Patent Application
20090091375
The present invention relates generally to integrated circuits and in particular to a method and system for minimizing the transition time between a circuit's operating modes.
Today, some computer architecture standards are configured such that a system has multiple operating modes. For example, Peripheral Component Interface (PCI) Express requires a normal operating mode and a low-power operating mode, known as electrical idle mode. During the electrical idle mode, a transmitter is required to have the same common-mode voltage as during normal operating mode. In the PCI Express, and other standards, the transmitter is required to be fully functional by meeting the amplitude, jitter, common-mode noise specifications within eight nano-seconds after the transmitter recovers from electrical idle mode.
A conventional solution is to completely turn off the transmitter and use an auxiliary low-power transmitter to set the common-mode voltage. This solution may be viable when it is possible to quickly turn the transmitter on and off. For example, the transmitter can be turned on and off quickly by turning off the data device. However, to turn off the data device completely, a bias voltage on the transmitter must be set to the appropriate power supply (zero volts for an NFET device). This approach is not feasible within some platforms that require a quick transition between operating modes. A large voltage excursion on the driver outputs can occur when the main driver is turned off while the auxiliary driver is turned on. For a low drive-strength auxiliary driver, the resulting voltage excursion can be impossible to overcome such that the required common-mode voltage is achieved within the specified time frame. Additionally, where the output amplitude requirement is large, a large power supply is required for the transmitter. To operate at high voltages, high-threshold devices are normally used. However, high-threshold devices are not suited for the task because high-threshold devices are slow transistors. Thus, regular, low-threshold devices are used. Accordingly, the voltage across the devices must be carefully handled such that the power supply is never seen across these devices. Thus, placing zero volts on an NFET differential pair gates is not possible. This also applies to PFET devices.
Another approach is to reduce the bias voltage just enough to turn off the transmitter and allow the data device to remain on. In addition, an auxiliary low-power transmitter is used to set the common-mode voltage. This approach is limited because when the bias voltage of the bias device is not zero, some leakage current is present. As a result, leakage current may cause an offset in the transmitter common-mode voltage. As one requirement of a multi-operating mode system is to have the same common-mode voltage during normal operating mode and electrical idle mode, only very small amounts of leakage relative to the auxiliary transmitter drive strength are acceptable. If the amount of power allowed in electrical idle mode is set, then it may not be possible to meet both the recovery time and the electrical idle mode voltage requirements.
An additional approach is to change the reference used to generate the bias voltage. Some multi-operating mode systems require a relatively accurate transmitter amplitude (+/−20%). Accurate amplitude generation generally requires a feedback system to set the bias voltage. However, the speed of the feedback system may be limited due to the stability issue of feedback loops. Thus, a feedback network will typically not be able to meet the recovery time requirements.
Thus, what is needed is a method for minimizing the transition time between a circuit's operating modes. The present invention addresses such a need.
A system is disclosed. The system includes a first circuit, the first circuit includes a bias device for allowing the first circuit to transition between a first mode and a second mode. The system further includes a second circuit which controls the bias device. The second circuit provides a bias voltage at a sub-threshold voltage level to the bias device when the first device is in one of the first and the second mode. The second circuit provides a bias voltage at a threshold voltage level or higher when the first device is in one of the first and the second mode. Accordingly, the transition time between modes of the first circuit is minimized.
The present invention relates generally to integrated circuits and in particular to a method and system for minimizing the transition time between a circuit's operating modes. The following description is presented to enable one having ordinary skill in the art to make and use the embodiment and is provided in the context of a patent application and the generic principles and features described herein will be apparent to those skilled in the art. Thus, the present embodiment 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.
A system and method in accordance with the present invention can be utilized to allow a circuit to transition from one mode to another mode by controlling a bias voltage to the circuit. The circuit can quickly transition utilizing a bias device to receive a bias voltage at a voltage above the threshold voltage level in one mode and receiving a sub-threshold voltage level in another mode. Under normal conditions, a FET bias voltage is set to the appropriate power supply voltage to turn the device off. However, if some leakage current is acceptable, then the FET bias voltage does not need to be entirely discharged to the power supply. Since the voltage difference between the sub-threshold voltage level to the normal bias voltage is less than the difference between the power supply and the normal bias voltage, the circuit recovers to the normal bias voltage level more rapidly. Accordingly, the bias voltage at the sub-threshold voltage level is maintained enough above zero volts or ground such that the circuit can transition between modes more quickly than if the bias voltage was zero volts when the circuit transitioned to the electrical idle mode.
Additionally, the bias device may also be controlled such that a common-mode voltage of the driver is the same while the circuit operates in both the normal operating mode and the electrical idle mode given a much increased load resistance (i.e. a low current, low power mode).
A system and method in accordance with the present invention operates advantageously utilizing a current mode logic (CML) transmitter circuit that has multiple operating modes. Although the present invention will be described herein below in the context of a CML transmitter circuit, many other types of circuits could be utilized and that would be within the spirit and scope of the present invention.
In addition, the bias device 101 receives a bias voltage from a circuit coupled to the CML transmitter circuit 100. The bias device 101 receives a bias voltage that is at either a sub-threshold voltage level when the circuit is in the electrical idle mode or receives a bias voltage that is at or above the threshold voltage level when the circuit is in the normal operating mode to allow for a quicker transition between modes than when utilizing conventional transmitters. To describe this operation in detail, refer now to the following description in conjunction with the accompanying figures.
As described above, the control circuit 201 provides a bias voltage to the bias device 101 via paths 203, 204. For an embodiment, only one of paths 203, 204 are closed at one time to allow a bias voltage to pass to bias device 101. Thus, when the control circuit 201 provides a bias voltage to the bias device 101 via path 203, only path 203 is closed and path 204 is open, which inhibits a bias voltage from being provided to the bias device 101, via path 204. Alternatively, when the control circuit 201 provides a bias voltage to the bias device 101 via path 204, only path 204 is closed and path 203 is open, which inhibits a bias voltage from being provided to the bias device 101 via path 203.
Additionally, the control circuit 201 can be utilized to quickly transition the CML transmitter circuit 100 from the electrical idle mode to the normal operating mode. As described above, when the CML transmitter circuit 100 is in the electrical idle mode, the CML transmitter circuit 100 operates at a sub-threshold voltage level. Because the sub-threshold voltage level is defined as a voltage maintained enough above zero volts, the CML transmitter circuit 100 transitions to the normal operating mode from the electrical idle mode more quickly than most conventional circuits. For an embodiment, the sub-threshold voltage level for a FET device ranges from Vth−0.2V and the normal bias level ranges from Vth+0.2V, where Vth is the device threshold voltage A typical value of an NFET Vth in a 90 nm process is 0.4V. Subsequently, if the control circuit 201 provides a bias voltage of 0.2V to the bias device 101 for idle mode, the CML transmitter circuit 100 will transition to the normal operating mode and have bias level 0.6V. Since the voltage excursion is smaller, i.e. 0.4V, than in conventional circuits (i.e. 0V to 0.6V), the CML transmitter circuit 100 can transition more quickly from the electrical idle mode to the normal operating mode.
During the transition between the normal operating mode and the electrical idle mode, the common-mode voltage of the CML transmitter circuit 100 maintains the same voltage level. For an embodiment, the common-mode voltage maintains the same voltage level by varying the impedance of the variable resistive loads 102a, 102b. For example, when the CML transmitter circuit 100 operates in the normal operating mode, the impedance of the variable resistive loads 102a, 102b is approximately 50 ohms. Alternatively, when the CML transmitter circuit 100 operates in the electrical idle mode, the impedance of the variable resistive loads 102, 102b is approximately 2,000 ohms. Accordingly, the impedance of the variable resistive loads 102a, 102b is higher when the CML transmitter circuit 100 operates in the electrical idle mode than when the CML transmitter circuit 100 operates in the normal operating mode.
It is an advantage of a system and method in accordance with the present invention to provide a means for utilizing a bias device for transitioning a circuit between a normal operating mode and an electrical idle mode within a short time period.
It is another advantage to provide a means for controlling a bias voltage of a device such that a common-mode voltage of a device is the same while the circuit operates in both normal electrical idle modes given a much increased load resistance.
It is yet another advantage to limit a change in a bias voltage of a device such that a threshold voltage level can be achieved quickly upon exiting a low power mode.
Although the present embodiment, has been described in accordance with the embodiments shown, one having ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present embodiment. For example, although the present invention has been described in the context of a CML transmitter circuit, many other circuits that transition between two or more operating modes via a current source could be utilized and that would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one having ordinary skill in the art without departing from the spirit and scope of the appended claims.
