Bidirectional Level Shifting Interface Circuit

The invention relates generally to electronic interface circuits and deals more particularly with a bidirectional level shifting interface circuit for use between two different digital circuits operating at different voltage levels.

Many systems utilize two different circuit technologies which operate with a common ground (binary zero level) but different binary one or high voltage levels. Such circuits require an interface to permit bidirectional communication between the two circuits. For example, new CMOS technology operates at lower voltage than older CMOS technologies to improve performance and density. However, it may be advantageous in some applications to provide in one system a combination of both technologies. In such a system, each printed circuit board or other circuit carrying card typically contains only one technology and is connected by a transmission line to another board or card containing a different technology. If one of the CMOS technologies operates between 3.4 volts and ground and the other operates between 5 volts and ground, a 3.4 volt binary one signal from the first technology cannot reliably drive the second technology. This is because the second technology can have a maximum 3.1 volt threshold and there are losses in greater margin of drive voltage. Also, if a 5 volt signal were applied directly to the 3.4 volt receiver, the 5 volt signal could breakdown the 3.4 volt receiver. Thus, an interface is required to drive the respective receiving circuit with a proper voltage level.

Heretofore it was known to provide a level shifting transceiver between the two circuit boards to provide the appropriate voltage for each technology and a non-level shifting transceiver at the I/O of each circuit. Such a level shifting transceiver comprises two buffer gates connected in opposite directions back-to-back, and an inverter gate to enable one buffer gate while disabling the other (to provide a high impedance output) depending on the direction of communication. One of the buffer gates was biased at the 5 volt level and the other buffer gate was biased at the 3.4 voltage level. However, there are problems with such a level shifting transceiver because it requires a separate enable signal and adds a propagation delay through the respective buffer gate.

U.S. Pat. No. 4,216,390 to Stewart discloses a unidirectional level shifting circuit which comprises an N Channel FET. A source of the FET Channel is connected to first relatively low voltage digital circuit via a first inverter circuit, and a drain of the FET Channel is connected to a second relatively high voltage digital circuit via a second inverter circuit. Thus, a relatively low voltage, binary one communication signal of the first digital circuit passes through the first inverter circuit, the channel of the FET and the second inverter circuit to the output of the level shifting circuit. This output is also connected to an input of a first latch and the output of the first latch is connected to the input of the second inverter circuit. Thus, when the relatively low voltage binary one signal passes to the output of the level shifting circuit, it is latched to the relatively low voltage. This level shifting circuit also includes another FET which has its source connected to the high voltage supply and its drain to the second inverter. This FET is activated by a separate enable signal to deliver the relatively high voltage to the output of the level shifting circuit via the second inverter after the relatively low voltage passes to the output of the level shifting circuit.

A general object of the present invention is to provide a bidirectional level shifting interface circuit for use between two different circuits operating at different voltage levels.

A more specific object of the present invention is to provide an interface circuit of the foregoing type which introduces a minimal propagation delay therethrough.

Another specific object of the present invention is to provide an interface circuit of the foregoing type which does not require an enable or other control signal to operate.

The invention resides in bidirectional level shifting interface circuit comprising first and second I/O ports and a first FET having a drain-source channel connected between the first and second I/O ports. The first I/O port is connected via a transmission line to a third I/O port of a first digital circuit operating at a relatively low supply voltage, and the second I/O port is connected via a transmission line to a fourth I/O port of a second digital circuit operating at a relatively high supply voltage. The drain-source channel passes communication signals in each direction between the first and second digital circuits. To pass binary zero signals, the FET serves in both directions in the current sinking path. To pass a binary one signal from the second digital circuit, the FET can clamp the binary one voltage to a level compatible with the first digital circuit. To pass a binary one signal from the first digital circuit, the relatively low voltage signal passes through the FET and doubles in magnitude at the receiver within the second digital circuit due to a high input impedance of the receiver relative to the characteristic impedance of the transmission line. Also, the interface circuit includes a latching circuit which is biased by the relatively high voltage supply, has an output connected to the second I/O port, and has a control input responsive to the voltage at the second I/O port, for latching the second I/O port at sufficient voltage to drive the second digital circuit at binary one level following the transmission of a binary one signal by said first digital circuit.

According to one feature of the invention, the latching circuit comprises a P Channel FET and the interface circuit further comprises an inverter circuit having a control input connected to the second I/O port and an inverted output connected to the gate of the P Channel FET. Consequently, when the second I/O port receives a reflection from the receiver of the second digital circuit, the inverted output exhibits a binary zero voltage to activate the P Channel FET and thereby latch the second I/O port at sufficient voltage to drive the second digital circuit at binary one level .

FIG. 1 is a schematic, block diagram of an electronic system utilizing two different circuit technologies, a bidirectional level shifting interface circuit therebetween according to the present invention, and two transmission lines which connect the interface circuit to the two different circuits.

FIG. 2 is a detailed circuit diagram of the bidirectional level shifting interface circuit of FIG. 1.

Referring now to the drawings in detail wherein like reference numerals indicate like elements throughout the several views, FIG. 1 illustrates a digital system comprising a first digital circuit 12, a second digital circuit 16, and a bidirectional level shifting interface circuit 10 in accordance with the present invention. Circuit 10 is interposed between the two digital circuits, and transmission lines 14 and 18 interconnect the interface circuit to digital circuits 12 and 16, respectively. Digital circuit 12 operates between a relatively high, binary one voltage V.sub.DDH, for example, 5 volts, and ground, and digital circuit 16 operates between a relatively low binary one voltage V.sub.DDL, for example 3.4 volts, and the same ground. In the aforesaid example, both digital circuits 12 and 16 utilize CMOS technology, and each includes a non-level shifting transceiver input/output (I/O) circuit.

The non-level shifting transceiver of circuit 12 comprises a transmitter 20, receiver 19, and input 21 to enable the transmitter only during transmission. When disabled, transmitter 20 exhibits a high impedance output to prevent loading of the transmission line. When enabled, transmitter 20 exhibits an output impedance which equals a characteristic impedance of transmission line 14. Both transmitter 20 and receiver 19 are biased with the relatively high supply voltage V.sub.DDH characteristic of circuit 12.

The transceiver of digital circuit 16 comprises a transmitter 23, a receiver 22 and an input 25 which is used to enable the transmitter only during transmission. When disabled, transmitter 23 exhibits a high impedance output to prevent loading of the transmission line, and when enabled, transmitter 23 exhibits an output impedance which is much less than the characteristic impedance of transmission line 18. By way of example, each of the transmission lines has a characteristic impedance of 80 ohms, and the output impedance of transmitter 23 when enabled is 20 ohms. Both transmitter 23 and receiver 22 are biased with the relatively low supply voltage V.sub.DDL characteristic of circuit 16.

FIG. 2 illustrates in detail the circuitry of interface circuit 10. Circuit 10 includes I/O ports 24 and 26 connected respectively to transmission lines 18 and 14. Circuit 10 further comprises N Channel FET 30 which channel is connected between and directly to ports 24 and 26 to pass communication signals in each direction between digital circuits 12 and 16 with minimal propagation delay. Experiments have revealed only a 0.9 nanosecond delay through the channel. Preferably, the channel is wide (for example 2000 micrometers) to minimize the on resistance such that the on resistance is much less than the characteristic impedance of transmission lines 14 and 18. For example, if each transmission line has a characteristic impedance of 80 ohms, the on resistance of the FET Channel should be approximately 10 ohms or less. The gate of FET 30 is biased to approximately V.sub.DDL, the relatively low operating voltage of digital circuit 16. In the illustrated embodiment, the gate is biased to approximately 0.3 volts above the V.sub.DDL by a voltage divider comprising resistor 34 (for example 500 ohms) and resistor 36 (for example 2K ohms). Capacitor 40 (for example 10 picafarads), stabilizes the gate voltage. When either transmitter 20 or 23 is enabled and presents a binary zero or ground voltage at its output, the drain-source channel of FET 30 serves in the current sinking path to provide the necessary interface.

However, interface circuit 10 includes additional circuitry to assist in the current sinking function when either transmitter 20 or 23 is enabled and exhibits binary zero voltage. This additional circuitry includes N Channel FET 50 (having for example, an on resistance of 300 ohms) and a series resistor 52 (having for example 500 ohms), and an inverter circuit 53 connected between output port 26 and the gate of FET 50. Inverter circuit 53 comprises P Channel FET 54 and N Channel FET 56 having a common gate connection at port 26 and a common output connection to the gate of FET 50. Consequently, when either transmitter outputs binary zero (approximately ground) voltage, the output of inverter 53 is binary one and activates N Channel FET 50. The channel of FET 50 is connected to I/O port 24 (or optionally to I/O port 26) to assist transmitter 20 or transmitter 23 in sinking the binary zero current. Resistor 52 is large enough to permit subsequent transmission of a binary one signal.

When transmitter 20 outputs its binary one level signal, of approximately 4.0 volts, approximately one half that amount is transmitted along transmission line 14 due to the output impedance of transmitter 20. When the binary one signal reaches FET 30, there is a small loss in voltage due to the channel on-resistance of FET 30. Then, the binary one signal proceeds along transmission line 18 toward receiver 22. Transmitter 23 is disabled during transmission by transmitter 20. Consequently, the input impedance is the input impedance of receiver 22 which is very large, for example, several hundred thousand ohms. Consequently, the voltage at the input of receiver 22 doubles. This is due to the fact that the energy of the binary one signal transmitted along transmission line 18 is provided by a magnetic and an electric field, and at the input of receiver 22, all the energy of the magnetic field is converted to the electric field energy. In the aforesaid example, the doubling of the voltage at the input of receiver 22 yields approximately 4 volts which is more than enough voltage to provide the binary one signal with receiver 22 but not so much voltage to break down receiver 22.

Also, when the binary one signal from transmitter 20 reaches receiver 22, a reflection is caused due to the high input impedance of receiver 22. This reflection propagates back through the channel of FET 30 and triggers inverter 53. Inverter 53 outputs binary zero voltage to activate FET 60 and assist in driving receiver 22. The activation of FET 60 prevents multiple reflections of the binary one signal between the I/O ports of digital circuits 12 and 16. The channel of FET 60 is connected at one end to V.sub.DDH via resistor 64 and at the other end to I/O port 24. By way of example, the channel of FET 60 has an on-resistance of 100 ohms and resistor 64 is a 100 ohm resistor. FET 30 limits the voltage of the binary one signal which passes through its channel to a level below the gate voltage to prevent overdrive of receiver 22. It should also be noted that during the transmission of the binary one signal from transmitter 20, the input of inverter 53 is high, the output of inverter 23 is low, and consequently FET 50 is shut off to prevent loading of the binary one signal.

When transmitter 23 transmits a binary one level signal, most of this voltage (for example, 2.5 volts) is received by transmission line 18 due to the low output impedance of transmitter 23. This signal passes through the channel of FET 30 where it looses a small amount of energy due to the on-resistance of the channel. Then, the signal travels along transmission line 14 toward receiver 19. At this time, receiver 19 is enabled and transmitter 20 is disabled by an appropriate signal applied to gate 21. The signal doubles in amplitude at the input of receiver 19 due to the high input impedance of receiver 19 to approximately 4.0 volts to drive receiver 19. Because of the impedance miss match at the input of receiver 19, there is also a reflection toward interface circuit 10, which reflection activates inverter 53 to provide a binary zero signal at the output of inverter 53. This binary zero signal activates P Channel FET 60. Thus, the voltage output from port 26 is pulled-up and clamped to approximately V.sub.DDH to thoroughly drive receiver 19. It should be noted that the gate voltage of FET 30 prevents the voltage at port 24 from exceeding V.sub.DDL to prevent reverse bias of transmitter 23 and breakdown of receiver 22.

In the illustrated embodiment, the threshold voltage of inverter 53 is greater than the voltage of the binary one signal transmitted by circuit 16 directly to I/O port 26. This makes it easier to deactivate inverter 53 when either transmitter transmits a binary zero signal. However, optionally, the threshold voltage of inverter 53 can be reduced such that the voltage of the binary one signal transmitted by transmitter 23 is sufficient to directly trigger inverter 53 to provide a binary zero output and thereby activate FET 60 to latch the output voltage (before the reflection is sensed by the input of inverter 53).

By way of example, resistors 62 and 52 can be designed with a minimum dimension printed polysilican line so that if the printed dimension narrows due to process tolerances, the resistances of resistor 62 and resistor 52 will increase but the on resistance of FET 50 and FET 60 will decrease to compensate for the tolerance.

Based on the foregoing, a bidirectional level shifting interface circuit embodying the present invention has been disclosed. However, numerous modifications and substitutions can be made without deviating from the scope of the invention. For example, the interface circuit 10 can be biased by different binary one voltages other than the 3.4 volt V.sub.DDL and 5.0 volt V.sub.DDH. Therefore, the invention has been disclosed by way of illustration not limitation, and reference should be made to the following claims to determine the scope of the invention.