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AR# 21762

Spartan-3 DCM - When does the DCM start to acquire lock?

Description

When does the DCM start to acquire lock?

Solution

The DCM starts to acquire lock as soon as the GHIGH_b signal is released. The de-assertion of AGHIGH is part of the Last Frame Write (LFRM) configuration command. De-asserting GHIGH_b essentially enables the input clock path from the clock pad to the CLKIN pin of the DCM and the clock feedback path. Although GWE, GSR, and GTS are active, these signals have no effect on the DCM. As long as the reset pin of the DCM is not being asserted, DCM will start to acquire lock at this time.

If the DCM has external feedback, the feedback clock will not be present at the DCM CLKFB pin until GTS is released during the startup sequence. Consequently, it is possible that the DCM has been trying to acquire lock for quite awhile without a stable input clock or without the feedback clock present. This is why it is always recommended that you reset your DCM after the startup sequence is complete. Without the reset pulse, the DCM might not lock at all or it might not lock at the most optimal point, which potentially introduces slightly more jitter since there is greater clock cycle latency through the DCM.

AR# 21762
Date Created 09/04/2007
Last Updated 12/15/2012
Status Active
Type General Article