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The oscillator output is optionally available after configura-
tion. Any two of four resynchronized taps of a built-in
divider are also available. These taps are at the fourth,
ninth, fourteenth and nineteenth bits of the divider. There-
fore, if the primary oscillator output is running at the nomi-
nal 8 MHz, the user has access to an 8 MHz clock, plus any
two of 500 kHz, 16kHz, 490Hz and 15Hz (up to 10% lower
for low-voltage devices). These frequencies can vary by as
much as -50% or +25%.
These signals can be accessed by placing the OSC4
library element in a schematic or in HDL code (see
Figure
25).
The oscillator is automatically disabled after configuration if
the OSC4 symbol is not used in the design.
Programmable Interconnect
All internal connections are composed of metal segments
with programmable switching points and switching matri-
ces to implement the desired routing. A structured, hierar-
chical matrix of routing resources is provided to achieve
efficient automated routing.
The XC4000E and XC4000X share a basic interconnect
structure. XC4000X devices, however, have additional
routing not available in the XC4000E. The extra routing
resources allow high utilization in high-capacity devices. All
XC4000X-specific routing resources are clearly identified
throughout this section. Any resources not identified as
XC4000X-specific are present in all XC4000 Series
devices.
This section describes the varied routing resources avail-
able in XC4000 Series devices. The implementation soft-
ware automatically assigns the appropriate resources
based on the density and timing requirements of the
design.
Interconnect Overview
There are several types of interconnect.
• CLB routing is associated with each row and column of
the CLB array.
• IOB routing forms a ring (called a VersaRing) around
the outside of the CLB array. It connects the I/O with the
internal logic blocks.
March 30, 1998 (Version 1.5)
• Global routing consists of dedicated networks primarily
designed to distribute clocks throughout the device with
minimum delay and skew. Global routing can also be
used for other high-fanout signals.
Five interconnect types are distinguished by the relative
length of their segments: single-length lines, double-length
lines, quad and octal lines (XC4000X only), and longlines.
In the XC4000X, direct connects allow fast data flow
between adjacent CLBs, and between IOBs and CLBs.
Extra routing is included in the IOB pad ring. The XC4000X
also includes a ring of octal interconnect lines near the
IOBs to improve pin-swapping and routing to locked pins.
XC4000E/X devices include two types of global buffers.
These global buffers have different properties, and are
intended for different purposes. They are discussed in
detail later in this section.
CLB Routing Connections
A high-level diagram of the routing resources associated
with one CLB is shown in
represent routing present only in XC4000X devices.
Table 15
shows how much routing of each type is available
in XC4000E and XC4000X CLB arrays. Clearly, very large
designs, or designs with a great deal of interconnect, will
route more easily in the XC4000X. Smaller XC4000E
designs, typically requiring significantly less interconnect,
do not require the additional routing.
Figure 28 on page 4-31
is a detailed diagram of both the
XC4000E and the XC4000X CLB, with associated routing.
The shaded square is the programmable switch matrix,
present in both the XC4000E and the XC4000X. The L-
shaped shaded area is present only in XC4000X devices.
As shown in the figure, the XC4000X block is essentially an
XC4000E block with additional routing.
CLB inputs and outputs are distributed on all four sides,
providing maximum routing flexibility. In general, the entire
architecture is symmetrical and regular. It is well suited to
established placement and routing algorithms. Inputs, out-
puts, and function generators can freely swap positions
within a CLB to avoid routing congestion during the place-
ment and routing operation.
Figure
26. The shaded arrows
4-29
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