ESD Suppression on Microcontrollers at Board Level

Importance of EMC Testing for Microcontrollers

EMC testing identifies potential EMC issues encountered in practice with microcontrollers. Electrostatic discharges are particularly important for systems that contain microcontrollers. ESD can be generated in practice in various ways, such as static electricity accumulating on people or frictional electricity developing on conveyor belts, packaging machines, etc.

EMC Resilience Issues in Microcontrollers

On a microcontroller board, during EMC testing according to IEC 61000-4-2 / HMM, an EMC issue appears. The microcontroller on the board crashes after an ESD generator discharge on a metal rail.

Problem Description

The metal spacer, where the discharge was applied, is adjacent to the microcontroller in the area of the PLL circuits and quartz oscillator. The interference can lead to two types of faults:

• The quartz oscillator circuits usually stop for a few microseconds or milliseconds.
• The PLL power supply, Vss PLL and Vdd PLL, is a weak point in the PLL circuit. In case of interference, the PLL circuit usually locks up at the maximum possible frequency, which is higher than the permissible operating frequency of the microcontroller, causing it to crash.

Failure Analysis and Testing Methods

The failure of the microcontroller is visualized by the LED that indicates life turning off. Interference in the quartz oscillator circuit usually causes the microcontroller to stop for just a few microseconds. In exceptional cases, the microcontroller may also stop for a few milliseconds (crystal oscillator error). This fault affects all time-synchronous signal sequences of the microcontroller, particularly data transmission systems, USB, or LAN networks. It also affects simple UART systems. Due to the microcontroller stopping, individual data bits are not transmitted. Such faults are corrected by USB or LAN correction procedures. Data transmission is slowed down due to persistent error correction/retransmission with frequent interference. The transmission link can be disrupted by strong interference. In the example (Figure 1), a UART converter connected to the microcontroller is connected to USB. The microcontroller stopping due to quartz oscillator interference will lead to UART system disruptions, which can be transmitted to a PC and visualized. This allows for analysis of the effects caused by quartz oscillator errors. These data errors could be displayed on a PC during the ESD test of the microcontroller board. This means that the microcontroller's quartz oscillator likely stops for a few hundred microseconds after applying ESD. Relevant error registers can be read in the microcontroller if the appropriate hardware, firmware, and software are available. This data can be examined to determine if there were faults in the quartz oscillator or PLL. If so, it confirms the initial considerations about the fault cause (quartz oscillator and PLL fault). In any case, even if error registers cannot be read, the microcontroller board should be examined.

Testing Method

To analyze the faults, a field source is used, which generates local fields with the required intensity. These local fields must match the fields occurring under ESD influence in practical applications. In this example, a field source generating a field beam with a diameter of about 4 mm was used. The rise time of the pulse emitted by the field source is 1 ns, which is close to the real ESD event. The field source must be able to induce an impulsive voltage up to 20 V in the conductor loops (Figure 2).

EMC Solutions for Problem Solving

For practical field situations, the following possible solutions can be derived:

• Reposition the IC on the microcontroller board to increase the distance of the PLL and crystal oscillator circuits from the metal spacer.
• Modify the mechanical design by relocating the metal spacer to a non-critical area.
• Use a spacer made of non-conductive material (plastic spacer).
• Design a plastic enclosure so that the metal spacer is no longer accessible to the ESD generator's tip.
• Shield the relevant ICs.
• Use a suitably robust IC.

Conclusions and Recommendations

Programmers should have experience with EMC and practical skills in using field sources to attenuate interference, eliminate faults, and effectively harden modules. Solid knowledge of EMC cause-and-effect relationships is also useful. Participation in a specialized EMC experimental seminar is beneficial if EMC issues need to be frequently addressed. Additionally, in particularly difficult situations, seeking help from an experienced EMC consultant who can quickly and effectively identify the fault cause is advisable.