Chapter 1: Hardware Design Using System Generator
Synchronization Mechanisms
System Generator does not make implicit synchronization mechanisms available. Instead, synchronization is the responsibility of the designer, and must be done explicitly.
Valid Ports
System Generator provides several blocks (in particular, a FIFO) that can be used for synchronization. Several blocks provide input (respectively, output) ports that specify when an input (resp., output) sample is valid. Such ports can be chained, affording a primitive form of flow control. Blocks with such ports include the FFT, FIR, and Viterbi.
Indeterminate Data
Indeterminate values are common in many hardware simulation environments. Often they are called “don’t cares” or “Xs”. In particular, values in System Generator simulations can be indeterminate. A dual port memory block, for example, can produce indeterminate results if both ports of the memory attempt to write the same address simultaneously.
What actually happens in hardware depends upon effectively random implementation details that determine which port sees the clock edge first. Allowing values to become indeterminate gives the system designer greater flexibility. Continuing the example, there is nothing wrong with writing to memory in an indeterminate fashion if subsequent processing does not rely on the indeterminate result.
HDL modules that are brought into the simulation through HDL co-simulation are a common source for indeterminate data samples. System Generator presents indeterminate values to the inputs of an HDL co-simulating module as the standard logic vector 'XXX . .
. XX'.
Indeterminate values that drive a Gateway Out become what are called NaNs. (NaN abbreviates “not a number”.) In a Simulink scope, NaN values are not plotted. Conversely,
NaNs that drive a Gateway In become indeterminate values. System Generator provides an Indeterminate Probe block that allows for the detection of indeterminate values. This probe cannot be translated into hardware.
In System Generator, any arithmetic signal can be indeterminate, but Boolean signals cannot be. If a simulation reaches a condition that would force a Boolean to become indeterminate, the simulation is halted and an error is reported. Many Xilinx blocks have control ports that only allow Boolean signals as inputs. The rule concerning indeterminate
Booleans means that such blocks never see an indeterminate on a control port
A UFix_1_0 is a type that is equivalent to Boolean except for the above restriction concerning indeterminate data.
Block Masks and Parameter Passing
The same scoping and parameter passing rules that apply to ordinary Simulink blocks apply to System Generator blocks. Consequently, blocks in the Xilinx Blockset can be parameterized using MATLAB variables and expressions. This capability makes possible highly parametric designs that take advantage of the expressive and computational power of the MATLAB language.
Block Masks
In Simulink, blocks are parameterized through a mechanism called masking. In essence, a block can be assigned mask variables whose values can be specified by a user through dialog
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System-Level Modeling in System Generator
box prompts or can be calculated in mask initialization commands. Variables are stored in a mask workspace. A mask workspace is local to the blocks under the mask and cannot be accessed by external blocks.
Note:
It is possible for a mask to access global variables and variables in the base workspace. To access a base workspace variable, use the MATLAB evalin function. For more information on the
MATLAB and Simulink scoping rules, refer to the manuals titled Using MATLAB and Using Simulink from The MathWorks, Inc.
Parameter Passing
It is often desirable to pass variables to blocks inside a masked subsystem. Doing so allows the block’s configuration to be determined by parameters on the enclosing subsystem. This technique can be applied to parameters on blocks in the Xilinx blockset whose values are set using a listbox, radio button, or checkbox. For example, when building a subsystem that consists of a multiply and accumulate block, you can create a parameter on the subsystem that allows you to specify whether to truncate or round the result. This parameter will be called trunc_round as shown in the figure below.
As shown below, in the parameter editing dialog for the accumulator and multiplier blocks, there are radio buttons that allow either the truncate or round option to be selected.
In order to use a parameter rather than the radio button selection, right click on the radio button and select: “Define With Expression”. A MATLAB expression can then be used as the parameter setting. In the example below, the trunc_round parameter from the
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Chapter 1: Hardware Design Using System Generator subsystem mask can be used in both the accumulator and multiply blocks so that each block will use the same setting from the mask variable on the subsystem.
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