FLEX 6000 Programmable Logic Device Family Data Sheet
Either the counter enable or the up/down control may be used for a given
counter. Moreover, the synchronous load can be used as a count enable by
routing the register output into the data input automatically when
requested by the designer.
The second LE of each LAB has a special function for counter mode; the
carry-in of the LE can be driven by a fast feedback path from the register.
This function gives a faster counter speed for counter carry chains starting
in the second LE of an LAB.
The Altera software implements functions to use the counter mode
automatically where appropriate. The designer does not have to decide
how the carry chain will be used.
Internal Tri-State Emulation
Internal tri-state emulation provides internal tri-states without the
limitations of a physical tri-state bus. In a physical tri-state bus, the
tri-state buffers’ output enable (OE) signals select which signal drives the
bus. However, if multiple OE signals are active, contending signals can be
driven onto the bus. Conversely, if no OE 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 Altera 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 LAB-wide signals LABCTRL1 and LABCTRL2. The LE
register has an asynchronous clear that can implement an asynchronous
preset. Either LABCTRL1 or LABCTRL2 can control the asynchronous clear
or preset. Because the clear and preset functions are active-low, the Altera
software automatically assigns a logic high to an unused clear or preset
signal. The clear and preset logic is implemented in either the
asynchronous clear or asynchronous preset mode, which is chosen during
design entry (see Figure 8).
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