Embedded Design Handbook

ID 683689
Date 8/28/2023
Public
Document Table of Contents

6.3.2.1.3. Device Isolation

Using device isolation techniques, you can disable features of devices on your PCB that cause your design to fail. Typically designers use device isolation for early revisions of the PCB, and then remove these capabilities before shipping the product.

Most designs use crystal oscillators or other discrete components to create clock signals for the digital logic. If the clock signal is distorted by noise or jitter, failures may occur. To guard against distorted clocks, you can route alternative clock pins to your FPGA. If you include SMA connectors on your board, you can use an external clock generator to create a clean clock signal. Having an alternative clock source is very useful when debugging clock-related issues.

Sometimes the noise generated by a particular device on your board can cause problems with other devices or interfaces. Having the ability to reduce the noise levels of selected components can help you determine the device that is causing issues in your design. The simplest way to isolate a noisy component is to remove the power source for the device in question. For devices that have a limited number of power pins, if you include 0 ohm resistors in the path between the power source and the pin. You can cut off power to the device by removing the resistor. This strategy is typically not possible with larger devices that contain multiple power source pins connecting directly to a board power plane.

Instead of removing the power source from a noisy device, you can often put the device into a reset state by driving the reset pin to an active state. Another option is to simply not exercise the device so that it remains idle.

A noisy power supply or ground plane can create signal integrity issues. With the typical voltage swing of digital devices frequently below a single volt, the power supply noise margin of devices on the PCB can be as little as 0.2 volts. Power supply noise can cause digital logic to fail. For this reason it is important to be able to isolate the power supplies on your board. You can isolate your power supply by using fuses that are removed so that a stable external power supply can be substituted temporarily in your design.