The output macrocell sends its output back to the switch
matrix, via internal feedback, and to the I/O cell. The
feedback is always available regardless of the configu-
ration of the I/O cell. This allows for buried combinatorial
or registered functions, freeing up the I/O pins for use as
inputs if not needed as outputs. The basic output
macrocell configurations are shown in Figure 4.
The clock/latch-enable for each individual output mac-
rocell can be driven by one of four signals. Two of the
signals are provided by the global clock pin CLK
0
/
LE
0
;
either polarity may be chosen. The other two signals
come from a product term provided for each output
macrocell. Either polarity of the logic generated by the
product term can be chosen. The global clock pin is also
available as an input, although care must be taken when
a signal acts as both clock and input to the same device.
Each individual output macrocell also has a product
term for asynchronous reset and a product term for
asynchronous preset. This means that any register or
latch may be reset or preset without affecting any other
register or latch in the device. The functionality of the
flip-flops with respect to initialization is illustrated in
Table 3.
Table 3. Asynchronous Reset/Preset Operation
AR
0
0
1
1
AP
0
1
0
1
CLK/LE
X
X
X
X
Q+
See Table 12
1
0
0
The input macrocell (Figure 5) consists of a flip-flop that
can be used to provide registered or latched inputs. The
flip-flop can be clocked by either polarity of one of the
two global clock/latch-enable pins.
Reset or preset are not provided for these flip-flops. If
combinatorial inputs are desired, this macrocell is not
used, and the feedback from the I/O pin is used directly.
Both the I/O pin feedback and the output of the input
register or latch are always available to the switch
matrix.
Possible input macrocell configurations are shown in
Figure 6.
The I/O Cell
The I/O cell (Figure 7) provides a three-state output
buffer. The three-state control is provided by an
individual product term for each I/O cell. Depending on
the logic programmed onto this product term, the I/O pin
can be configured as an output, an input, or a
bidirectional pin. The feedback from the I/O pin is always
available to the switch matrix, regardless of the state of
the output buffer or the output macrocell.
MACH215-12/15/20
7
LATTICE [ LATTICE SEMICONDUCTOR ]
