Generating a Global Oversampling Clock

On this page…
Why Use a Global Oversampling Clock? Requirements for the Oversampling Factor Specifying the Oversampling Factor From the GUI Specifying the Oversampling Factor From the Command Line Resolving Oversampling Rate Conflicts

Why Use a Global Oversampling Clock?

In many designs, the DUT is not self-contained. For example, consider a DUT that is part of a larger system that supplies timing signals to all components under control of a global clock. The global clock typically runs at a higher rate than some of the components under its control. By specifying such a global oversampling clock, you can integrate your DUT into a larger system without using Upsample or Downsample blocks.

To generate global clock logic, you specify an oversampling factor. The oversampling factor expresses the desired rate of the global oversampling clock as a multiple of the base rate of your model.

When you specify an oversampling factor, the coder generates the global oversampling clock and derives the required timing signals from clock signal. Generation of the global oversampling clock affects only generated HDL code. The clock does not affect the simulation behavior of your model.

Back to Top

Requirements for the Oversampling Factor

When you specify the oversampling factor for a global oversampling clock, note these requirements:

• The oversampling factor must be an integer greater than or equal to 1.

• The default value is 1. In the default case, the coder does not generate a global oversampling clock.

• Some DUTs require multiple sampling rates for their internal operations. In such cases, all other rates in the DUT must divide evenly into the global oversampling rate. For more information, see Resolving Oversampling Rate Conflicts .

Back to Top

Specifying the Oversampling Factor From the GUI

You can specify the oversampling factor for a global clock from the GUI as follows:

1. Select the HDL Code Generation > Global Settings pane in the Configuration Parameters dialog box.

2. For Oversampling factor in the Clock settings section, enter the desired oversampling factor. In the following figure, Oversampling factor specifies a global oversampling clock that runs at ten times the base rate of the model.

   

3. Click Generate on the HDL Code Generation pane to initiate code generation.

   The coder reports the oversampling clock rate:

   ### Begin VHDL Code Generation
   ### MESSAGE: The design requires 10 times faster clock with respect to the base rate = 1.
   ### Working on symmetric_fir_tc as hdlsrc\symmetric_fir_tc.vhd
   ### Working on sfir_fixed/symmetric_fir as hdlsrc\symmetric_fir.vhd
   ### HDL Code Generation Complete.

Back to Top

Specifying the Oversampling Factor From the Command Line

You can specify the oversampling factor for a global clock from the command line by setting the 'Oversampling', N property in the makehdl command. The following example specifies an oversampling factor of 7:

>> makehdl(gcb,'Oversampling', 7)
### Generating HDL for 'sfir_fixed/symmetric_fir'
### Starting HDL Check.
### HDL Check Complete with 0 errors, 0 warnings and 0 messages.


### Begin VHDL Code Generation
### MESSAGE: The design requires 7 times faster clock with respect to the base rate = 1.
### Working on symmetric_fir_tc as hdlsrc\symmetric_fir_tc.vhd
### Working on sfir_fixed/symmetric_fir as hdlsrc\symmetric_fir.vhd
### HDL Code Generation Complete.

Back to Top

Resolving Oversampling Rate Conflicts

The HDL realization of some designs is inherently mulitrate, even though the original Simulink model is single-rate. As an example, consider the simplevectorsum_cascade model (also discussed in Example: Latency).

This model consists of a subsystem, vsum, driven by a vector input of width 10, with a scalar output. The following figure shows the root level of the model.

The device under test is the vsum subsystem, shown in the following figure. The subsystem contains a Sum block, configured for vector summation.

The simplevectorsum_cascade model specifies a cascaded implementation (SumCascadeHDLEmission) for the Sum block. The generated HDL code for a cascaded vector Sum block implementation runs at two effective rates: a faster (oversampling) rate for internal computations and a slower rate for input/output. The coder reports that the inherent oversampling rate for the DUT is five times the base rate:

>> dut = 'simplevectorsum_cascade/vsum';
>> makehdl(dut);
### Generating HDL for 'simplevectorsum_cascade/vsum'
### Starting HDL Check.
### HDL Check Complete with 0 errors, 0 warnings and 0 messages.


### The code generation and optimization options you have chosen have introduced
    additional pipeline delays. 
### The delay balancing feature has automatically inserted matching delays for
    compensation.
### The DUT requires an initial pipeline setup latency. Each output port
    experiences these additional delays
### Output port 0: 1 cycles

### Begin VHDL Code Generation
### MESSAGE: The design requires 5 times faster clock with respect to the
    base rate = 1.
...

In some cases, the clock requirements for such a DUT conflict with the global oversampling rate. To avoid oversampling rate conflicts, verify that all subrates in the model divide evenly into the global oversampling rate.

For example, if you request a global overampling rate of 8 for the simplevectorsum_cascade model, the coder displays a warning and ignores the requested oversampling factor. The coder instead respects the oversampling factor that the DUT requests:

>> dut = 'simplevectorsum_cascade/vsum';
>> makehdl(dut,'Oversampling',8);
### Generating HDL for 'simplevectorsum/vsum'
### Starting HDL Check.
### HDL Check Complete with 0 errors, 0 warnings and 0 messages.

### The code generation and optimization options you have chosen have introduced
    additional pipeline delays. 
### The delay balancing feature has automatically inserted matching delays for
    compensation.
### The DUT requires an initial pipeline setup latency. Each output port
    experiences these additional delays
### Output port 0: 1 cycles

### Begin VHDL Code Generation
### WARNING: The design requires 5 times faster clock with respect to 
        the base rate = 1, which is incompatible with the oversampling 
        value (8). Oversampling value is ignored.
...

An oversampling factor of 10 works in this case:

>> dut = 'simplevectorsum_cascade/vsum';
>> makehdl(dut,'Oversampling',10);
### Generating HDL for 'simplevectorsum_cascade/vsum'
### Starting HDL Check.
### HDL Check Complete with 0 errors, 0 warnings and 0 messages.


### The code generation and optimization options you have chosen have introduced
    additional pipeline delays. 
### The delay balancing feature has automatically inserted matching delays for
    compensation.
### The DUT requires an initial pipeline setup latency. Each output port
    experiences these additional delays
### Output port 0: 1 cycles

### Begin VHDL Code Generation
### MESSAGE: The design requires 10 times faster clock with respect to 
    the base rate = 1.
...

Back to Top

Example: Model with a Multirate DUT

Generating Multicycle Path Information Files

© 1984-2012- The MathWorks, Inc.    -   Site Help   -   Patents   -   Trademarks   -   Privacy Policy   -   Preventing Piracy   -   RSS