R
XC3000 Series Field Programmable Gate Arrays
Configurable Logic Block
The array of CLBs provides the functional elements from
which the user’s logic is constructed. The logic blocks are
arranged in a matrix within the perimeter of IOBs. For
example, the XC3020A has 64 such blocks arranged in 8
rows and 8 columns. The development system is used to
compile the configuration data which is to be loaded into
the internal configuration memory to define the operation
and interconnection of each block. User definition of CLBs
and their interconnecting networks may be done by auto-
matic translation from a schematic-capture logic diagram or
optionally by installing library or user macros.
Each CLB has a combinatorial logic section, two flip-flops,
and an internal control section. See
There are:
five logic inputs (A, B, C, D and E); a common clock input
(K); an asynchronous direct RESET input (RD); and an
enable clock (EC). All may be driven from the interconnect
resources adjacent to the blocks. Each CLB also has two
outputs (X and Y) which may drive interconnect networks.
Data input for either flip-flop within a CLB is supplied from
the function F or G outputs of the combinatorial logic, or the
block input, DI. Both flip-flops in each CLB share the asyn-
chronous 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.
DI
DATA IN
0
MUX
F
DIN
G
RD
QX
A
B
LOGIC
VARIABLES
C
D
E
QY
F
DIN
G
0
MUX
1
D
Q
QY
COMBINATORIAL
FUNCTION
G
G
Y
CLB OUTPUTS
F
F
QX
X
1
D
Q
7
EC
ENABLE CLOCK
1 (ENABLE)
RD
K
CLOCK
DIRECT
RESET
RD
0 (INHIBIT)
(GLOBAL RESET)
X3032
Figure 5: Configurable Logic Block.
Each CLB includes a combinatorial logic section, two flip-flops and a program memory controlled multiplexer selection of
function. It has the following:
- five logic variable inputs A, B, C, D, and E
- a direct data in DI
- an enable clock EC
- a clock (invertible) K
- an asynchronous direct RESET RD
- two outputs X and Y
November 9, 1998 (Version 3.1)
7-9
XILINX [ XILINX, INC ]
