R
XC5200 Series Field Programmable Gate Arrays
can also be independently disabled for any flip-flop. CLR is
active High. It is not invertible within the CLB.
Three-State Buffers
The XC5200 family has four dedicated Three-State Buffers
(TBUFs, or BUFTs in the schematic library) per CLB (see
Figure 9). The four buffers are individually configurable
through four configuration bits to operate as simple
non-inverting buffers or in 3-state mode. When in 3-state
mode the CLB output enable (TS) control signal drives the
enable to all four buffers. Each TBUF can drive up to two
horizontal and/or two vertical Longlines. These 3-state buff-
ers can be used to implement multiplexed or bidirectional
buses on the horizontal or vertical longlines, saving logic
resources.
STARTUP
GR
GTS
PAD
Q2
Q3
Q1Q4
IBUF
DONEIN
CLK
X9009
Figure 8: Schematic Symbols for Global Reset
Global Reset
The 3-state buffer enable is an active-High 3-state (i.e. an
active-Low enable), as shown in Table 4.
A separate Global Reset line clears each storage element
during power-up, reconfiguration, or when a dedicated
Reset net is driven active. This global net (GR) does not
compete with other routing resources; it uses a dedicated
distribution network.
Table 4: Three-State Buffer Functionality
IN
X
T
1
0
OUT
Z
GR can be driven from any user-programmable pin as a
global reset input. To use this global net, place an input pad
and input buffer in the schematic or HDL code, driving the
GR pin of the STARTUP symbol. (See Figure 9.) A specific
pin location can be assigned to this input using a LOC
attribute or property, just as with any other user-program-
mable pad. An inverter can optionally be inserted after the
input buffer to invert the sense of the Global Reset signal.
Alternatively, GR can be driven from any internal node.
IN
IN
Another 3-state buffer with similar access is located near
each I/O block along the right and left edges of the array.
The longlines driven by the 3-state buffers have a weak
keeper at each end. This circuit prevents undefined float-
ing levels. However, it is overridden by any driver. To
ensure the longline goes high when no buffers are on, add
an additional BUFT to drive the output High during all of the
previously undefined states.
Using FPGA Flip-Flops and Latches
Figure 10 shows how to use the 3-state buffers to imple-
ment a multiplexer. The selection is accomplished by the
buffer 3-state signal.
The abundance of flip-flops in the XC5200 Series invites
pipelined designs. This is a powerful way of increasing per-
formance by breaking the function into smaller subfunc-
tions and executing them in parallel, passing on the results
through pipeline flip-flops. This method should be seriously
considered wherever throughput is more important than
latency.
TS
To include a CLB flip-flop, place the appropriate library
symbol. For example, FDCE is a D-type flip-flop with clock
enable and asynchronous clear. The corresponding latch
symbol is called LDCE.
CLB
CLB
LC3
LC2
LC1
LC0
In XC5200-Series devices, the flip-flops can be used as
registers or shift registers without blocking the function
generators from performing a different, perhaps unrelated
task. This ability increases the functional capacity of the
devices.
The CLB setup time is specified between the function gen-
erator inputs and the clock input CK. Therefore, the speci-
fied CLB flip-flop setup time includes the delay through the
function generator.
Horizontal
Longlines
X9030
Figure 9: XC5200 3-State Buffers
7-90
November 5, 1998 (Version 5.2)
ETC [ ETC ]
