Flip-flop
The ATF15xxAE Family’s flip-flop has very flexible data and control functions. The data input
can come from either the XOR gate, from a separate product term or directly from the I/O pin.
Selecting the separate product term allows creation of a buried registered feedback within a
combinatorial output or vice-versa. (This enhanced function is automatically implemented by
the fitter software). The flip-flop can be configured for D, T, JK and SR operation, and changes
state on the clock’s rising edge. It can also be configured as a flow-through latch. In this mode,
data passes through when the clock is high and is latched when the clock is low.
When a GCK signal is used as the clock, one of the macrocell product terms can be selected
as a clock enable. When the clock enable function is active and the enable signal (product
term) is low, all clock edges are ignored. The flip-flop has asynchronous reset and preset. The
flip-flop’s asynchronous reset signal (AR) can be either the Global Clear (GCLEAR), a product
term, or always off. AR can also be a logic OR of GCLEAR with a product term. The asynchro-
nous preset (AP) can be a product term or always off.
Output Buffer
The ATF15xxAE Family macrocell output can be selected as registered or combinatorial. The
extra buried feedback signal can be either combinatorial or registered signal regardless of
whether the output is combinatorial or registered. (This enhanced function is automatically
implemented by the fitter software) Feedback of a buried combinatorial output allows the cre-
ation of a second latch within a macrocell.
The output enable multiplexer (MOE) controls the output enable signals. Any buffer can be
permanently enabled for simple output operation. Buffers can also be permanently disabled to
allow use of the pin as an input. In this configuration, all the macrocell resources are still avail-
able, including the buried feedback, expander and CASCADE logic. The output enable for
each macrocell can be selected as one of six global OE signals or a product term. In addition,
some product term feedbacks can generate one of the global output enables.
The buffer has a fast/slow slew rate option to control EMI and an open-collector option which
enables the device to provide control signals such as an interrupt that can be asserted by any
of the several devices.
Programmable
Pin-keeper Option
for Inputs and I/Os
The ATF15xxAE Family offers the option of programming all input and I/O pins with pin-keeper
circuits enabled. When any pin is driven high or low and then subsequently left floating, the pin
keeper circuit will hold it at that previous high or low-level. This circuitry prevents unused input
and I/O lines from floating to intermediate voltage levels, which causes unnecessary power
consumption and system noise. The pin-keeper circuits eliminate the need for external pull-up
resistors and eliminate their DC power consumption.
6
ATF15xxAE Family
2398E–12/01
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