1.7 System components  (Page 3/3)

$\mathrm{R(Source)\; =}\frac{\mathrm{Z(S)\; -\; Z(t)}}{\mathrm{Z(S)\; +\; Z(t)}}$ (1a)

$\mathrm{R(Load)\; =}\frac{\mathrm{Z(L)\; -\; Z(t)}}{\mathrm{Z(L)\; +\; Z(t)}}$ (1b)

Where:

R(Source) = the reflection coefficient at the source

R(Load) = the reflection coefficient at the load

Z(S) = impedance of the source

Z(L)= impedance of the load

Z(t) = impedance of the transmission line

Inspection of the reflection coefficients shows that if the impedance of the source driving the transmission line is matched with the impedance of the transmission line itself, there is no reflection. The same holds true for the load. Similarly, if the impedances are not matched, you can expect reflections to occur, resulting in distortions of the signal being propagated on the transmission line.

Signal characteristics such as the rise and fall time (slew rate) of the signal can also impact the integrity of the signal being transmitted. Slew-rate limitation also limits the data transmission rate of a given bus configuration. Therefore, bus interface products are designed to optimize not only bus impedance matching but signal slew rate as well.

In practice, perfect matching is difficult to achieve because of the trade-offs between drive currents, output voltages and frequencies. You can optimize the performance of the bus and minimize the impact of these distortions by carefully selecting the line drivers or bus interface devices. For buses that follow industry standards such as LVDS, PECL and many others, devices exist that provide a robust switching solution. The "SLL Advanced Bus Interface Logic Products Selection Guide and Reference," http://www.ti.com/lit/sg/scyt126/scyt126.pdf , provides an in-depth guide to the various options available.

Finally, when dealing with buses and signal-integrity challenges, one of the best things that you can do is use an oscilloscope to observe the impact of the various options available on the actual signal. Experiment with different drivers and termination schemes to determine the best solution for your project.

Basic functions

Basic functions incorporate logic gates, multiplexers, analog switches and many other relatively simple functions. This category of devices is discussed separately from integrated solutions, primarily because basic functions tend to be relatively small circuits that perform a simple action.

Examples of these types of devices include simple logic gates (NAND, AND, OR), stand-alone registers and flip-flops, and clock drivers. Use keywords that describe the function you need in the www.ti.com search engine to view the available options.

As with all products, it's imperative that you read the data sheet to confirm that the recommended operating conditions meet your application. First, look at the supply voltage ratings and input voltage levels to ensure that the device will function in your system. Then review the output current properties to confirm that the device can drive the load you need to drive. For storage elements such as flip-flops and register files, frequency characteristics are also key design criteria.

Conclusion

System components encompass a large variety of products. We recommend that you explore the options available to you at www.ti.com . Indeed, one of the pleasures of the engineering design process is sorting out the available solutions to meet your project’s needs.