 to 50 MHz clock coming in.

    Knowing what the reconfig block does, I don't think they would require the reconfig_clk to be different for PCIe.

    Here is the link to using a pll for reconfig_clk: http://www.altera.com/support/kdb/solutions/rd12172009_309.html
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    For PCIe core, fixedclk must be 125MHz which is used to control transceiver data path of Offset Cancellation process. There is no other frequency allows for fixedclk. Whereas, reconfig_clk can be from 37.5 to 50MHz that is used to control the reconfig block controller.