説明

General Description:

It may not be obvious that the HIGH and LOW qualifiers for the

PERIOD constraint define the length and polarity "first clock

pulse", not simply the duty cycle. This becomes relevant when

using the PERIOD in conjunction with the OFFSET constraint. For

example, as far as the duty cycle is concerned, the following

two PERIOD constraints are identical:

1) NET clock PERIOD = 30ns high 18;

2) NET clock PERIOD = 30ns low 12;

But if an OFFSET constraint is used, such as:

NET qoutpad OFFSET = OUT 20 AFTER clockpad;

then the two PERIOD constraints will yield different results in TRCE.

~~~~~~~~~~~~~~~~~~~~~~~~

For 1), this means the first clock pulse is high for 18ns,

followed by a low level for 12ns. Since it is implied there

will be an edge at time 0, the OFFSET constraint will

be calculated with that assumption.

====================================================================

Timing constraint: COMP "qoutpad" OFFSET = OUT 20.000 nS AFTER COMP "clockpad" ;

1 item analyzed, 0 timing errors detected.

Minimum allowable offset is 16.587ns.

--------------------------------------------------------------------

Slack: 3.413ns path clockpad to qout relative to

12.627ns delay constraint qout to qoutpad and

20.000ns offset clockpad to qoutpad

~~~~~~~~~~~~~~~~~~~~~~~

For 2), this means the clock is low for 12ns initially,

followed by a 18ns high. In this case, the OFFSET constraint

will now be calculated assuming the edge will come at

time=12ns. 12ns will be subtracted from the allowable offset

(20ns).

====================================================================

Timing constraint: COMP "qoutpad" OFFSET = OUT 20.000 nS AFTER COMP "clockpad" ;

1 item analyzed, 1 timing error detected.

Minimum allowable offset is 28.587ns.

--------------------------------------------------------------------

Slack: -8.587ns path clockpad to qout relative to

12.627ns delay constraint qout to qoutpad and

8.000ns offset clockpad to qoutpad

Notice the "8.000ns" on the last line; this is 20.000ns-12ns.

The "Minimum allowable offset" is now 12ns more than 1);

28.587ns = 16.587ns + 12ns.

There is also the case with OFFSET IN BEFORE/AFTER. This will result in addition instead of subtraction of the offset clock to flop delay. Therefore you would have 20.000ns+12.000ns=32.000ns.

ソリューション

The resolution to this issue is never use the LOW

qualifier when using offset. Always use the HIGH

qualifier to specify the duty cycle.