2-11
Speed Is Enhanced Two Ways
Delays in LCA-based designs are layout dependent. While
this makes it hard to predict a worst-case guaranteed
performance, there is a rule of thumb designers can
consider — the system clock rate should not exceed one
third to one half of the specified toggle rate. Critical
portions of a design, shift registers and simple counters,
can run faster — approximately two thirds of the specified
toggle rate.
The XC4000 family can run at synchronous system clock
rates of up to 60 MHz. This increase in performance over
the previous families stems from two basic improve-
ments: improved architecture and more abundant routing
resources.
Improved Architecture
More Inputs
: The versatility of the CLB function genera-
tors improves system speed significantly. Table 3 shows
how the XC4000 families implement many functions more
efficiently and faster than is possible with XC3000 devices.
A 9-bit parity checker, for example, can be implemented in
one CLB with a propagation delay of 7 ns. Using a
XC3000-family device, the same function requires two
CLBs with a propagation delay of 2 x 5.5 ns = 11 ns. One
XC4000 CLB can determine whether two 4-bit words are
identical, again with a 7-ns propagation delay. The ninth
input can be used for simple ripple expansion of this
identity comparator (25.5 ns over 16 bits, 51.5 ns over
32 bits), or a 2-layer identity comparator can generate the
result of a 32-bit comparison in 15 ns, at the cost of a single
extra CLB. Simpler functions like multiplexers also benefit
from the greater flexibility of the XC4000-families CLB. A
16-input multiplexer uses 5 CLBs and has a delay of only
13.5 ns.
More Outputs:
The CLB can pass the combinatorial
output(s) to the interconnect network, but can also store
the combinatorial result(s) or other incoming data in one or
two flip-flops, and connect their outputs to the interconnect
network as well. With XC3000-families CLBs the designer
has to make a choice, either output the combinatorial
function or the stored value. In the XC4000 families, the flip
flops can be used as registers or shift registers without
blocking the function generators from performing a differ-
ent, perhaps unrelated task. This increases the functional
density of the devices.
When a function generator drives a flip-flop in a CLB, the
combinatorial propagation delay
overlaps completely
with
the set-up time of the flip-flop. The set-up time is specified
between the function generator inputs and the clock input.
This represents a performance advantage over competing
technologies where combinatorial delays must be added
to the flip-flop set-up time.
Fast Carry:
As described earlier, each CLB includes high-
speed carry logic that can be activated by configuration.
The two 4-input function generators can be configured as
a 2-bit adder with built-in hidden carry that can be ex-
panded to any length. This dedicated carry circuitry is so
fast and efficient that conventional speed-up methods like
carry generate/propagate are meaningless even at the
16-bit level, and of marginal benefit at the 32-bit level.
A 16-bit adder requires nine CLBs and has a combinatorial
carry delay of 20.5 ns. Compare that to the 30 CLBs and
50 ns, or 41 CLBs and 30 ns in the XC3000 family.
The fast-carry logic opens the door to many new applica-
tions involving arithmetic operation, where the previous
generations of FPGAs were not fast and/or not efficient
enough. High-speed address offset calculations in micro-
processor or graphics systems, and high-speed addition in
digital signal processing are two typical applications.
Faster and More Efficient Counters:
The XC4000-fami-
lies fast-carry logic puts two counter bits into each CLB and
runs them at a clock rate of up to 42 MHz for 16 bits,
whether the counters are loadable or not. For a 16-bit
Table 3. Density and Performance for Several Common Circuit Functions
XC3000 (-125) XC4000 (-5)
16-bit Decoder From Input Pad 15 ns 4 CLBs 12 ns 0 CLBs
24-bit Accumulator 17 MHz 46 CLBs 32 MHz 13 CLBs
State Machine Benchmark* 18 MHz 34 CLBs 30 MHz 26 CLBs
16:1 Multiplexer 16 ns 8 CLBs 16 ns 5 CLBs
16-bit Unidirectional Max Density 20 MHz 16 CLBs 40 MHz 8 CLBs
Loadable Counter Max Speed 34 MHz 23 CLBs 42 MHz 9 CLBs
16-bit U/D Counter Max Density 20 MHz 16 CLBs 40 MHz 8 CLBs
Max Speed 30 MHz 27 CLBs 40 MHz 8 CLBs
16-bit Adder Max Density 50 ns 30 CLBs 20.5 ns 9 CLBs
Max Speed 30 ns 41 CLBs 20.5 ns 9 CLBs
* 16 states, 40 transitions, 10 inputs, 8 outputs
