Fanout Delay?

[Jim]

Hello

Here is part of the timing constraints output for my design:


Timing constraint: Default OFFSET IN BEFORE for Clock 'mst_clk'
Offset: 9.162ns (Levels of Logic = 2)
Source: mst_rst
Destination: idsel_ff_Mtrien_q
Destination Clock: mst_clk rising

Data Path: mst_rst to idsel_ff_Mtrien_q
Gate Net
Cell:in->out fanout Delay Delay Logical Name (Net Name)
---------------------------------------- ------------
IBUF:I->O 13 0.797 2.000 mst_rst_IBUF (mst_rst_IBUF)
LUT1:I0->O 89 0.468 5.320 I_INV_mst_rst (N493)
FDPRE 0.577 idsel_ff_Mtrien_q
----------------------------------------
Total 9.162ns (1.842ns logic, 7.320ns route)
(20.1% logic, 79.9% route)


mst_rst is an asychronus reset for the whole system. And idsel_ff is a flip
flop, which has a asynchronus reset and a synchronus output enable.

If the reset is asynchrnous then how come it is associated with the rising
edge of the clock? And is the reason for the large time of 9.162ns due to
the high fanout?

Are there any techniques for dealing with fanout times (other than place and
route)?

TIA

 

[rickman]

Jim wrote:

Hello

Here is part of the timing constraints output for my design:

Timing constraint: Default OFFSET IN BEFORE for Clock 'mst_clk'
Offset: 9.162ns (Levels of Logic = 2)
Source: mst_rst
Destination: idsel_ff_Mtrien_q
Destination Clock: mst_clk rising

Data Path: mst_rst to idsel_ff_Mtrien_q
Gate Net
Cell:in->out fanout Delay Delay Logical Name (Net Name)
---------------------------------------- ------------
IBUF:I->O 13 0.797 2.000 mst_rst_IBUF (mst_rst_IBUF)
LUT1:I0->O 89 0.468 5.320 I_INV_mst_rst (N493)
FDPRE 0.577 idsel_ff_Mtrien_q
----------------------------------------
Total 9.162ns (1.842ns logic, 7.320ns route)
(20.1% logic, 79.9% route)

mst_rst is an asychronus reset for the whole system. And idsel_ff is a flip
flop, which has a asynchronus reset and a synchronus output enable.

If the reset is asynchrnous then how come it is associated with the rising
edge of the clock? And is the reason for the large time of 9.162ns due to
the high fanout?

Are there any techniques for dealing with fanout times (other than place and
route)?

I am not sure what you are looking for. The info above is telling you
the amount of setup time you need for the reset signal relative to the
rising edge of the clock on the FF. It does not include the delay for
the clock from pin to FF which will reduce your extermal pin to pin
setup time (so you need to know the min clock routing delay, not the
max; or treat this as zero). The async reset must be released some
point prior to a clock edge to be sure the FF will then operate
correctly. If you don't care about all the FFs being released on the
same clock edge, then this time is not useful.

--

Rick "rickman" Collins

rick.collins@XYarius.com
Ignore the reply address. To email me use the above address with the XY
removed.

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