Clock Conditioning Circuits in IGLOO and ProASIC3 Devices
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Use quadrant global region assignments by finding the clock net associated with the CCC
macro under the Nets tab and creating a quadrant global region for the net, as shown in
External I/O–Driven CCCs
The above-mentioned recommendation for proper layout techniques will ensure the correct
assignment. It is possible that, especially with External I/O–Driven CCC macros, placement of the
CCC macro in a desired location may not be achieved. For example, assigning an input port of an
External I/O–Driven CCC near a particular CCC location does not guarantee global assignments to
the desired location. This is because the clock inputs of External I/O–Driven CCCs can be assigned to
any I/O location; therefore, it is possible that the CCC connected to the clock input will be routed to
a location other than the one closest to the I/O location, depending on resource availability and
placement constraints.
Clock Placer
The clock placer is a placement engine for low-power flash devices that places global signals on the
chip global and quadrant global networks. Based on the clock assignment constraints for the chip
global and quadrant global clocks, it will try to satisfy all constraints, as well as creating quadrant
clock regions when necessary. If the clock placer fails to create the quadrant clock regions for the
global signals, it will report an error and stop Layout.
The user must ensure that the constraints set to promote clock signals to quadrant global networks
are valid.
Cascading CCCs
The CCCs in low-power flash devices can be cascaded. Cascading CCCs can help achieve more
accurate PLL output frequency results than those achievable with a single CCC. In addition, this
technique is useful when the user application requires the output clock of the PLL to be a multiple
of the reference clock by an integer greater than the maximum feedback divider value of the PLL
(divide by 128) to achieve the desired frequency.
For example, the user application may require a 280 MHz output clock using a 2 MHz input
Figure 4-29 Quadrant Clock Assignment for a Global Net