XC2C512 Datasheet, PDF (4/14 Pages) Xilinx, Inc – Industry’s best 0.18 micron CMOS CPLD

CoolRunner-II CPLD Family
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Function Block
The CoolRunner-II CPLD Function Blocks contain 16 mac-
rocells, with 40 entry sites for signals to arrive for logic cre-
ation and connection. The internal logic engine is a 56
product term PLA. All Function Blocks, regardless of the
number contained in the device, are identical. For a
high-level view of the Function Block, see Figure 2.
MC1
MC2
40
PLA
16
Out
To AIM
MC16
3
Global Global
Set/Reset Clocks
DS090_02_101001
Figure 2: CoolRunner-II CPLD Function Block
At the high level, it is seen that the product terms (p-terms)
reside in a programmable logic array (PLA). This structure
is extremely flexible, and very robust when compared to
fixed or cascaded product term function blocks.
Classic CPLDs typically have a few product terms available
for a high-speed path to a given macrocell. They rely on
capturing unused p-terms from neighboring macrocells to
expand their product term tally, when needed. The result of
this architecture is a variable timing model and the possibil-
ity of stranding unusable logic within the FB.
The PLA is different — and better. First, any product term
can be attached to any OR gate inside the FB macrocell(s).
Second, any logic function can have as many p-terms as
needed attached to it within the FB, to an upper limit of 56.
Third, product terms can be re-used at multiple macrocell
OR functions so that within a FB, a particular logical product
need only be created once, but can be re-used up to 16
times within the FB. Naturally, this plays well with the fitting
software, which identifies product terms that can be shared.
The software places as many of those functions as it can
into FBs, so it happens for free. There is no need to force
macrocell functions to be adjacent or any other restriction
save residing in the same FB, which is handled by the soft-
ware. Functions need not share a common clock, common
set/reset or common output enable to take full advantage of
the PLA. Also, every product term arrives with the same
time delay incurred. There are no cascade time adders for
putting more product terms in the FB. When the FB product
term budget is reached, there is a small interconnect timing
penalty to route signals to another FB to continue creating
logic. Xilinx design software handles all this automatically.
Macrocell
The CoolRunner-II CPLD macrocell is extremely efficient
and streamlined for logic creation. Users can develop sum
of product (SOP) logic expressions that comprise up to 40
inputs and span 56 product terms within a single function
block. The macrocell can further combine the SOP expres-
sion into an XOR gate with another single p-term expres-
sion. The resulting logic expression’s polarity is also
selectable. As well, the logic function can be pure combina-
torial or registered, with the storage element operating
selectably as a D or T flip-flop, or transparent latch. Avail-
able at each macrocell are independent selections of glo-
bal, function block level or local p-term derived clocks, sets,
resets, and output enables. Each macrocell flip-flop is con-
figurable for either single edge or DualEDGE clocking, pro-
viding either double data rate capability or the ability to
distribute a slower clock (thereby saving power). For single
edge clocking or latching, either clock polarity may be
selected per macrocell. CoolRunner-II macrocell details are
shown in Figure 3. Note that in Figure 3, standard logic
symbols are used except the trapezoidal multiplexers have
input selection from statically programmed configuration
select lines (not shown). Xilinx application note XAPP376
gives a detailed explanation of how logic is created in the
CoolRunner-II CPLD family.
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www.xilinx.com
DS090 (v1.7) October 2, 2003
1-800-255-7778
Preliminary Product Specification