Classic EPLD Family Data Sheet
The eight product terms of the programmable-AND array feed the 8-input
OR
gate, which then feeds one input to an
XOR
gate. The other input to the
XOR
gate is connected to a programmable bit that allows the array output
to be inverted. Altera’s MAX+PLUS II software uses the
XOR
gate to
implement either active-high or active-low logic, or De Morgan’s
inversion to reduce the number of product terms needed to implement a
function.
Programmable Registers
To implement registered functions, each macrocell register can be
individually programmed for D, T, JK, or SR operation. If necessary, the
register can be bypassed for combinatorial operation. During design
compilation, the MAX+PLUS II software selects the most efficient register
operation for each registered function to minimize the logic resources
needed by the design. Registers have an individual asynchronous clear
function that is controlled by a dedicated product term. These registers
are cleared automatically during power-up.
In addition, macrocell registers can be individually clocked by either a
global clock or any input or feedback path to the
AND
array. Altera’s
proprietary programmable I/O architecture allows the designer to
program output and feedback paths for combinatorial or registered
operation in both active-high and active-low modes. These features make
it possible to simultaneously implement a variety of logic functions.
Output Enable/Clock Select
Figure 2
shows the two operating modes (Modes 0 and 1) provided by the
output enable/clock (OE/CLK) select. The
OE/CLK
select, which is
controlled by a single programmable bit, can be individually configured
for each macrocell. In Mode 0, the tri-state output buffer is controlled by
a single product term. If the output enable is high, the output buffer is
enabled. If the output enable is low, the output has a high-impedance
value. In Mode 0, the macrocell flipflop is clocked by its global clock input
signal.
In Mode 1, the output enable buffer is always enabled, and the macrocell
register can be triggered by an array clock signal generated by a product
term. This mode allows registers to be individually clocked by any signal
on the
AND
array. With both true and complement signals in the
AND
array,
the register can be configured to trigger on a rising or falling edge. This
product-term-controlled clock configuration also supports gated clock
structures.
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