Digital voltage shifters and systems using the same

Abstract: A digital voltage shifter is provided which includes an input buffer 200 having an input for receiving data logic high signals at a first voltage, a true output and a complementary output. A static random access memory cell 220 is also included which operates in response to a voltage supply providing a second voltage differing from the first voltage and having a first input coupled to the true output of the input buffer and a second input coupled to the complementary output of the input buffer. An output driver 230 is further included which operates in response to the second supply voltage and is coupled to an output of the memory cell, the output driver outputting the received logic signals at the second voltage.

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates in general to electronic devices and systems and in particular to digital voltage shifters and systems using the same. BACKGROUND OF THE INVENTION In many electronic systems it becomes necessary to shift the voltage levels of signals travelling between circuits or devices operating at different supply voltages. For example, a digital integrated circuit may include core logic circuitry operating in conjunction with a 3 volt voltage supply and associated input/output circuitry operating in conjunction with a 5 volt voltage supply. The logic high signals output from the core logic to the I/0 circuitry will be at approximately 3 volts, and therefore shifting upward is required to insure that such signals meet the input voltage threshold and noise immunity requirements of the 5 volts I/O circuitry. In the other direction, logic high signals travelling from the I/O circuitry to the core logic circuitry will be at approximately 5 volts and therefore shifting downward is required to insure that the maximum input voltage of the core circuitry is not be exceeded. There are numerous other instances where voltage shifting, upwards or downwards, is required. Currently available voltage shifting circuits are typically of an analog design. Such analog circuits, especially those employing rationed inverters, consume a substantial amount of power. Given the importance of power conservation in most electronic circuits and systems, the need has arisen for improved voltage shifting circuitry and methods which minimize power consumption. US-A-4 039 862 describes a level shift circuit, in which each one of two switching transistors, driven by complementary input signals, has its conduction path connected between a different one of two output terminals and a first point of potential. Connected between each output terminal and a second point of potential are the conduction paths of a load transistor responsive to the signal at the other output terminal, and an input signal responsive transistor. When the switching transistor connected to one output terminal is being turned on, the effective impedance of the input signal responsive transistor connected between, that output and the second point of potential is increased. US-A-4 914 318 describes a latch circuit having a dual n-type driver transistors to provide an output which is compatible to TTL and CMOS. A complementary pair of input signals is coupled to drive a pair of input transistors which are enabled by a clocking signal. The input transistors are driven by a crosscoupled inverters such that the output of each inverter is coupled to a gate of one or the other of the n-type driver transistors. SUMMARY OF THE INVENTION The subject matter of the present invention is defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: FIGURE 1 is a functional block diagram of a typical electronic system in which voltage shifting is required; FIGURE 2 is an electrical schematic diagram of a digital voltage shifter according to the principles of the present invention; and FIGURE 3 is an electrical schematic diagram of a second digital voltage shifter according to the principles of the present invention. FIGURE 4 is a functionnal block diagram of a bidirectional level shifting buffer using the voltage shifter of FIGURE 3. DETAILED DESCRIPTION OF THE INVENTION The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGURES 1-3 of the drawings, in which like numbers designate like parts. FIGURE 1 is a functional block diagram of a digital integrated circuit system 100 employing a digital voltage shifter 101 embodying the principles of the present invention. It should be noted that system 100 is illustrative of the uses of voltage shifter 101 and that the use of voltage shifter 101 is not limited thereto; voltage shifter 101 may be employed in any one of a number of devices, circuits and systems in which voltage shifting is desired. System 100 includes core processing circuitry 102 which outputs logic high digital data at a voltage level of approximately +3 volts as a function of a +3 volt power supply. Core logic circuitry 102 performs the primary data processing functions of system 100. System 100 also includes input/output circuitry 103 which provides an interface with associated systems, circuits or devices. In a digital integrated circuit application, I/O circuitry 103 receives data from and transmits data to the device bond pads. I/O circuitry 103 operates in conjunction with a +5 volt power supply and inputs and outputs logic high data at approximately the +5 volt level. Assuming for the purposes of discussion that data is being transmitted from core circuitry 102 to I/O circuitry 103 for output. In this case, voltage shifter 101 shifts upward the +3 volt logic high signals to the +5 volt level required to meet the input voltage threshold requirements of I/O circuitry 103. As will become apparent in the discussion below, voltage shifter can also be used to down shift signals (data) being transmitted from I/O circuitry 102 at the +5 volt level to the +3 volt level required by core logic circuitry 102. FIGURE 2 is an electrical schematic diagram of a preferred embodiment of voltage shifter 101. Voltage shifter 101 generally includes an input buffer section 200, a load section 210, a static random access memory (SRAM) cell 220, and an output driver section 230. Each of these sections will be discussed in detail below; however, it should be recognized at this point that the transistor types and voltages shown in FIGURE may be reversed in alternate embodiments. It should also be recognized that the embodiment of voltage shifter 101 shown in FIGURE 2 may be used to shift the voltage level of a data stream either up or down, depending on the supply voltage connections. Input buffer section 200 includes n-channel transistors 201, 202 and 203 and p-channel transistor 204. The gates of n-channel transistors 201 and 202 and p-channel transistor 204 are coupled to the input signal source data in which in the example where data is being transferred from core logic 102 to I/O circuitry 103 system 100 is core circuitry 102. One source/drain of each n-channel transistor 201 and 203 is coupled to SRAM section 220 (these source/drain terminals of transistors 201 and 203 respectively comprise the complementary and true outputs of input buffer section 200). One source/drain and the tank of p-channel transistor 204 are coupled to Voltage Supply B (+3 volts for the voltage up-shifting case in system 100). The other source/drain of p-channel transistor 204 and the other source/drain of n-channel transistor 202 are coupled together and to the gate of n-channel transistor 203 (transistors 202 and 204 form an invertor with their coupled gates being the invertor input and their coupled source/drains being the invertor output). The other source/drains of n-channel transistors 201, 202 and 203 are coupled to V SS (zero volts) or ground. In the case where the voltage of voltage supply B is less than the voltage of voltage supply A, the threshold voltages V L of transistors 201, 202 and 203 are preferably selected to allow for a switching point of Voltage B/2 for the transistor 202/transistor 204 inverter. Load section 210 includes a p-channel transistor 211 and an n-channel transistor 212. Both source/drains and the tank of p-channel transistor 211 are coupled to Voltage Supply A (+5 volts for the voltage up-shifting case in system 100). Both source/drains of n-channel transistor 212 are coupled to V SS (zero volts) or ground. The gates of both load transistors 211 and 212 are coupled together and to SRAM section 220. SRAM section 220 includes a pair of p-channel transistors 221 and 222 and a pair of n-channel transistors 223 and 224. One source/drain and the tank of each p-channel transistor 221 and 222 is coupled to Voltage Supply A. The other source/drain of p-channel transistor 221 is coupled to the gates of transistors 211 and 212 of load section 210, one source/drain of n-channel transistor 223, the true output of input buffer section 200, and the gates of transistors 222 and 224. The other source/drain of p-channel transistor 222 is coupled to output driver section 230, one source/drain of n-channel transistor 224, and the complementary output of input buffer 200, and the gates of transistors 221 and 223. The other source/drains of n-channel transistors 223 and 224 are coupled to ground V SS (zero volts). The gates of transistors 211 and 212 represent one input to SRAM section 220 and the gates of transistors 222 and 224 another input thereto. Output driver section 230 includes a p-channel transistor 231 and an n-channel transistor 232. One source/drain and the tank of p-channel transistor 231 are coupled to Voltage Supply A. The other source/drain of transistor 231 is coupled to one source/drain of n-channel 232, such connection also providing the output of voltage shifter 101. The other source/drain of n-channel transistor 232 is coupled to ground. The gates of transistors 231 and 232 are coupled to the source/drains of transistors 222 and 224 (the output of SRAM section 220). The operation of the embodiment of voltage shifter 101 described in FIGURE 2 can now be described. When a logic high signal is received at the input DATA IN, n-channel transistors 201 and 202 turn on and transistors 202 and 203 turn off. When transistor 201 turns on, the gate of p-channel transistor 221 of SRAM section 220 is pulled down and transistor 221 turns on. Transistor 224, along with transistor 201, pulls down the gate of output driver transistor 231. Transistor 213 turns on and the output DATA OUT is pulled to approximately the voltage of Voltage Supply B. When a logic low signal is received at the input DATA IN, transistors 201 and 202 of input buffer 200 turn off and transistors 204 and 203 turn on. When transistor 203 turns on, the gate of transistor 222 is pulled low thereby turning on transistor 222. The voltage of Voltage Supply A passed by transistor 222 turns on n-channel output drive transistor 232 and the output DATA OUT is pulled low. Additionally, n-channel transistor 223 turns on which helps pull down the gate of transistor 222. The embodiment of voltage shifter 101 depicted in FIGURE 2 is preferably employed to drive a unidirectional bus crossing power supply boundaries. As discussed above, the circuit of FIGURE 2 may be used, depending on the voltages provided by Supply A and Supply B, to either shift-up the voltage of data being sent from core circuitry 102 to I/O circuitry 103 via a unidirectional bus or shift down the voltage of data being sent from I/O circuitry 103 to core circuitry 102 via a unidirectional bus. The Voltage shifter embodiment 300 depicted in FIGURE 3 however may be used to shift voltages on a multiplexed bus crossing power supply boundaries. In voltage shifter 300 of FIGURE 3, an additional p-channel transistor 301 and an additional n-channel transistor 302 have been included in output driver section 230. Transistor 301 includes a source/drain coupled to Voltage Supply A, another source/drain coupled to transistor 231 and a gate coupled to a control signal 1. Transistor 302 includes a source/drain coupled to transistor 232, another source/drain coupled to ground and a gate coupled to the complement of control signal 1 ( 1 ). The output driver section 230 of voltage shifter 300 allows the data output DATA OUT to be coupled to a multiplexed bus since DATA OUT can be set to a high impedance state while a second device is driving the bus. Specifically, when 1 is set high (i.e., to a voltage equal to the voltage of Voltage Supply A) and 1 is set low the DATA OUT output is in a high impedance state. When control signal 1 is low and 1 is high, the voltage at the output DATA OUT is controlled by transistor 231 and 232 as described above. FIGURE 4 depicts a bi-directional level shifting buffer 400 employing a pair of voltage shifters 401 and 402 each constructed in accordance with embodiment of FIGURE 3. For illustration purposes, it will be assumed that buffer 400 is interfacing a 5 volt data bus 403 and a 3 volt data bus 404. When CLKA is set to 5 volts, CLKA bar to 0 volts, CLKB to 0 volts and CLKB bar to 3 volts, data received on 3 volt bus 404 is shifted up to 5 volts and transferred to 5 volts bus 403. When CLKA is set to 0 volts, CLKA bar to 5 volts, CLKB to 3 volts and CLKB bar to 0 volts, data received on 5 volt bus 403 is shifted down to 3 volts and transferred to 3 volt bus 404. The embodiments of the present invention provide substantial advantages over the prior art voltage shifting circuits and methods. Among other things, the use of digital techniques lessens the possibility of dwelling on a transistor gate and consuming a large amount of power. Further, the principles of the present invention provide for a voltage shifter with an output that can be switched into a high impedance state such that the voltage shifter into a high impedance state such that the voltage shifter may be coupled to a multiplexed bus.

Claims: A digital voltage shifter for shifting the voltage of logic high data signals from a first voltage to a second voltage including a first transistor (201) of a first type having a control terminal for receiving said signals at said first voltage, a first current path terminal and a second current path terminal coupled to a low voltage, and an invertor (202, 204) having an input for receiving said signals ; further comprising : a second transistor (203) of said first type having a control terminal coupled to an output of said invertor, a first current path terminal and a second current path terminal coupled to said low voltage ; a third transistor (221) of a second type having a first current path terminal coupled to a voltage supply providing said second voltage and a control terminal coupled to said first current path terminal of said first transistor (201) ; a fourth transistor (223) of said first type having a first current path terminal coupled to a second current path terminal of said third transistor, a second current path terminal coupled to said low voltage, and a control terminal coupled to said first current path terminal of said first transistor (201) ; a fifth transistor (222) of said second type having a first current path terminal coupled to said voltage supply, a second current path terminal coupled to said first current path terminal of said first transistor (201), and a control terminal coupled to said first current path terminal of said second transistor (203) ; and a sixth transistor (224) of said first type having a first current path terminal coupled to said second current path terminal of said fifth transistor (222), a second current path terminal coupled to said low voltage, and a control terminal coupled to said first current path terminal of said second transistor (203), and wherein a node at the coupling of said second current path terminal of said fifth transistor (222) and said first current path terminal of said sixth transistor (224) comprises an undriven output of said voltage shifter ; and wherein said fifth transistor (222) and said fourth transistor (223) are turned off when a logic high signal is received by said first transistor (201) thereby saving power. The voltage shifter of claim 1, wherein said transistors comprise field effect transistors, said control terminals comprising gates and said current path terminals comprising source/drains. The voltage shifter of claim 1, wherein each of said transistors of said first type is operable to conduct current between said respective first and second current path terminals in response to the application of a positive voltage to said respective control terminals. The voltage shifter of claim 3, wherein each of said transistors of said first type comprises an n-type field effect transistor. The voltage shifter of claim 1, wherein each of said transistors of said second type is operable to conduct current between said respective first and second current path terminals in response to the application of a voltage of approximately zero volts to said respective control terminal. The voltage shifter of claim 5, wherein each of said transistors of said second type comprises a p-channel field effect transistor. The voltage shifter of claim 1 and further comprising an output driver (230) including : a first output transistor (231) of said first type having a first current path terminal coupled to said voltage supply and control terminal coupled to said voltage supply and control terminal coupled to said output of said memory cell (220) ; a second output transistor of said second type (232) having a first current path terminal coupled to a second current path terminal of said first output transistor (231), a second current path terminal coupled to said low voltage, and a gate coupled to said output of said memory cell (220), a node at the coupling of said memory cell (220), a node at the coupling of said second current path terminal of said second output transistor and said first current path terminal of said second output transistor comprising a voltage shifter output. The voltage shifter of claim 1 and further comprising an output driver including : a first output transistor (301) of said first type having a first current path terminal coupled to said voltage supply and a control terminal coupled to a first control signal source ; a second output transistor (231) of said first type having a first current path terminal coupled to a second current path terminal of said first output transistor and a control terminal coupled to said output of said memory cell ; a third output transistor (232) of said second type having a first current path terminal coupled to a second current path terminal of said second output transistor, and a control terminal coupled to said output of said memory cell, a node at the coupling of said second current path terminal of said second output transistor and said first current path terminal of said third output transistor comprising a voltage shifter output ; and a fourth output transistor (302) of said second type having a first current path terminal coupled to a second current path terminal of said third output transistor, a second current path terminal coupled to said low voltage, and a control terminal coupled to a second control signal source ; The voltage shifter of claim 8, wherein a control signal (1) generated by said second control signal source is the complement of a control signal (  1) generated by said first control signal source. The voltage shifter of claim 1 wherein said invertor comprises a first transistor (202) of a first type and a second transistor (204) of a second type each having a gate for receiving said signals, said first transistor (202) having a first source/drain coupled to a voltage supply outputting at said first voltage, a second source/drain coupled to a first source/drain of said second transistor, said second transistor (204) having a second source/drain coupled to said low voltage. The voltage shifter of claim 1 and further comprising a load (210) including : a first transistor (211) of said first type having first and second source/drains coupled to said voltage supply, and a second transistor (212) of said second type having first and second source/drains coupled to said low voltage and a gate coupled to a gate of said first transistor (211) of said load (210) and said second source/drain of said first transistor (221) of said memory cell. The voltage shifter of claim 10, wherein said first transistor of said first type comprise an p-channel transistor and said second transistor of said second type comprise a n-channel transistor. The voltage shifter of claim 1, wherein said transistors of said first type comprise n-channel transistors and said transistors of said second type comprise p-channel transistors, or wherein said transistors of said first type comprise p-channel transistors and said transistors of said second type comprise n-channel transistors.

Inventors: RUNAS MICHAEL E, RUNAS, MICHAEL E.
Applicants: CIRRUS LOGIC INC, CIRRUS LOGIC, INC.
Application year: 1995
Main classification: H03K19
IPC codes: H03K3