Abstract:
A level shifter for use in a dual power supply circuit operating from a VDD power supply and a VDDH power supply greater than the VDD power supply. The level shifter indicates to a status circuit in the VDDH power supply domain that the VDD power supply is enabled. The level shifter detects when the VDD power supply is on and sets an enable signal to the status circuit. The level shifter also detects when the VDD power supply is off and clears the enable signal to the status circuit.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention is generally directed to large scale integrated circuits and, in particular, to power-on reset circuits for use in integrated circuits having dual power supply domains. 
   BACKGROUND OF THE INVENTION 
   In recent years, there have been great advancements in the speed, power, and complexity of integrated circuits, such as application specific integrated circuit (ASIC) chips, random access memory (RAM) chips, microprocessor (uP) chips, and the like. These advancements have made possible the development of system-on-a-chip (SOC) devices. A SOC device integrates into a single chip all (or nearly all) of the components of a complex electronic system, such as a wireless receiver (i.e., cell phone, a television receiver, and the like). SOC devices greatly reduce the size, cost, and power consumption of the overall system. 
   Reductions in power consumption are particularly important in SOC devices. SOC devices are frequently used in portable devices that operate on battery power. Since maximizing battery life is a critical design objective in a portable device, it is essential to minimize the power consumption of SOC devices that may be used in the portable device. Furthermore, even if an SOC device is not used in a portable device, minimizing power consumption is still an important objective. The increased use of a wide variety of electronic products by consumers and businesses has caused corresponding increases in the electrical utility bills of homeowners and business operators. The increased use of electronic products also is a major contributor to the increased electrical demand that has caused highly publicized power shortages in the United States, particularly California. 
   To minimize power consumption in electronic devices, particularly SOC devices, many manufacturers have reduced the voltage levels at which electronic components operate. Low power integrated circuit (IC) technology operating at +3.3 volts replaced IC technology operating at +5.0 volts. The +3.3 volt IC technology was, in turn, replaced by +1.8 volt IC technology in many applications, particularly microprocessor and memory applications. 
   However, as the operating voltage of an integrated circuit is reduced, the noise margins of the integrated circuit are also reduced. Thus, an integrated circuit operating at +1.8 volts has smaller noise margins than a circuit operating at +3.3 volts. In deep submicron VLSI designs, two voltage sources for a chip design are common. One voltage source is an internal core power supply voltage (i.e., VDD) that has a lower swing voltage than the second voltage source, which provides the input/output (I/O) S pad ring voltage (i.e., VDDH). Common range values may include a VDD of 1-1.8 volts and a VDDH range of 2.3-3.6 volts. 
   Many processing systems implement states in which the output power supply, VDDH, is powered up while the internal core power supply, VDD, is zero. In order to allow circuits in the VDDH power supply domain to know the status of the VDD power supply domain, a power status signal in the VDD power supply domain is level shifted and latched into the higher VDDH power supply domain. The power status signal is a power-on reset (POR) signal that is detected and latched by the level shifting circuit. The POR signal indicates that the VDD power supply is ON. 
   Unfortunately, however, in many systems, if the VDD power supply is cycled ON and OFF several times, the latching circuit in the level shifter is not cleared. Thus, if the VDD power supply is turned OFF, the level shifter will falsely indicate that the VDD power supply is present. Some conventional level shifting circuits that clear the VDD status signal each time that VDD is turned OFF consume an excessive amount of current. 
     FIG. 4  illustrates conventional level shifter  400  according to one embodiment of the prior art. Level shifter  400  comprises N-channel transistors  402 ,  404 ,  405 ,  406  and  408 , P-channel transistors  412 - 418 , and capacitor  420 . Level shifter  400  operates between VDDH=+3.3 volts and VSS=ground. The inputs to level shifter  400  are the VDD power supply and the IN signal. Level shifter generates the POWER VALID signal on the OUT node. 
   The IN signal indicates that the VDD power supply is enabled. When VDD goes high, the IN signal goes high shortly thereafter. When VDD goes low, the IN signal goes low shortly thereafter. 
   P-channel transistor  414  and N-channel transistor  404  form a first inverter stage. P-channel transistor  415  and N-channel transistor  406  form a second inverter stage. Finally, P-channel transistors  416 ,  417 , and  418  and N-channel transistor  408  form a third inverter stage. 
   When the VDD power supply is on, VDD is a Logic 1 (+1.8 volts) and the IN signal also is a Logic 1 (+1.8 volts). When the IN signal goes to Logic 1, N-channel transistor  402  is on and the INT 1 * node is pulled down to ground (i.e., Logic 0). This turns on P-channel transistor  413 . When the IN signal is Logic 1, N-channel transistor  404  is on and P-channel transistor  414  is off. This pulls the gate of N-channel transistor  405  to ground, thereby turning off N-channel transistor  405 . Since P-channel transistor  413  is on and N-channel transistor  405  is off, the INT 1  node is pulled up to the VDDH power supply rail. This ensures that P-channel transistor  412  is turned off. Thus the input stage latches the INT 1  node to a Logic 1 level equal to VDDH and latches the INT 1 * node to Logic 0. 
   Since INT 1  is VDDH, capacitor  420  charges up to VDDH. This turns on N-channel transistor  406  and turns off P-channel transistor  415 , so that the INT 2  node is pulled low (i.e., Logic 0). The Logic 0 on INT 2  node turns on P-channel transistors  416 ,  417  and  418  and turns off N-channel transistor  408 . This drives the OUT node high, so that POWER VALID is Logic 1. 
   At some point, the VDD power supply may turn off, so that the VDD power supply rail at the source of P-channel transistor  414  goes to Logic 0 (i.e., ground). The IN signal goes to Logic 0 a fraction of a second after VDD turns off. Unfortunately, the Logic 0 value of the IN signal does not propagate through the first inverter formed by P-channel transistor  414  and N-channel transistor  404 . This is because the VDD power supply rail provides power to the first inverter, and VDD has turned off. 
   As a result, when the IN signal goes to Logic 0, the gate of N-channel transistor  405  does not go to Logic 1. Since N-channel transistor  405  is stuck in the off position, the INT 1  node is stuck at Logic 1. Therefore, if the VDD power supply is cycled on and off, the INT 1  node in the latching circuit in level shifter  400  is not cleared and the OUT signal is stuck at Logic 1. Thus, if the VDD power supply is turned off, level shifter  400  falsely indicates that the VDD power supply is still present. 
   Therefore, there is a need in the art for integrated circuits in which one power supply domain be powered up while internal core circuitry is not powered up. More particularly, there is a need for an improved level shifter circuit that indicates the presence of a valid VDD power supply to a higher VDDH power supply domain that clears itself whenever the VDD power supply signal is turned OFF. 
   SUMMARY OF THE INVENTION 
   To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a level shifter for use in a dual power supply circuit operating from a VDD power supply and a VDDH power supply greater than the VDD power supply. According to an advantageous embodiment of the present invention, the level shifter is capable of indicating to a status circuit in the VDDH power supply domain that the VDD power supply is enabled, wherein the level shifter detects when the VDD power supply is on and sets an enable signal to the status circuit and wherein the level shifter detects when the VDD power supply is off and clears the enable signal to the status circuit. 
   According to one embodiment of the present invention, the level shifter receives the VDD power supply voltage and a VDD status signal, wherein the VDD status signal indicates that the VDD power supply voltage is present. 
   According to another embodiment of the present invention, the level shifter clears the enable signal to the status circuit if either the VDD power supply voltage is off or the VDD status signal indicates that the VDD power supply voltage is not present. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
   Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain is words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
       FIG. 1  illustrates a processing system which comprises an exemplary system-on-a-chip (SOC) device according to one embodiment of the present invention; 
       FIG. 2  illustrates power-on reset (POR) status circuitry according to one embodiment of the present invention; 
       FIG. 3  illustrates an exemplary level shifter according to an advantageous embodiment of the present invention; and 
       FIG. 4  illustrates a conventional level shifter according to one embodiment of the prior art. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 4 , discussed herein, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged integrated circuit. 
     FIG. 1  illustrates processing system  100 , which comprises exemplary system-on-a-chip (SOC) device  105  according to one embodiment of the present invention. SOC device  105  is a single integrated circuit comprising processor core  110 , graphics rendering block  120 , (optional) display control circuit  130 , memory  140 , bandwidth matching-clock synchronization interface  150 , peripheral interface  160 , split transaction, unidirectional bus interface (IF) unit  170  (or bus IF unit  170 ), and bus control processor  180 . Processor core  110  contains internal level one (L 1 ) cache  115 . Peripheral interface  160  communicates with external device  190 . 
   Processing system  100  is shown in a general level of detail because it is intended to represent any one of a wide variety of electronic products, particularly consumer appliances. Display controller  130  is described above as optional because not all end-products require the use of a display. Likewise, graphics rendering block  120  may also be optional. 
   For example, processing system  100  may be a printer rendering system for use in a conventional laser printer. Processing system  100  also may represent selected portions of the video and audio compression-decompression circuitry of a video playback system, such as a video cassette recorder or a digital versatile disk (DVD) player. In another alternative embodiment, processing system  100  may comprise selected portions of a cable television set-top box or a stereo receiver. 
   Bus. IF unit  170  provides high-speed, low latency communication paths between the components coupled to bus IF unit  170 . Each component coupled to bus IF unit  170  is capable of initiating or servicing data requests via four unidirectional bus interfaces: two request buses and a two data buses. The request bus contains address lines, byte enable lines (32-bit or 64-bit data reads), cycle type lines, and routing information for transactions. The data bus contains data lines, byte enable lines (for data writes), completion status lines, and routing information to associate the data bus packets with the appropriate request bus packet. As noted, the four buses are unidirectional and point-to-point to minimize loading and timing variations. In addition, bus IF unit  170  provides a diagnostic bus, power management controls, clocks, reset signals, and a scan interface. 
   Bus IF unit  170  implements a transaction protocol that defines the mechanism for transferring packets between devices coupled to bus IF unit  170 . In addition, the transaction protocol defines the control for clocks and power management. The packet protocol standardizes the system level interactions between devices coupled to bus IF unit  170 . The hardware requirements for mapping transactions, arbitrating packets, and maintaining coherency is specified in the packet protocol. 
   Bandwidth matching-clock synchronization interface  150  comprise a queue that bridges ports on bus IF unit  170  that have different widths or different frequencies, or both. Bus control processor  180  controls certain operations of bus IF unit  170  related to clock timing, power management, and diagnostic features. Peripheral interface  160  is a bus device used for chip-to-chip combination between SOC device  105  and an external peripheral device, such as external device  190 . 
   In an advantageous embodiment of the present invention, SOC device  105  may use two power supplies: an internal low voltage supply (e.g., VDD=+1.8 volts) to power internal logic and an input/output (I/O) high voltage supply (e.g., VDDH=+3.3 volts) to power I/O lines that interface with external circuitry. For example, processor core  110  and bus IF unit  170  may operate at VDD=+1.8 volts and the output stage of peripheral interface  160  may operate at VDDH=+3.3 volts. Additionally, +3.3 volt circuitry may be used within SOC device  105  to drive selected internal address and data lines. For example, if memory (i.e., RAM)  140  is large and separated from bus IF unit  170 , the address and data lines of memory  140  may be driven by +3.3V power supply rails. 
   The present invention provides a level shifting circuit capable of transferring a power status signal from the VDD power domain to the higher VDDH power supply domain. The power status signal is a power-on reset (POR) signal that is detected and latched by a level shifting circuit. The POR signal indicates that the VDD power supply is ON. The latching circuit translates the POR signal to the higher voltage domain. If the VDD power supply is cycled ON and OFF several times, the latching circuit according to the principles of the present invention is cleared each time that VDD is turned OFF. 
     FIG. 2  illustrates power-on reset (POR) status circuitry  200  according to one embodiment of the present invention. POR status circuit  200  comprises power-on reset (POR) detector  210 , filter  220  and level shifter  230 . POR detector  210  may be any conventional circuit that detects with the PWR 2  input (i.e., VDD) is set high and output a high voltage (i.e., +1.8 volt) on the PWR ON output. Filter  220  prevents “glitches” caused by noise spikes in the PWR ON output from reaching level shifter  230 . Thus, a stable Logic 1 is issued to level shifter  230 , which responds by setting the POWER VALID signal to a Logic 1 in the VDDH=+3.3 volt power supply domain. 
     FIG. 3  illustrates exemplary level shifter  230  according to an advantageous embodiment of the present invention. Level shifter  230  comprises N-channel transistors  301 - 308 , P-channel transistors  311 - 318 , and capacitor  320 . Level shifter  230  operates between VDDH=+3.3 volts and VSS=ground. The inputs to level shifter  230  are the VDD power supply and the IN signal, which is coupled to the OUT signal from filter  220 . Level shifter generates the POWER VALID signal on the OUT node. 
   The gate and the drain of N-channel transistor  301  and the gate and the drain of N-channel transistor  303  are connected to the VDDH power supply rail. In this configuration, N-channel transistors  301  and  303  cause threshold voltage drops between the VDDH power supply rail and the sources of P-channel transistors  311  and  314 , respectively. The threshold voltage drops ensure that P-channel transistors  311  and  314  turn completely off and have leakage currents that are nearly zero. 
   P-channel transistor  314  and N-channel transistor  304  form a first inverter stage. P-channel transistor  315  and N-channel transistor  306  form a second inverter stage. Finally, P-channel transistors  316 ,  317 , and  318  and N-channel transistor  308  form a s third inverter stage. 
   When the VDD power supply is ON, VDD is a Logic 1 (+1.8 volts) and the IN signal also is a Logic 1 (+1.8 volts). VDD equal to Logic 1 turns on N-channel transistor  307  and turns off P-channel transistor  311 . When the IN signal goes to Logic 1, N 10  channel transistor  302  is on and the INT 1 * node is pulled down to ground (i.e., Logic 0). This turns on P-channel transistor  313 . 
   When the IN signal is Logic 1, N-channel transistor  304  is on and P-channel transistor  314  is off. This pulls the gate of N-channel transistor  305  to ground, thereby turning off N-channel is transistor  305 . Since P-channel transistor  313  is on and N-channel transistor  305  is off, the INT 1  node is pulled up to the VDDH power supply rail. This ensures that P-channel transistor  312  is turned off. Thus the input stage latches the INT 1  node to a Logic 1 level equal to VDDH and latches the INT 1 * node to Logic 0. 
   Since INT 1  is VDDH, capacitor  320  charges up to VDDH. This turns on N-channel transistor  306  and turns off P-channel transistor  315 , so that the INT 2  node is pulled low (i.e., Logic 0) through N-channel transistor  307 . The Logic 0 on INT 2  node turns on P-channel transistors  316 ,  317  and  318  and turns off N-channel transistor  308 . This drives the OUT node high, so that POWER VALID is Logic 1. 
   The IN signal goes to Logic 0 whenever VDD goes to Logic 0 (i.e., ground) in that the high value of the IN signal is set by VDD. The IN signal goes high (i.e., to the value of VDD at that time) when a sampling circuit sampling VDD indicates that VDD is high enough to be considered valid. 
   At some point, the VDD power supply may go low, so that the IN signal goes to Logic 0 (i.e., ground) and the VDD input goes to Logic 0 (i.e., ground). VDD equal to Logic 0 turns off N-channel transistor  307  and turns on P-channel transistor  311 . When the IN signal goes to Logic 0, N-channel transistor  302  is off and the INT 1 * node is pulled high by N-channel transistor  301  and P-channel transistor  311 . This turns off P-channel transistor  313 . When the IN signal is Logic 1, N-channel transistor  304  is off and P-channel transistor  314  is on. This pulls the gate of N-channel transistor  305  up to Logic 1, thereby turning on N-channel transistor  305 . Since P-channel transistor  313  is off and N-channel transistor  305  is on, the INT 1  node is pulled down to the VSS power supply rail (i.e., ground). This discharges capacitor  320  through N-channel transistor  305 . This turns on P-channel transistor  312 , which pulls the INT 1 * node up to the VDDH power supply rail. Thus, the input stage latches the INT 1  node to a Logic 0 level and latches the INT 1 * node to Logic 1. 
   Since INT 1  is pulled down to ground, N-channel transistor  306  is off and P-channel transistor  315  is on. This pulls the INT 2  node up to the VDDH power supply rail (i.e., Logic 1). The Logic 1 on the INT 2  node turns off P-channel transistors  316 ,  317  and  318  and turns on N-channel transistor  308 . This drives the OUT node low, so that POWER VALID is Logic 0. 
   Although the present invention has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.