I/O Port Properties
To prevent electrostatic discharge (ESD) attacks (discussed in part 1), it is recommended to design ESD protection devices onto any connectors or I/O pins that are exposed (such as keypads, buttons, switches, or displays). ESD protection can simply be in the form of clamping diodes or Transient Voltage Suppressor (TVS) devices. Manufacturers include Vishay, Fairchild, and Semtech.
All unused I/O pins should be disabled or set to a fixed state. As discussed in the earlier section on PLDs and FPGAs, unused I/O pins could also be configured to detect probing or tampering by setting them to inputs and waiting for a level change. If one is detected, the device can assume it is being probed and perform a countermeasure or response. This type of detection mechanism would only work while the device is active, which is the most likely time for probing by an attacker to occur.