Design Considerations for Electrical
Fast Transient (EFT) Immunity
www.cypress.com Document No. 001-80994 Rev. *H 9
4 Performance Criteria
The loss of functionality or degradation of performance of a controller, relative to the performance as defined by its
specifications, per IEC 61000-4-4, can be categorized into the following criteria
Table 2. Performance Criteria
Normal performance after the test is within limits as specified by the manufacturer.
Temporary loss of function or degradation of the performance during the test; the controller
recovers to its normal performance without any intervention after the test.
Temporary loss of function or degradation of the performance during the test; the controller
recovers to its normal performance with intervention after the test.
Loss of function or degradation of the performance during the test; the controller does not recover
owing to damage.
It is important to note that a condition assessed as failure for a particular system may not be a failure with respect to
another system. For example, an industrial process controller cannot tolerate intermittent device resets during EFT
testing but a user interface may. Therefore, give due consideration to the end-application requirements and its ability
to tolerate transient noise.
5 Troubleshooting and Methods to Improve EFT Immunity of a Failing
System
Effective troubleshooting of problems due to transient-induced noise is nontrivial even though it is often overlooked as
part of system compliance testing and bring-up. Designers should ascertain the probable causes for each failure
observed as part of compliance testing. The designer and the person responsible for running compliance tests must
work together to isolate the causes for the failures.
Most failures that occur in systems employing microcontrollers can be quickly identified if due cognizance is paid to
the type of failure. You should identify the failure modes discussed in the previous section occurring in a system
during or after EFT tests and their causes.
Once you understand the probable causes for the associated failures, take a step-by-step approach to debug the
cause. Some tips for troubleshooting are:
1. When looking for a reset-related issue, probe the supply pins of the controller during the test and look for obvious
reasons. Place a debug routine in the firmware that can visually indicate a reset.
2. In the case of a latch-up, observe whether the current drawn by the controller is beyond its normal ratings. If that
is not the case, look for a firmware freeze by introducing debug routines.
3. For analog or clock-related issues, probe the associated I/O lines for noise or glitches. Pay attention to clock
stretches if the communications to the device are hampered.
4. For flash/RAM corruptions, place a firmware debug routine such as a port pin toggling and monitor the pin status
to check if the firmware flow is as intended. Read susceptible memory contents during and after the tests.
Note that it is advised to use oscilloscopes with isolated earth grounds. Failing to do so might cause the oscilloscope
earth ground to shunt transient noise thus giving wrong readings. In a typical oscilloscope, the signal ground is
connected to the earth ground internally. Refer to Figure 7.