EPF8452AQC160-3 Datasheet, PDF (13/62 Pages) Altera Corporation – Programmable Logic Device Family

FLEX 8000 Programmable Logic Device Family Data Sheet
Internal Tri-State Emulation
Internal tri-state emulation provides internal tri-stating without the
limitations of a physical tri-state bus. In a physical tri-state bus, the
tri-state buffers’ output enable signals select the signal that drives the bus.
However, if multiple output enable signals are active, contending signals
can be driven onto the bus. Conversely, if no output enable signals are
active, the bus will float. Internal tri-state emulation resolves contending
tri-state buffers to a low value and floating buses to a high value, thereby
eliminating these problems. The MAX+PLUS II software automatically
implements tri-state bus functionality with a multiplexer.
Clear & Preset Logic Control
Logic for the programmable register’s clear and preset functions is
controlled by the DATA3, LABCTRL1, and LABCTRL2 inputs to the LE. The
clear and preset control structure of the LE is used to asynchronously load
signals into a register. The register can be set up so that LABCTRL1
implements an asynchronous load. The data to be loaded is driven to
DATA3; when LABCTRL1 is asserted, DATA3 is loaded into the register.
3
During compilation, the MAX+PLUS II Compiler automatically selects
the best control signal implementation. Because the clear and preset
functions are active-low, the Compiler automatically assigns a logic high
to an unused clear or preset.
The clear and preset logic is implemented in one of the following six
asynchronous modes, which are chosen during design entry. LPM
functions that use registers will automatically use the correct
asynchronous mode. See Figure 7.
■ Clear only
■ Preset only
■ Clear and preset
■ Load with clear
■ Load with preset
■ Load without clear or preset
Altera Corporation
13