On Spartan-3A, Spartan-3AN, or Spartan-3A DSP - when constraining a clock to 250 MHz on the CLKIN input of a DCM_SP, Timing Analyzer returns the following warning:
"WARNING:Timing:3328 - Timing Constraint "TS_CLKIN_250MHz = PERIOD TIMEGRP "CLKIN_250MHz" 4 ns HIGH 50%;" fails the minimum period check for the output clock inst_clock_block/Inst_dcm_250in/CLK0_BUF from DCM inst_clock_block/Inst_dcm_250in/DCM_SP_INST because the period constraint value (4000 ps) is less than the minimum internal period limit of 4166 ps. Please increase the period of the constraint to remove this timing failure."
This warning implies that the maximum CLKIN frequency is 240 MHz; however, the data sheet for each of these devices says the max frequency for the -4 speed grade is 250 MHz. Which value is correct for the maximum clock input frequency in the lowest (-4) speed grade?
This Answer Record applies to Spartan-3A, Spartan-3AN, and Spartan-3A DSP
The warning can be ignored as the correct CLKIN maximum frequency is 250 MHz, as indicated by the data sheet.
For Spartan-3A DSP, the discrepancy in the speedfiles will be corrected in the new version of the speedfiles, version 1.32, available through the 10.1 Service Pack 2 (due out in late June 2008).
For Spartan-3A and Spartan-3AN, there is no planned fix at this time. A fix will be scheduled for the next available update.
Please Note: All other timing numbers reported by Timing Analyzer for the Spartan-3A family DCM are accurate. Please also see the device data sheets for these values:
Spartan-3A Data Sheet (DS529, v1.6):
Spartan-3AN Data Sheet (DS557, v3.0):
Spartan-3A DSP Data Sheet (DS610, v2.0):