ACEX 1K Programmable Logic Device Family Data Sheet
Clearable Counter Mode
The clearable counter mode is similar to the up/ down counter mode, but
it supports a synchronous clear instead of the up/ down control. The clear
function is substituted for the cascade-in signal in the up/ down counter
mode. Two 3-input LUTs are used; one generates the counter data, and the
other generates the fast carry bit. Synchronous loading is provided by a
2-to-1 multiplexer. The output of this multiplexer is ANDed with a
synchronous clear signal.
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 OEsignals are active, contending signals can be
driven onto the bus. Conversely, if no OEsignals 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.
13
Clear & Preset Logic Control
Logic for the programmable register’s clear and preset functions is
controlled by the DATA3, LABCTRL1, and LABCTRL2inputs to the LE. The
clear and preset control structure of the LE asynchronously loads signals
into a register. Either LABCTRL1or LABCTRL2can control the
asynchronous clear. Alternatively, the register can be set up so that
LABCTRL1implements an asynchronous load. The data to be loaded is
driven to DATA3; when LABCTRL1is asserted, DATA3is loaded into the
register.
During compilation, the 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
modes chosen during design entry:
■
■
■
■
■
■
Asynchronous clear
Asynchronous preset
Asynchronous clear and preset
Asynchronous load with clear
Asynchronous load with preset
Asynchronous load without clear or preset
Altera Corporation
23
ALTERA [ ALTERA CORPORATION ]
