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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

Table 4. ST Behavior Classes

Any ST Microcontroller under the “acceptance limits” is rejected as a fail. The “target level” is

the level used by ST to define good EMS performance.

Class B could be caused by:

– a parasitic reset correctly managed by the firmware (preferable case).

– deprogramming of a peripheral register or memory recovered by the application.

– a blocked status, recovered by a Watchdog or other firmware implementation.

Class C could be caused by:

– deprogramming of a peripheral register or memory not recovered by the application.

– a blocked application status requiring an external user action.

Table 5 shows ST target and acceptance limits.

Table 5. F_ESD / FTB target level & acceptance limit

Between “Acceptance limit” and “Target Level”, the device is relatively susceptible to noise.

Special care during system design should be taken to avoid susceptibility issues.

Table 6 shows how F_ESD / FTB test results are presented in ST datasheets.

Table 6. Example of F_ESD / FTB test results

Class A

Class B

Class C

Class D

No failure detected

Failure detected but self

recovery after distur-

bance

Needs an external user

action to recover normal

functionality

Normal functionality

cannot be recovered

Acceptance limit

Target Level

F_ESD

0.5kV

>1kV

FTB

0.5kV

>1.5kV

Symbol

Ratings

Conditions

Severity/Criteria

Voltage limits to be applied on any I/O

pin to induce a functional disturbance

2/A, 3/B

Fast transient voltage burst limits to

be applied through 100pF on VSS and

VDD pins to induce a functional distur-

bance

3/B