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242 www.xilinx.com Development System Reference Guide
Chapter 12: TRACE
R
OFFSET IN Detail Path Clock Path
The second section is the clock path. In this example the clock starts at an IOB, goes to a
DCM, comes out CLK0 of the DCM through a global buffer (BUFGHUX). It ends at a clock
pin of a FF.
The Tdcmino is a calculated delay. This is the equation:
Clock Path: wclk_in to wr_addr[2]
Location Delay type Delay(ns) Logical Resource(s)
------------------------------------------------- ----------------
D7.I Tiopi 0.825 wclk_in
write_dcm/IBUFG
DCM_X0Y1.CLKIN net (fanout=1) 0.798 write_dcm/IBUFG
DCM_X0Y1.CLK0 Tdcmino -4.297 write_dcm/CLKDLL
BUFGMUX3P.I0 net (fanout=1) 0.852 write_dcm/CLK0
BUFGMUX3P.O Tgi0o 0.589 write_dcm/BUFG
SLICE_X3Y11.CLK net (fanout=41) 0.748 wclk
------------------------------------------------- ----------------
Total -0.485ns (-2.883ns logic,
2.398ns route)
----------------------------------------------------------------------
OFFSET In with Phase Shifted Clock
In the following example, the clock is the CLK90 output of the DCM. The clock arrival time
is 2.5 ns. The rclk_90 rising at 2.500 ns. This number is calculated from the PERIOD on
rclk_in which is 10ns in this example. The 2.5 ns affects the slack. Because the clock is
delayed by 2.5 ns, the data has 2.5 ns longer to get to the destination.
If this path used the falling edge of the clock, the destination clock would say, falling at 00
ns 7.500 ns (2.5 for the phase and 5.0 for the clock edge). The minimum allowable offset
can be negative because it is relative to the initial edge of the clock. A negative minimum
allowable offset means the data can arrive after the initial edge of the clock. This often
occurs when the destination clock is falling while the initial edge is defined as rising. This
can also occur on clocks with phase shifting.
Example:
======================================================================
Timing constraint: OFFSET = IN 4 nS BEFORE COMP "rclk_in" ;
2 items analyzed, 0 timing errors detected.
Minimum allowable offset is 1.316ns.