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XC4000E and XC4000X Series Field Programmable Gate Arrays
~100 kΩ
"Weak Keeper"
Figure 23: 3-State Buffers Implement a Multiplexer
Wide Edge Decoders
Dedicated decoder circuitry boosts the performance of
wide decoding functions. When the address or data field is
wider than the function generator inputs, FPGAs need
multi-level decoding and are thus slower than PALs.
XC4000 Series CLBs have nine inputs. Any decoder of up
to nine inputs is, therefore, compact and fast. However,
there is also a need for much wider decoders, especially for
address decoding in large microprocessor systems.
An XC4000 Series FPGA has four programmable decoders
located on each edge of the device. The inputs to each
decoder are any of the IOB I1 signals on that edge plus one
local interconnect per CLB row or column. Each row or col-
umn of CLBs provides up to three variables or their compli-
ments., as shown in
Figure
High output (resistor pull-up) when the AND condition of
the selected inputs, or their complements, is true. This is
analogous to a product term in typical PAL devices.
Each of these wired-AND gates is capable of accepting up
to 42 inputs on the XC4005E and 72 on the XC4013E.
There are up to 96 inputs for each decoder on the
XC4028X and 132 on the XC4052X.
also be split in two when a larger number of narrower
decoders are required, for a maximum of 32 decoders per
device.
The decoder outputs can drive CLB inputs, so they can be
combined with other logic to form a PAL-like AND/OR struc-
ture. The decoder outputs can also be routed directly to the
chip outputs. For fastest speed, the output should be on the
same chip edge as the decoder. Very large PALs can be
emulated by ORing the decoder outputs in a CLB. This
decoding feature covers what has long been considered a
weakness of older FPGAs. Users often resorted to external
PALs for simple but fast decoding functions. Now, the ded-
icated decoders in the XC4000 Series device can imple-
ment these functions fast and efficiently.
To use the wide edge decoders, place one or more of the
WAND library symbols (WAND1, WAND4, WAND8,
WAND16). Attach a DECODE attribute or property to each
WAND symbol. Tie the outputs together and attach a PUL-
4-28
D
A
BUFT
A
24. Each decoder generates a
The decoders may
Z = D
• A + D
• B + D
• C + D
• N
A
B
C
N
D
D
B
C
BUFT
B
C
LUP symbol. Location attributes or properties such as L
(left edge) or TR (right half of top edge) should also be used
to ensure the correct placement of the decoder inputs.
INTERCONNECT
IOB
.I1
A
Figure 24: XC4000 Series Edge Decoding Example
Figure 25: XC4000 Series Oscillator Symbol
On-Chip Oscillator
XC4000 Series devices include an internal oscillator. This
oscillator is used to clock the power-on time-out, for config-
uration memory clearing, and as the source of CCLK in
Master configuration modes. The oscillator runs at a nomi-
nal 8 MHz frequency that varies with process, Vcc, and
temperature. The output frequency falls between 4 and 10
MHz.
D
N
BUFT
BUFT
N
X6466
IOB
.I1
C
B
OSC4
F8M
F500K
F16K
F490
F15
X6703
March 30, 1998 (Version 1.5)
(
C) .....
(A • B • C) .....
(A • B • C) .....
(A • B • C) .....
X2627
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