XC3000FM Datasheet, PDF (9/50 Pages) Xilinx, Inc – XC3000 Logic Cell Array Families

asynchronous RD which, when enabled and High, is
dominant over clocked inputs. All flip-flops are reset by the
active-Low chip input, RESET, or during the configuration
process. The flip-flops share the enable clock (EC) which,
when Low, recirculates the flip-flops’ present states and
inhibits response to the data-in or combinatorial function
inputs on a CLB. The user may enable these control inputs
and select their sources. The user may also select the
clock net input (K), as well as its active sense within each
CLB. This programmable inversion eliminates the need to
route both phases of a clock signal throughout the device.
Flexible routing allows use of common or individual CLB
clocking.
The combinatorial-logic portion of the CLB uses a 32 by 1
look-up table to implement Boolean functions. Variables
selected from the five logic inputs and two internal block
flip-flops are used as table address inputs. The combina-
torial propagation delay through the network is indepen-
dent of the logic function generated and is spike free for
single input variable changes. This technique can gener-
ate two independent logic functions of up to four variables
each as shown in Figure 5a, or a single function of five
variables as shown in Figure 5b, or some functions of
seven variables as shown in Figure 5c. Figure 6 shows a
modulo-8 binary counter with parallel enable. It uses one
CLB of each type. The partial functions of six or seven
variables are implemented using the input variable (E) to
dynamically select between two functions of four different
variables. For the two functions of four variables each, the
independent results (F and G) may be used as data inputs
to either flip-flop or either logic block output. For the single
function of five variables and merged functions of six or
seven variables, the F and G outputs are identical. Sym-
metry of the F and G functions and the flip-flops allows the
interchange of CLB outputs to optimize routing efficiencies
of the networks interconnecting the CLBs and IOBs.
Programmable Interconnect
Programmable-interconnection resources in the Logic
Cell Array provide routing paths to connect inputs and
outputs of the IOBs and CLBs into logic networks. Inter-
connections between blocks are composed of a two-layer
grid of metal segments. Specially designed pass transis-
tors, each controlled by a configuration bit, form program-
mable interconnect points (PIPs) and switching matrices
used to implement the necessary connections between
selected metal segments and block pins. Figure 7 is an
example of a routed net. The XACT development system
provides automatic routing of these interconnections. In-
teractive routing (Editnet) is also available for design
optimization. The inputs of the CLBs or IOBs are multiplex-
ers which can be programmed to select an input network
from the adjacent interconnect segments. Since the
switch connections to block inputs are unidirec-
tional, as are block outputs, they are usable only for
block input connection and not for routing. Figure 8
illustrates routing access to logic block input variables,
control inputs and block outputs. Three types of metal
resources are provided to accommodate various network
interconnect requirements.
• General Purpose Interconnect
• Direct Connection
• Longlines (multiplexed busses and wide AND gates)
General Purpose Interconnect
General purpose interconnect, as shown in Figure 9,
consists of a grid of five horizontal and five vertical metal
segments located between the rows and columns of logic
and IOBs. Each segment is the height or width of a logic
block. Switching matrices join the ends of these segments
and allow programmed interconnections between the
metal grid segments of adjoining rows and columns. The
switches of an unprogrammed device are all non-
conducting. The connections through the switch matrix
may be established by the automatic routing or by using
Editnet to select the desired pairs of matrix pins to be
connected or disconnected. The legitimate switching
matrix combinations for each pin are indicated in Figure 10
and may be highlighted by the use of the Show-Matrix
command in the XACT system.
Figure 7.
An XACT view of routing resources used to form a typical
interconnection network from CLB GA.
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