The present invention is related to input circuits in integrated circuits and, more particularly, to tristate input circuits useful in BiCMOS integrated circuits.
A common problem with integrated circuits is the limited number of pins which are available to make the electrical connections between an integrated circuit and the outside world. As integrated circuits become more complex and more capable of performing advanced functions, physical constraints keep the number of available pins low. The low pin number can often create a communications bottleneck.
The present invention provides for a technique by which an input pin responds to three states of voltage signals, in contrast to two-state pins in most integrated circuit. This "tristate" technique is particularly adapted to BiCMOS integrated circuits.
BiCMOS integrated circuits are semiconductor devices in which bipolar technology is combined with complementary metal oxide semiconductor (CMOS) technology. In such an BiCMOS integrated circuit, the higher speed, greater power-consuming bipolar transistor circuits are located at the suitable locations in the semiconductor device to use the speed and drive capabilities inherent in bipolar transistors. The CMOS circuits are used wherever higher packing densities and lower power consumption of CMOS circuits are suitable.
Some BiCMOS integrated circuit communicate with the outside world with signal levels appropriate for bipolar logic circuits. CMOS level signals are used within the device. A common bipolar logic used in BiCMOS devices is emitter-coupled logic (ECL) which has a signal range from -0.9 to -1.7 volts. On the other hand, CMOS signals swing in a 5-volt range.
The present invention provides for an input circuit which is responsive to signals having voltage levels below the ECL range. This allows the integrated circuit through the same pin to receive ECL signals for normal communication and special signals at voltage levels below the ECL range for special functions in the BiCMOS device, such as testing and the like.