1. Field of the Invention
This invention relates to integrated circuits (ICs). More particularly, it relates to a design technique and method to better protect electrical circuits such as an integrated circuit from electrical over stress (EOS) damage, particularly useful in the design of a IEEE 802A “Firewire” high speed serial interface.
2. Background of Related Art
The “Firewire” interface is a high performance serial bus developed by Apple Computer and Texas Instruments, and standardized as the IEEE 1394a standard. The Firewire interface as currently standardized can connect up to 63 devices in a tree-like daisy chain configuration, and transmit data at up to 400 megabits per second (Mb/s). The Firewire high speed serial interface bus is now implemented in many personal computer systems that support plug and play and peer-to-peer communication between peripheral devices.
The Firewire interface is often used with digital video devices because of the need to transfer extremely large amounts of data in real time as a videotape is played. To upload a digital movie from a digital video camera to a typical personal computer, the digital video camera may be temporarily plugged into a Firewire interface port on the PC or other computing device.
The Firewire interface is capable of passing extremely large amounts of data. However, as the inventors of the present application have appreciated, Firewire devices are prone to electrical over stress damage. This is because when a ‘hot’ or powered device is plugged into another device, all connections in the Firewire cable are designed for substantially simultaneous contact. In reality, the cable may be inserted at a slight angle by the unwary user, thus making contact with some connections in the Firewire cable slightly before others.
Ideally, a ground wire should be the first connection to make contact, thus providing an electrical sink for any current that may be passed on signal connections. However, Firewire cables are not designed to ensure grounded contact before other electrical connections are made.
FIG. 6 shows a conventional Firewire circuit that is susceptible to damage due to electrical over stress conditions.
In particular, as shown in FIG. 6, a Firewire interface plug 300 in a device such as a personal computer (PC) has multiple connections, including power and signal connections that are carried through a suitable Firewire cable to another Firewire device (e.g., a digital video camera). The Firewire interface plug 300 includes connections such as (8 volt-30 volt) power Vdd 301, an electrical ground Vg or Vss 303, and a plurality of input and output signals, only one of which TPout 302 is represented in FIG. 3 for clarity.
If the unregulated power supply line (up to 30 volts) 301 of the Firewire connector ‘makes’ (or connects) first followed by connection of the signal pin (e.g., TPout 302), before the board/integrated circuit ground connection 303 is made, the voltage regulator 310, being absent of a ground, will likely pass a 30 volt signal to the Vdd of the integrated circuit for some short period of time. In such a situation, the integrated circuit output pin TPout 302 will become an alternate ground for this 30 volt signal, with current flowing through the diode D1.
Electrostatic Discharge (ESD), another potentially harmful electrical signal, has been accommodated in many conventional integrated circuits, ICs, including those relevant to a Firewire device. For instance, the conventional IC shown in FIG. 6 includes an Electrostatic Discharge shunt 307.
If present, the ESD shunt 307 is turned on such as by an RC time constant-based control, and typically shunts power to ground for a very short period of time appropriate for ESD events. Activation of an ESD shunt 307 for approximately 2 microseconds is typical in the event of an EOS event. During these 2 microseconds, the ESD shunt 307 will partially clamp the voltage. An ESD shunt 307 must operate for a short period of time, to then allow the circuit to properly power up afterwards.
ESD shunts 307 are suitable and appropriate for shunting surges associated with ESD events. The present inventors have appreciated another electrical stressing event, electrical over stress and have presented a technique which solves problems associated therewith.
In particular, electrical over stress conditions are different from ESD events. One possible set of conditions arises from differences in the order in which connections are made between conductors of a cable and a connector to which the cable is being mated. Either the cable or the connector may be powered. Although cable connections can be performed quickly, EOS conditions nevertheless cause serious damage not solved by conventional ESD protection.
For instance, if an electrical over stress event lasts longer than 2-3 microseconds (which it typically does), full ungrounded supply voltage will be applied between the integrated circuit power rail Vdd and the signal pin TPout as shown in FIG. 6. This causes electrical over stress in transistor P1, diode D2, or both. Eventually, damage may also occur in the OUT drive circuits 320 themselves.
Conventional electrostatic discharge shunts or clamps 307 are designed to handle an ESD event, and are not suitable for alleviating the potential for damage resulting from a typical EOS event. Conventional ESD voltage triggered clamps 307 are based on RC time constants relative to the duration of a typical ESD event. Thus, ESD clamps 307 are adequate only for ESD protection, since ESD voltage spikes typically last only a few microseconds.
However, ESD is typically a very short term event, whereas electrical over stress in comparison can be a much longer term event.
Nevertheless, Firewire has become a well-followed standard, particularly among digital video devices and personal computers. Thus, the design of the now well-standardized Firewire cable cannot easily be changed without making obsolete many, many devices already in the marketplace.
There is a need to accommodate and improve upon the danger of electrical over stress damage caused by the insertion and unplugging of a cable, particularly a serial data cable such as a Firewire cable.