2.9.1 Non-5V Tolerant Input Pins

A quick review of the absolute maximum rating in the 51 Electrical Characteristics will indicate that the voltage on any non-5V tolerant pin may not exceed VDD + 0.4V unless the input current is limited to meet the respective injection current specifications defined by electrical spec parameters DI_19, DI_21, and DI_23 in 51.12 I/O Pin Electrical Specifications. The following figure shows an example of a remote circuit using an independent power source, which is powered while connected to a PIC32C non-5V tolerant circuit that is not powered.

Without proper signal isolation, on non-5V tolerant pins, the remote signal can power the PIC32C device through the high side ESD protection diodes. Besides violating the absolute maximum rating specification when VDD of the PIC32C device is restored and ramping up or ramping down, it can also negatively affect the internal Power-on Reset (POR) and Brown-out Reset (BOR) circuits, which can lead to improper initialization of internal PIC32C logic circuits. In these cases, it is recommended to implement digital or analog signal isolation as depicted in the following figure, as appropriate or use series I/O pin resistors when possible to limit the injection current to less than 5mA/pin and/or 20mA total for the whole MCU. This is indicative of all industry microcontrollers and not just Microchip products.