OR2C12A-2S240I View Datasheet(PDF) - Agere -> LSI Corporation
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Manufacturer
OR2C12A-2S240I Datasheet PDF : 192 Pages
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Data Sheet
June 1999
ORCA Series 2 FPGAs
Clock Distribution Network (continued)
Alternatively, the clock can be routed from the spine to
the branches by using the BIDIs instead of the long-line
drivers. This results in added delay in the clock net, but
the clock skew is approximately equal to the clock
routed using the long-line drivers. This method can be
used to create a clock that is used in only one quad-
rant. The XH lines act as a clock spine, which is then
routed to perpendicular XH lines (the branches) using
the BIDIHs.
Clock signals, such as the output of a counter, can also
be generated in PLCs and routed onto an XL line,
which then acts as a clock spine. Although the clock
can be generated in any PLC, it is recommended that
the clock be located as close to the center of the FPGA
as possible to minimize clock skew.
The following equation can be used to determine pin
names:
Pad number = P[RL][TB]n ± (i x 4)[A – D]
Where i = 1—8, and n is the current PIC number.
For more information, please refer to Utilizing the
ORCA® OR2C/TxxA Clock Distribution Network Appli-
cation Note (AP97-055FPGA).
SEE DETAIL A
CLK PIN
SEE DETAIL B
CLOCK
BRANCHES
Selecting Clock Input Pins
Any user I/O pin on an ORCA FPGA can be used as a
very fast, low-skew clock input. Choosing the first clock
pin is completely arbitrary, but using a pin that is near
the center of an edge of the device (as shown in Fig-
ures 34 and 35) will provide the lowest skew clock net-
work. The pin-to-pin timing numbers in the Timing
Characteristics section of this data book assume that
the clock pin is in one of the four PICs at the center of
any side of the device.
Once the first clock pin has been chosen, there are
only two sets of pins (within the center four PICs on
each side of the device) that should not be chosen as
the second clock pin: a pin from the same PIC, and/or a
pin from the PIC on the exact opposite edge of the die
(i.e., if a pin from a PIC on the top edge is chosen for
the first clock, the same PIC on the bottom edge should
not be chosen for the second clock).
These rules should be followed iteratively until a total of
eight clocks (or other global signals) have been
selected: four from the left/right sides of the device, and
four from the top/bottom sides of the device. If more
than eight clocks are needed, then select another pin
outside the center four PICs to use primary-clock rout-
ing, use secondary clock routing for any pin, or use
local clock routing.
If it is desired to use a pin for one of the first eight
clocks that is not within the center four PICs of any side
of the device and primary clock routing is desired, the
pad names (see Pin Information) of the two clock pins
on the top or bottom of the device cannot be a multi-
plier of four PICs away. The same rule applies to clock
pins on the left or right side of the device.
Lucent Technologies Inc.
CLOCK SPINE
PA
PB
PC
PD
DT
DT
DT
DT
DETAIL A
HCK
VCK
PFU
DETAIL B
5-4481(F).r2
Figure 35. Secondary Clock Distribution
39
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