Another strage timing problem

[Kload]

Hi again,

Following up on my earlier post, I have come across another unusual
timing problem. It seems I have timing paths that pass through flip
flops. I've attached an example from the timing report. Using FPGA
Editor to examine the implementation I have been able to confirm that
oath addrRange<1> and addrRange<2> are flip flop outputs. In both cases
the inputs seems to come through the carry logic, though and XOR gate,
through a multiplexer to the D input of the flip flop.

Thanks in advance for your help.

--------------------------------------------------------------------------------
Slack: -2.537ns path ROBAVGSEG2/endAddr<8> to
ROBAVGSEG2/dataSum<16> relative to
20.000ns delay constraint

Path ROBAVGSEG2/endAddr<8> to ROBAVGSEG2/dataSum<16> contains 10 levels
of logic:
Path starting from Comp: CLB_R22C54.S0.CLK (from CLK180)
To Delay type Delay(ns) Physical Resource
Logical Resource(s)
------------------------------------------------- --------
CLB_R22C54.S0.YQ Tcko 1.372R ROBAVGSE/eAddr<8>
ROBAVGSE/eAddr_reg<8>
CLB_R21C58.S1.F1 net (fanout=6) 2.258R ROBAVGSE/eAddr<8>
CLB_R21C58.S1.X Tilo 0.738R ROBAVGSE/syn6957
ROBAVGSE/C2679
CLB_R25C58.S1.G3 net (fanout=1) 1.633R ROBAVGSE/syn6957
CLB_R25C58.S1.Y Tilo 0.738R ROBAVGSE/C1432
ROBAVGSE/C2677
CLB_R25C58.S1.F3 net (fanout=2) 0.130R ROBAVGSE/syn6963
CLB_R25C58.S1.X Tilo 0.738R ROBAVGSE/C1432
ROBAVGSE/C2676
CLB_R23C55.S1.F4 net (fanout=16) 1.975R ROBAVGSE/C1432
CLB_R23C55.S1.X Tilo 0.738R ROBAVGSE/C1291/N21
ROBAVGSE/C2663
CLB_R24C59.S1.F1 net (fanout=1) 1.871R ROBAVGSE/C1291/N21
CLB_R24C59.S1.COUT Topcyf 1.445R ROBAVGSE/addrRange<1>
ROBAVGSE/C1281/C4/C0
ROBAVGSE/C1281/C4/C2
ROBAVGSE/C1281/C5/C2
CLB_R23C59.S1.CIN net (fanout=1) 0.000R ROBAVGSE/C1281/C5/C2/O
CLB_R23C59.S1.Y Tciny 0.590R ROBAVGSE/addrRange<2>
ROBAVGSE/C1281/C6/C2
ROBAVGSE/C1281/C7/C1
CLB_R25C60.S1.F2 net (fanout=6) 1.577R ROBAVGSE/N7566
CLB_R25C60.S1.X Tilo 0.738R ROBAVGSE/syn2456
ROBAVGS2/C2558
CLB_R26C67.S0.G2 net (fanout=7) 1.766R ROBAVGSE/syn2456
CLB_R26C67.S0.Y Tilo 0.738R ROBAVGSE/C92/N85
ROBAVGSE/C2458
CLB_R21C75.S1.CE net (fanout=9) 2.544R ROBAVGSE/C92/N85
CLB_R21C75.S1.CLK Tceck 0.948R ROBAVGSE/dSum<16>
ROBAVGSE/dSum_reg<16>

 

[John_H]

Greetings, Kload.

The timing report that you see does not show your flops driving NET
elements, but their inclusion in the timing report is misleading for
new engineers. The Xilinx devices work with "slices" which are
treated as a single element within the tools to some degree. Each
slice can contain many smaller parts such as LUTs, registers, MUXCY
elements, XORCYs, MUXF5s, and so on. The first name in one "group"
from the CLB timing element (such as Tilo or Topcyf) corresponds to
the name the Xilinx tool gave to the entire slice. If you have the
regular XIlinx tools (rather than just Webpack) you can use the FPGA
Editor to look at the place & route implementation inside the device.
If you find the ROBAVGSE/addrRange<1> COMPonent and open it up to view
the innerds of that slice, you'll see the register you were worried
about as well as the other piece-parts that make up that slice to
include the named elements:
ROBAVGSE/C1281/C4/C0
ROBAVGSE/C1281/C4/C2
ROBAVGSE/C1281/C5/C2
Which actually *are* part of your carry chain.

The "net" items are the actual signals that propagate out of the
slice. Your flop isn't part of the carry chain logic, but is packed
in the same slice as that change, affecting performance very little.

Your difficulties with this timing path include not-so-great delay
paths for the rather large amount of logic. If you can figure out how
to reduce the levels of logic (number of LUTs passed through as
indicated by the Tilo times) or can get the slices to pack closer
together with floorplanning, RLOCed macros, or better placement
methods (each is a slightly advanced design aspect) you should get
better times. I've kept the "rule of thumb" that a 50% logic / 50%
route is a "good" mix and longer routes can be cut down toward that
50/50 mix. Your numbers show on the bottom of your report item as
"(39.0% logic, 61.0% route)".

Enjoy the journey.
- John_H