MATLAB DESIGN HDL CODER RELEASE NOTES User's Guide Page 23

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System Generator for DSP User Guide www.xilinx.com 23
UG640 (v 12.2) July 23, 2010
System-Level Modeling in System Generator
description of its implementation and hardware resource requirements. Individual
documentation for each block is also provided in the topic Xilinx Reference Blockset
.
Signal Types
In order to provide bit-accurate simulation of hardware, System Generator blocks operate
on Boolean and arbitrary precision fixed-point values. By contrast, the fundamental scalar
signal type in Simulink is double precision floating point. The connection between Xilinx
blocks and non-Xilinx blocks is provided by gateway blocks. The gateway in converts a
double precision signal into a Xilinx signal, and the gateway out converts a Xilinx signal into
double precision. Simulink continuous time signals must be sampled by the Gateway In
block.
Most Xilinx blocks are polymorphic, i.e., they are able to deduce appropriate output types
based on their input types. When full precision is specified for a block in its parameters
dialog box, System Generator chooses the output type to ensure no precision is lost. Sign
extension and zero padding occur automatically as necessary. User-specified precision is
usually also available. This allows you to set the output type for a block and to specify how
quantization and overflow should be handled. Quantization possibilities include unbiased
rounding towards plus or minus infinity, depending on sign, or truncation. Overflow
options include saturation, truncation, and reporting overflow as an error.
Note:
System Generator data types can be displayed by selecting Format > Port Data Types in
Simulink. Displaying data types makes it easy to determine precision throughout a model. If, for
example, the type for a port is Fix_11_9, then the signal is a two's complement signed 11-bit number
having nine fractional bits. Similarly, if the type is Ufix_5_3, then the signal is an unsigned 5-bit
number having three fractional bits.
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