Learn more about Real-Time Workshop Embedded Coder

Using Single-Rate and Multi-Rate Buses

On this page…
Introduction Techniques for Combining Multiple Rates Larger Buses and Multiple Rates Specifying Sample Time Rates

Introduction

Nonvirtual buses do not support multiple rates. Virtual buses support multiple rates as long as the bus does not cross any root level inport or outport. The best techniques for optimizing a bus that contains signals that initially have different rates can depend on the type of the bus and the number of signals.

Back to Top

Techniques for Combining Multiple Rates

The simplest bus contains only two signals. The next figure shows two examples of two-element buses. The first example shows a virtual bus created from two signals that have different rates. The second example shows a nonvirtual bus created from the same two signals. The Sample Time Legend shows the different signal rates:

The signals with different rates in the first example can be combined into a virtual bus, because virtual buses support multiple rates. However, a multirate virtual bus cannot connect to a root-level output port. The bus therefore passes through a Rate Transition block that converts it to a single-rate bus, then connects to the Outport. This technique is preferable only for virtual buses that contain no more than two signals. See Larger Buses and Multiple Rates.

The signals with different rates in the second example cannot initially be combined into a nonvirtual bus, because nonvirtual buses do not support multiple rates. One of the signals therefore passes through a Rate Transition block, which converts it to have the same rate as the other signal, then connects to the Bus Creator block. The signals can then combine into a single-rate nonvirtual bus, which can connect to the root-level outport without further conversion.

Back to Top

Larger Buses and Multiple Rates

When you create a multirate virtual bus that contains more than two signals, you can convert the bus to single-rate by applying a Rate Transition block to the output of the Bus Creator block. However, The MathWorks recommends using a Rate Transition block on each input signal to give full control over the output rate. As the next figure shows, when a single Rate Transition block is used the block sets all the signals to the fastest rate (D1):

Note that the preferred techniques for a virtual bus with more than two signals, and the required technique for a nonvirtual bus with any number of signals, are the same. Note also that, in the preceding figure, the blocks that perform rate transition are not actual Rate Transition blocks, but other blocks that can change the signal rate as part of some other operation. The identity of the blocks that perform rate transition makes no difference. All that matters is that the signal rates match when required.

Back to Top

Specifying Sample Time Rates

The sample time for buses should be specified through the signals that define the bus. If the sample times do not match, use Rate Transition (or equivalent) blocks to create a uniform rate, as shown in the previous figures. The signal rates should not be set by specifying Sample Time values in a Bus Creator block's bus object. Instead, set the sample time for each signal before inputting it to the Bus Creator, and set each Sample Time in the corresponding bus object to -1 (inherited), as shown in the next figure:

Back to Top

Optimizing Virtual and Nonvirtual Buses

Setting Bus Signal Initial Values

© 1984-2009- The MathWorks, Inc.    -   Site Help   -   Patents   -   Trademarks   -   Privacy Policy   -   Preventing Piracy   -   RSS