The methods described in this application report do not apply when the main supply (typically DVCC) is unpowered and a voltage is applied on a GPIO pin. This condition can cause unintended power sourcing to the MCU through the ESD structures on the GPIOs. This is a concern in both the power up and power down scenarios. If the main supply and a GPIO are powered at the same time, ensure that delays in the main supply (due, for example, to inline capacitance or LDO delays) are accounted for and that a GPIO is not powered before the main supply.

Due to the physical architecture of the ESD diodes, which is explained in the following sections, other secondary side effects can appear if the MSP microcontroller is not properly powered. Improper power scenarios include sensors that are sourced by a different supply or communication lines that are driven by other ICs in the application, if these signals are connected to the MCU while the main supply of the MCU is off. During the power-down scenario, a side effect is that the device can be back powered through the ESD diodes. However, this is not the intended powering scheme for the MSP microcontroller and can lead to erroneous and unpredictable behavior, due to the current limitations of the ESD diodes and the bypassing of the intended power path. In worst case scenarios, this can lead to physical damage to the device, unexpected execution, or memory corruption causing a malfunction in the application.

Most MSP microcontrollers specify the maximum ESD diode current in the Absolute Maximum Ratings section (see Figure 2-1). The diode current specified as ±2 mA is a constant current that flows through the ESD structure to the supply rails to protect the device. These ESD structures are triggered if a signal is applied that exceeds the maximum or minimum supply voltage specifications of the device. To follow the specification, the application must protect the device pin externally so that a signal does not exceed the ±2-mA specification.

In other words, voltages greater than the actual device supply (DVCC and AVCC) or lower than VSS can be applied, but the current that flows through the ESD diodes must be controlled.

Figure 2-1 Absolute Maximum Ratings of MSP430FR5969

While the current is specified as ±2 mA, the ESD structure can withstand much higher current levels, such as those that appear during typical ESD events according HBM or CDM. The cells are made to pass standard ESD tests like HBM or CDM as shown in Figure 2-2. During this high-voltage stress, current peaks in the range of several amperes flow through the ESD structures.

Figure 2-2 ESD Ratings of MSP430FR5969

However, the duration of the high-current pulse is quite short (in the range of several nanoseconds), which leads to less thermal stress and much less heating compared to a long-term high-current event. This is the main reason why the constant current through the ESD diodes is limited to ±2 mA for longer or constant operation.