sdram tunning

when i connect clock to pin 91(and therefor use pll number 4 )and no constrain on timing parameters(tsu, tco, tpd,th) with tsu=7.9 n

tco_max=6.8 n tco_min=1.39 n th=-7.6 n my program on the board is executed.

when i connect clock to pin 152 (and therefor use pll number 2) and

constrain on timing parameter with tsu=5.1 n tco_max=2.3 n

tco_min = 1.53 n , th=-4.8 n my program isn't executed.

timing parameter in the second case is beter from the first case but program

in the second case isn't executed .

in the all situation clock=27mhz sdram_clock = 81mhz .

if timing parameter cause this problem in first situation my program on the board dosn't execute.