SSTL voltage translator with dynamic biasing

As shown in FIG. 1, a typical computer system 10 includes at least a microprocessor 12 (often referred to and known as “CPU”) and some form of memory 14 . The microprocessor 12 has, among other components, arithmetic, logic, and control circuitry that interpret and execute instructions necessary for the operation and use of the computer system 10 . Specifically, FIG. 1 shows the computer system 10 having the microprocessor 12 , memory 14 , integrated circuits (ICs) 16 that have various functionalities, and communication paths 19 , i.e., buses and wires, that are necessary for the transfer of data among the aforementioned components of the computer system 10 .

In order to keep pace with improving technologies, computer system and circuit designers are constantly trying to improve and get the most out of their designs through the most cost-effective means. As faster versions of a particular CPU become available, a designer will often try to improve the throughput of their existing design by simply increasing the CPU clock frequency. However, after a certain point, the speed of the system's main memory becomes a limiting factor in optimizing the throughput of the system. To this end, designers have produced faster memories, which, in turn, has necessitated high-speed memory interfaces.

One type of design that has been used for high-speed memory interface applications involves the use of stub series termination logic (SSTL). SSTL is a standard created by the Joint Electron Device Engineering Council (JEDEC) to provide a termination scheme for high speed signaling in applications such as DDR-SDRAM. SSTL specifies particular switching characteristics such that high operating frequencies are available. As operating frequencies continue to increase and as the demand for faster memory interfaces has and continues to grow, the STTL interface standard continues to enjoy wide acceptance.

According to one aspect of the present invention, a computer system comprises a pre-driver stage comprising a voltage translator and an output buffer, wherein the voltage translator comprises: a first device that, dependent on a first bias signal, causes an output signal from the voltage translator to be pulled down, where the first bias signal is dependent on an input signal to the voltage translator; and a second device that, dependent on a second bias signal, causes the output signal to be pulled up, where the second bias signal is dependent on the input signal.

According to another aspect, a computer system comprises means for translating an input signal having a first voltage swing to an output signal having a second voltage swing, where the means for translating comprises: means for driving the output signal; and means for dynamically biasing the means for driving the output signal dependent on the input signal.

According to another aspect, a method for performing a SSTL interface operation comprises: inputting an input signal; dynamically generating a voltage on a first bias signal dependent on the input signal; dynamically generating a voltage on a second bias signal dependent on the input signal; and driving an output signal dependent on the first bias signal and the second bias signal.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

FIG. 1 shows a typical computer system.

FIG. 2 shows a block diagram of a SSTL interface.

FIG. 3 shows a block diagram of a SSTL interface pre-driver.

FIG. 4 shows a portion of a SSTL interface in accordance with an embodiment of the present invention.

SSTL interface design requires detailed consideration of the voltage levels supplied to different parts of the SSTL interface. Accordingly, with low power, and hence low voltage, devices, e.g., transistors, considerations must be made as to whether there is a chance for a large voltage to appear across the terminals of one or more of such low voltage devices. Such large voltages (breakdown voltages) can damage the device, thereby causing the SSTL interface, and, in turn, the entire microprocessor, to malfunction.

FIG. 2 shows a SSTL output interface 30 . A core signal 32 (from the microprocessor (not shown)) serves as an input to a pull-up pre-driver 34 and a pull-down pre-driver 36 . The core signal 32 , coming from the microprocessor (not shown), has a low voltage swing, e.g., 0 to 1 volt, and thus, the pull-up pre-driver stage 34 is used to convert the core signal 32 to a larger voltage swing signal referenced to a power supply voltage (not shown) of the SSTL output interface 30 , and the pull-down pre-driver stage 36 is used to convert the core signal 32 to a larger voltage swing signal references to ground. For example, for a core signal 32 that has a 1 volt voltage swing, e.g., between 0 and 1 volt, the pull-up pre-driver 34 , operating off of a 2.5 volt SSTL power supply, could be used to generate a 1.5 volt voltage swing signal, e.g., from 1 to 2.5 volts, and the pull-down pre-driver 36 , operating off the 2.5 volt SSTL power supply, could be used to generate a 1.5 volt voltage swing signal, e.g., from 0 to 1.5 volts.

The signals from the pull-up pre-driver 34 and the pull-down pre-driver 36 serve as inputs to output buffers 38 , 40 that output the signals from the SSTL interface 30 .

Now referring to FIG. 3, a SSTL interface pre-driver 33 (such as the pre-drivers 34 , 36 shown in FIG. 2) is typically formed by two stages: a voltage translator 35 and a gain stage 37 . The voltage translator stage 35 is used to translate the incoming core signal 32 to a particular voltage swing signal that serves as an input to the gain stage 37 , which, in turn, outputs the translated signal from the pre-driver 33 .

FIG. 4 shows an exemplary voltage translator 40 in accordance with an embodiment of the present invention. The voltage translator 40 shown in FIG. 4 may be used for a pull-up pre-driver stage. However, those skilled in the art will understand that the features, structure, and principles of the present invention are equally or similarly applicable to a pull-down pre-driver stage.

The voltage translator 40 uses a push-pull structure with a device stack 45 formed by PMOS devices 42 , 44 that are controlled by bias signals 46 , 48 , respectively, that dynamically change according to an input core signal 50 to the voltage translator 40 . Particularly, the device stack 45 uses a pair of cross-coupled PMOS devices 50 , 52 that are followed by PMOS devices 42 , 44 , which are then followed by NMOS devices 54 , 56 , and finally followed by NMOS devices 58 , 60 .

NMOS devices 54 , 56 , are controlled by bias voltage 62 . NMOS devices 58 , 60 are controlled by non-complemented and complemented versions of the input core signal 50 (via the inverter formed by transistors 64 , 66 ). The output 68 from the voltage translator 40 is coupled to the signal lines, i.e., wires, that are used to cross-couple PMOS devices 50 , 52 .

When the input core signal 50 goes ‘low,’ (1) a first bias voltage 70 , through PMOS device 72 (which is controlled by the input core signal 50 ), is provided to bias signal 46 , (2) NMOS device 58 is switched ‘off,’ (3) a third bias voltage 82 , through PMOS device 84 (which is controlled by the input core signal 50 ), is provided to bias signal 48 , and (4) an inverter formed by PMOS device 64 and NMOS device 66 outputs ‘high’ to NMOS device 60 and PMOS device 80 . The ‘high’ to NMOS device 60 switches NMOS device 60 ‘on’ and pulls the right side of the device stack 45 down, i.e., to ground. Dependent on the input core signal 50 being ‘low,’ the bias signal 48 to PMOS device 44 causes PMOS device 44 to conduct and pull the output 68 of the voltage translator 40 down via the biased NMOS device 56 and ‘on’ NMOS device 60 .

Moreover, when the output 68 of the voltage translator 40 goes ‘low,’ PMOS device 50 switches ‘on’ and allows Vdd to propagate to a terminal of PMOS device 42 . However, the providing of the first bias voltage 70 to bias signal 46 ensures that too large of a voltage is not placed across PMOS device 42 , thereby not damaging PMOS device 42 .

When the input core signal 50 goes ‘high,’ (1) a second bias voltage 74 , through PMOS device 76 (which is controlled by a complement of the input core signal 50 ), is provided to bias signal 46 , (2) NMOS device 58 is switched ‘on,’ (3) a fourth bias voltage 78 , through PMOS device 80 (which is controlled by a complement of the input core signal 50 ), is provided to bias signal 48 , and (4) the inverter formed by PMOS device 64 and NMOS device 66 outputs ‘low’ to NMOS device 60 and PMOS device 80 . The ‘on’ NMOS device 58 pulls the left side of the device stack 45 down, i.e., to ground. Dependent on the input core signal 50 being ‘high,’ the bias signal 46 to PMOS device 42 causes PMOS device 42 to conduct and pull the input of PMOS device 52 down via the biased NMOS device 56 and ‘on’ NMOS device 60 . When the input of PMOS device 52 is pulled down, PMOS device 52 switches ‘on’ and allows Vdd to propagate to the output 68 of the voltage translator 40 .

Moreover, when the output 68 of the voltage translator 40 goes ‘high,’ Vdd also propagates to a terminal of PMOS device 44 . However, the providing of the fourth bias voltage 78 to bias signal 48 ensures that too large of a voltage is not placed across PMOS device 44 , thereby not damaging PMOS device 44 .

Those skilled in the art will note that PMOS devices 72 , 76 perform a multiplexing function, and thus, form a first ‘multiplexer’ portion 75 of the voltage translator 40 . Further, those skilled in the art will note that PMOS devices 78 , 82 perform a multiplexing function, and thus, form a second ‘multiplexer’ portion 77 of the voltage translator 40 .

Those skilled in the art will understand that, in one or more embodiments, dynamic biasing may be used to control NMOS devices within the device stack 45 of the voltage translator 40 .

Advantages of the present invention may include one or more of the following. In one or more embodiments, because a voltage translator for a SSTL interface pre-driver uses dynamic biasing, voltage translation speed is increased, which, in turn, reduces potential crow-bar current at an output of the voltage translator.

In one or more embodiments, because a voltage translator for a SSTL interface pre-driver uses dynamic biasing, small devices, such as transistors, may be used in the implementation of the voltage translator.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.