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AR# 18390

10.1 Timing - Changing the DESKEW_ADJUST parameter does not affect the DCM value (Tdcmino)


Keywords: deskew, adjust, source, system, synchronous, external, feedback, 6.1i, de-skew

In certain cases, when I try to change the DESKEW_ADJUST value on the DCM to make my clock SOURCE_SYNCHRONOUS, I do not see any changes on Tdcmino in static timing analysis. What factors affect how DESKEW_ADJUST works?


The DESKEW_ADJUST parameter is used to change the DCM de-skewing from SYSTEM_SYNCHRONOUS to SOURCE_SYNCHRONOUS. It accomplishes this by removing an internal delay to the DCM that delays the FEEDBACK path. The FEEDBACK path is delayed in the SYSTEM_SYNCHRONOUS setting to account for the input path delay to the DCM (GCLKIOB to DCM). The DCM aligns the FEEDBACK path to the GCLKIOB Pad path, which results in a deskewing of the internal clock to the external clock pad. Setting the DCM to SOURCE_SYNCHRONOUS removes this delay so that the DCM will actually de-skew less.

However, there are situations in SYSTEM_SYNCHRONOUS mode in which the DCM does not add this delay to the FEEDBACK path. In these situations, the DESKEW_ADJUST parameter has no affect on the DCM timing (Tdcmino). Specifically, the DCM adds offset to the feedback path for EXTERNAL CLKIN and INTERNAL FEEDBACK to compensate for input delay. In all other cases, an offset is not added to the feedback path.

Consequently, the DESKEW_ADJUST attribute does not affect DCM timing (Tdcmino) for DCMs that are cascaded, have external feedback, or have an external CLKIN that does not come from a GCLKIOB. The FEEDBACK path is delayed to not account for the input path delay to the DCM. The DCM aligns the FEEDBACK path to the DCM CLKIN pin path, which results in a deskewing of the internal clock to the internal CLKIN pin. This behavior to calculate the DCM timing (Tdcmino) matches what occurs in hardware. For these situations, you can still adjust the output clocks of the DCM by using COURSE or FINE phase shifting.
AR# 18390
Date 06/09/2008
Status Active
Type General Article