MATLAB DESIGN HDL CODER RELEASE NOTES User's Guide Page 26

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26 www.xilinx.com System Generator for DSP User Guide
UG640 (v 12.2) July 23, 2010
Chapter 1: Hardware Design Using System Generator
Hardware Oversampling
Some System Generator blocks are oversampled, i.e., their internal processing is done at a
rate that is faster than their data rates. In hardware, this means that the block requires more
than one clock cycle to process a data sample. In Simulink such blocks do not have an
observable effect on sample rates.
One block that can be oversampled is the DAFIR FIR filter. An oversampled DAFIR
processes samples serially, thus running at a higher rate, but using less hardware.
Although blocks that are oversampled do not cause an explicit sample rate change in
Simulink, System Generator considers the internal block rate along with all other sample
rates when generating clocking logic for the hardware implementation. This means that
you must consider the internal processing rates of oversampled blocks when you specify
the Simulink system period value in the System Generator block dialog box.
Asynchronous Clocking
System Generator focuses on the design of hardware that is synchronous to a single clock.
It can, under some circumstances, be used to design systems that contain more than one
clock. This is possible provided the design can be partitioned into individual clock
domains with the exchange of information between domains being regulated by dual port
memories and FIFOs. System Generator fully supports such multi-clock designs, including
the ability to simulate them in Simulink and to generate complete hardware descriptions.
Details are discussed in the topic Generating Multiple Cycle-True Islands for Distinct
Clocks. The remainder of this topic focuses exclusively on the clock-synchronous aspects
of System Generator. This discussion is relevant to both single-clock and multiple-clock
designs.
Synchronous Clocking
As shown in the figure below, when you use the System Generator token to compile a
design into hardware, there are three clocking options for Multirate implementation: (1)
Clock Enables (the default), (2) Hybrid DCM-CE, and (3) Expose Clock Ports.
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