March 30, 1998 (Version 1.5)
4-27
or clear on reset and after configuration. Other than the glo-
bal GSR net, no user-controlled set/reset signal is available
to the I/O flip-flops. The choice of set or clear applies to
both the initial state of the flip-flop and the response to the
Global Set/Reset pulse. See
for a description of how to use GSR.
JTAG Support
Embedded logic attached to the IOBs contains test struc-
tures compatible with IEEE Standard 1149.1 for boundary
scan testing, permitting easy chip and board-level testing.
More information is provided in
.
Three-State Buffers
A pair of 3-state buffers is associated with each CLB in the
array. (See
.) These 3-state buffers
can be used to drive signals onto the nearest horizontal
longlines above and below the CLB. They can therefore be
used to implement multiplexed or bidirectional buses on the
horizontal longlines, saving logic resources. Programmable
pull-up resistors attached to these longlines help to imple-
ment a wide wired-AND function.
The buffer enable is an active-High 3-state (i.e. an active-
Low enable), as shown in
.
Another 3-state buffer with similar access is located near
each I/O block along the right and left edges of the array.
(See
.)
The horizontal longlines driven by the 3-state buffers have
a weak keeper at each end. This circuit prevents undefined
floating levels. However, it is overridden by any driver, even
a pull-up resistor.
Special longlines running along the perimeter of the array
can be used to wire-AND signals coming from nearby IOBs
or from internal longlines. These longlines form the wide
edge decoders discussed in
.
Three-State Buffer Modes
The 3-state buffers can be configured in three modes:
•
Standard 3-state buffer
•
Wired-AND with input on the I pin
•
Wired OR-AND
Standard 3-State Buffer
All three pins are used. Place the library element BUFT.
Connect the input to the I pin and the output to the O pin.
The T pin is an active-High 3-state (i.e. an active-Low
enable). Tie the T pin to Ground to implement a standard
buffer.
Wired-AND with Input on the I Pin
The buffer can be used as a Wired-AND. Use the WAND1
library symbol, which is essentially an open-drain buffer.
WAND4, WAND8, and WAND16 are also available. See
the
XACT Libraries Guide for further information.
The T pin is internally tied to the I pin. Connect the input to
the I pin and the output to the O pin. Connect the outputs of
all the WAND1s together and attach a PULLUP symbol.
Wired OR-AND
The buffer can be configured as a Wired OR-AND. A High
level on either input turns off the output. Use the
WOR2AND library symbol, which is essentially an open-
drain 2-input OR gate. The two input pins are functionally
equivalent. Attach the two inputs to the I0 and I1 pins and
tie the output to the O pin. Tie the outputs of all the
WOR2ANDs together and attach a PULLUP symbol.
Three-State Buffer Examples
shows how to use the 3-state buffers to imple-
ment a wired-AND function. When all the buffer inputs are
High, the pull-up resistor(s) provide the High output.
shows how to use the 3-state buffers to imple-
ment a multiplexer. The selection is accomplished by the
buffer 3-state signal.
Pay particular attention to the polarity of the T pin when
using these buffers in a design. Active-High 3-state (T) is
identical to an active-Low output enable, as shown in
Table 14: Three-State Buffer Functionality
IN
T
OUT
X
1
Z
IN
0
IN
P
U
L
L
U
P
Z = D
A
●
D
B
●
(D
C
+D
D
)
●
(D
E
+D
F
)
D
E
D
F
D
C
D
D
D
B
D
A
WAND1
WAND1
WOR2AND
WOR2AND
X6465
Figure 22: Open-Drain Buffers Implement a Wired-AND Function
