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22 Xilinx Development System
Xilinx System Generator v2.1 Reference Guide
In the Simulink environment, the Override with Doubles option allows you to
simulate the entire design in double precision floating point.
This option is useful in selecting fixed point widths or when debugging. If you detect
unacceptable quahtization errors with fixed point signals, you can choose to simulate
your entire design, or only specific blocks, using double precision floating point
signals and arithmetic operations. This option will help you discover which part of
your design is responsible for the unacceptable quantization error.
You may choose Override with Doubles on a particular block. You may also choose
this option for an entire sheet or an entire subsystem (the sheet plus underlying
hierarchy) by instantiating a System Generator token on the sheet, and choosing
Override with Doubles as one of the System Generator block’s configurable
parameters.
When the output of one block with Override with Doubles set is connected to the
input of another block where the option is also set, data samples are transmitted in
double precision.
You can easily identify which blocks are currently set to Override with Doubles.
When this option is set, affected Xilinx blocks are displayed in gray rather than the
normal blue or yellow.
Sample Period
Data streams are processed at a specific sample rate as they flow through Simulink.
Typically, each block detects the input sample rate and produces the correct sample
rate on its output. Xilinx blocks Up Sample and Down Sample provide a means to
increase or decrease sample rates.
Use Explicit Sample Period
If you select Use Explicit Sample Period rather than the default, you may set the
sample period required for all the block outputs. This is useful when implementing
features such as feedback loops in your design. In a feedback loop, it is not possible
for the System Generator to determine a default sample rate, because the loop makes
an input sample rate depend on a yet-to-be-determined output sample rate. System
Generator under these circumstances requires you to supply a hint to establish sample
periods throughout a loop.
The following image (the Concat block’s parameters dialog box) shows the options
with Use Explicit Sample Period selected.
Figure 2-3: Use Explicit Sample Period options (available if selected)