The present invention relates to semiconductor devices, and more particularly to an input buffer circuit. The following description is presented to enable one of ordinary skill in the art to make and use the invention, and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein.
An input buffer circuit in accordance with the present invention is disclosed. The input buffer circuit includes input transistors that are operable to receive single-ended input signals or differential input signals. The input buffer circuit also includes a reference transistor that is operable to receive a reference voltage that is the threshold voltage for receiving single-ended input signal. Having the cross point voltage of the differential signal that is higher than the threshold voltage of the single-ended signal results in an input buffer circuit that combines single-ended input and differential input functionalities without compromising performance. In one embodiment, the input transistors are operable to provide the input buffer circuit with compatibility to multiple input signal levels for different functions such as for normal operations and for testing. To more particularly describe the features of the present invention, refer now to the following description in conjunction with the accompanying figures.
Although the present invention disclosed herein is described in the context of input buffer circuits, the present invention may apply to other types of circuits, and still remain within the spirit and scope of the present invention.
In operation, generally, the selection node 58 is utilized to select either the differential input buffer circuit 52 or the single-ended input buffer circuit 54 via the mux 56. If the differential input buffer circuit 52 is selected, transistor Q1 is operable to receive a differential input signal via input node INP, and transistor Q2 is operable to receive a differential input signal via input node INN. If the single-ended input buffer circuit 54 is selected, transistor Q3 is operable to receive a single-ended input signal via input node TTL IN. Accordingly, mux 56 enables both types of input buffer circuits to be available.
As indicated above, one problem with this approach is that it requires more space and power on the chip because the system is designed with two separate input buffer circuits. However, this may be a desirable trade off since two different types of input buffer circuits are made available, even though only one input buffer circuit might be used at a given time. Another problem with this approach is that it requires one or more additional external pins per input buffer circuit. For example, two external pins INP and INN are required for the differential input buffer circuit, one external pin TTL IN is required for the single-ended input buffer circuit, and one external pin SEL is required to select the desired input buffer circuit.
In operation, transistors Q10 and Q12 are operable to selectively receive a single-ended input signal or differential input signals. For example, if the input buffer circuit 100 is operating in a single-ended input mode, transistor Q10 is operable, in conjunction with transistor Q14 and the reference voltage as the switching threshold voltage, to receive a single-ended input signal, and to drive their respective outputs accordingly. In this case, Q12 is turned off and not used. This is achieved through leaving Q12 unconnected or connected directly or indirectly to a voltage that is lower than the reference voltage, such as ground, and having the reference voltage be the threshold voltage. If the input buffer circuit 100 is operating in a differential input mode, both of the transistors Q10 and Q12 are operable to receive differential input signals and to drive their respective outputs accordingly. In this case, Q14 is turned off and not used. This is achieved through having the cross point, or the common mode voltage, of the differential signals higher than the reference voltage.
The input buffer circuit 200 of
In one embodiment, the resistors R14 and R16 couple to the transistors Q10 and Q12, respectively, and function as pull-down resistors for the inputs NP and INN, respectively, when either is not used. In one embodiment, the transistor Q16 functions as a balancing transistor to balance the load, similar to transistor Q12, to the two respective output nodes. Transistor Q16 remains off all the time.
Embodiments of the present invention, as examples, take advantage of the fact that PECL and CML cross point voltage levels are higher than TTL threshold voltages. In other words, differential cross point voltage levels are typically higher than single-ended threshold voltages. However, the present invention would work with different voltage levels and standards and is not limited only to the above mentioned voltage levels standards.
According to the circuit disclosed herein, the present invention provides numerous benefits. For example, embodiments of the present invention utilize the same elements to combine single-ended input and differential input functionalities. Embodiments of the present invention combine these single-ended input and differential input functionalities into one input buffer circuit without increasing power, size, complexity and number of external pins.
An input buffer circuit in accordance with the present invention has been disclosed. The input buffer circuit includes input transistors that are operable to receive single-ended input signals or differential input signals. The input buffer circuit also includes a reference transistor that is operable to receive a reference voltage that is the threshold voltage for receiving single-ended input signal. Having the cross point voltage of the differential signal that is higher than the threshold voltage of the single-ended signal results in an input buffer circuit that combines single-ended input and differential input functionalities without compromising performance.
The present invention has been described in accordance with the embodiments shown. One of ordinary skill in the art will readily recognize that there could be variations to the embodiments, and that any variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.