R
XC3000 Series Field Programmable Gate Arrays
Flexible routing allows use of common or individual CLB
clocking.
A
B
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 combinato-
rial propagation delay through the network is independent
of the logic function generated and is spike free for single
input variable changes. This technique can generate two
independent logic functions of up to four variables each as
shown in Figure 6a, or a single function of five variables as
shown in Figure 6b, or some functions of seven variables
as shown in Figure 6c. Figure 7 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 imple-
mented 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 vari-
ables, the F and G outputs are identical. Symmetry 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.
QX
Any Function
of Up to 4
Variables
QY
F
C
D
E
A
B
QX
Any Function
of Up to 4
Variables
QY
G
C
D
E
5a
A
B
QX
F
Any Function
of 5 Variables
QY
G
C
D
E
5b
A
B
QX
Any Function
of Up to 4
Variables
Programmable Interconnect
QY
C
D
Programmable-interconnection resources in the Field Pro-
grammable Gate Array provide routing paths to connect
inputs and outputs of the IOBs and CLBs into logic net-
works. Interconnections between blocks are composed of a
two-layer grid of metal segments. Specially designed pass
transistors, each controlled by a configuration bit, form pro-
grammable interconnect points (PIPs) and switching matri-
ces used to implement the necessary connections between
selected metal segments and block pins. Figure 8 is an
example of a routed net. The development system provides
automatic routing of these interconnections. Interactive
routing is also available for design optimization. The inputs
of the CLBs or IOBs are multiplexers which can be pro-
grammed to select an input network from the adjacent
interconnect segments. Since the switch connections to
block inputs are unidirectional, as are block outputs,
they are usable only for block input connection and not
for routing. Figure 9 illustrates routing access to logic
block input variables, control inputs and block outputs.
Three types of metal resources are provided to accommo-
date various network interconnect requirements.
F
M
U
X
A
B
G
QX
Any Function
of Up to 4
Variables
QY
C
D
FGM
Mode
E
5c
X5442
Figure 6: Combinational Logic Options
6a. Combinatorial Logic Option FG generates two func-
tions of four variables each. One variable, A, must be
common to both functions. The second and third variable
can be any choice of B, C, QX and QY. The fourth vari-
able can be any choice of D or E.
6b. Combinatorial Logic Option F generates any function
of five variables: A, D, E and two choices out of B, C, QX,
QY.
6c. Combinatorial Logic Option FGM allows variable E to
select between two functions of four variables: Both have
common inputs A and D and any choice out of B, C, QX
and QY for the remaining two variables. Option 3 can
then implement some functions of six or seven variables.
•
•
•
General Purpose Interconnect
Direct Connection
Longlines (multiplexed busses and wide AND gates)
7-10
November 9, 1998 (Version 3.1)
XILINX [ XILINX, INC ]
