Technical Field

The present invention relates to a TTL to CMOS input buffer designed to prevent current flow therethrough.

2. Description of the Prior Art

For many applications, it is desirable to provide a circuit which is capable of interfacing between transistor—transistor logic (TTL) levels and complementary MOS (CMOS) logic levels. In particular, TTL logic levels are nominally +2.4V for a logic "1" and 0.4V for a logic "0", and the associated CMOS levels are nominally +5.0V and 0.0V. The conventional interface circuit, also referred to in the art as an "input buffer circuit", comprises a p—channel MOS transistor and an n—channel MOS transistor connected in series between a positive power supply VDD (usually 5V) and ground. The gates of the devices are connected together and responsive to the TTL input signal. The drains of the transistors are also connected together and provide the CMOS output signal. In the ideal situation, one transistor of the pair will always be "off", preventing any current to flow through the pair of transistors from VDD to ground. However, this is not always the case. In particular, problems arise at the TTL input level for a logic "1", 2.4V, and when the TTL input level for a logic "0" is somewhat greater than 0.4V, for example, 0.08V. At these levels, both transistors may be "on" and a current will flow through the transistor pair to ground. One solution is disclosed in U.S. Patent

4,471,242. The current flow through the devices is eliminated in this arrangement by introducing a reference voltage to match the lowest level of a logic "1" of the

TTL input signal. This reference voltage is utilized in lace of VDD as the supply voltage to the p-channel transistor, thus preventing the p-channel transistor from turning "on" when its gate voltage is at the lowest TTL logic "1" input level. A problem with this arrangement, however, is that by reducing the supply voltage of the p- channel transistor, the operating range of the buffer circuit is also restricted. By lowering the voltage available to operate the transistors, therefore, the device will be inherently slower. For many applications, this is not acceptable.

As an alternative solution, the actual sizes of the p- and n—channel transistors may be modified to prevent the static current flow. However, this solution is not practical since it requires additional masking levels and, therefore, additional processing time. Further, it is difficult with this method to accurately control the device sizes so as to reproducibly provide the necessary threshold voltage. Therefore, a need remains in the prior art for a

TTL to CMOS input buffer which draws no static current, does not require additional processing steps, and is capable of operating over the entire 0-5V CMOS power supply level. Summary of the Invention

A TTL to CMOS input buffer is provided which utilizes a transition detector and an additional pair of MOS devices to prevent static current flow when the TTL logic "1" input signal is at relatively low levels. Brief Description of_the Drawings

FIG. 1 illustrates a simple prior art CMOS inverter which may be utilized to shift TTL logic level input signals to CMOS logic level output signals;

FIG. 2 is a graph illustrating the voltage transfer function, as well as static current flow, for the device illustrated in FIG. 1; and

FIG. 3 illustrates an exemplary TTL to CMOS input buffer without static current flow formed in accordance with the present invention.

Detailed Descrigtion A prior art CMOS input buffer 10 is illustrated in FIG. 1. Input buffer 10 includes a p-channel MOS transistor 12 and an n-channel MOS transistor 14 connected in series between a positive power supply (denoted VDD) and a negative power supply, generally, ground. The gates of transistors 12 and 14 are connected together and receive as an input signal V_ I-N _τ , the TTL input logic signal. Similarly,- the drains of transistors 12 and 14 are connected together and provide the CMOS logic output signal, here denoted ^v -ττr _j ι» ^Λs shown in FIG. 1, a logic "0" to a logic "1" transition in TTL relates to a step change in voltage from approximately 0.4V to approximately 2.4V. In operation, when the input signal is at the logic "0" value, p-channel transistor 12 will be turned "on" and n- channel transistor 14 will be turned "off". Therefore, the voltage at node 0 will be approximately the positive supply voltage VDD, where VDD is typically 5V. When the TTL input signal is a logic "1", transistor 12 will be turned less "on", transistor 14 turned "on", and the voltage at node 0 will drop to near 0V, or ground. FIG. 2 illustrates (continuous line curve) the transfer function

(V U-.-U--i- vs V-I..t-!) of the circuit illustrated in FIG. 1. When

V_ 1„ a is equal to OV VU-, _I U _Tr 1- will be at its maximum potential of VDD, or 5V. As V- ±..N. approaches 1.6V, both transistors

12 and 14 will be turned "on", and V will switch from VDD to ground. As can be seen by reference tc FIG. 2, the switch is not instantaneous, and there exists a predetermined voltage interval wherein V_ I.N _τ provides a sufficient gate voltage to both transistors so as to override their respective threshold voltages. This is the interval wherein static current id (dashed line curve) will flow from VDD through transistors 12 and 14 to ground. As stated above, it is necessary in many

applications to reduce, or even better, eliminate, this current flow. To achieve this, therefore, it is necessary to ensure that both transistors forming the CMOS inverter are never in conduction at the same time. Shown in FIG. 3 is a TTL to CMOS input buffer formed in accordance with the present invention which eliminates the static current flow is through the p- channel and n-channel transistors. Input buffer comprises a first CMOS inverter 30, a second CMOS inverter 38, a transition detector 44, and a voltage boosting circuit 50. The TTL logic input signal V_ I„N is applied as an input to the first CMOS inverter 30 via an n-channel transistor 22, where the source of transistor 22 is connected to receive input signal V_ _N . The gate of transistor 22 is controlled by a reference voltage Vrei., later described, and the drain of transistor 22 is connected to the input of first CMOS inverter 30, where this connection is illustrated as node A in FIG. 3. First CMOS inverter 30 comprises, as shown in FIG. 3 a p-channel transistor 32 and an n-channel transistor 34 connected in series between the positive power supply VDD and ground. The gates of transistors 32 and 34 are coupled together and connected to the drain of transistor 22 at node A. The drains of transistors 32 and 34 are also coupled together and provide the output of first inverter 30 at node B, where this output signal is defined as ^v _{0τ T1} » Output signal is provided at essentially VDD when input signal V_-_ is a logic "0" and at essentially ground when input signal V_ I-„N is a logic "1" . The output of first CMOS inverter 30 is subsequently applied as an input to second CMOS inverter 38. Second CMOS inverter 38 is similar in structure to first CMOS inverter 30 and comprises a p-channel transistor 40 and an n-channel transistor 42 connected between VDD and ground. The output of second CMOS inverter 33, denoted V , will be the opposite of the output from first CMOS inverter 30. The combination of

the two inverters 30 and 38 is a known TTL to CMOS buffer. It operates as follows.

When input signal V _τ „ is a logic "1", transistor

IN

22 must be "off". Therefore, the reference voltage V _f is chosen to ensure that transistor 22 will remain "off" when the TTL input signal is at its lowest logic "1" value of approximately 2V. Thus, the input to first inverter 30 at node A will be approximately 1.8V, including some noise margin. One of the advantages of utilizing transistor 22 that the input inverter transistors 32 and 34 do not have to be ratioed to accept TTL input levels, since device 22 enables node A to go fully to VCC. This logic "1" value turns "on" n-channel transistor 34 and turns "off" p- channel transistor 32, pulling the voltage appearing at node B, V , to ground. Consequently, this logic "0" value of V is applied as the input to second inverter 38, turning p-channel transistor 40 "on" and n-channel transistor 42 "off" and bringing the value of output signal _QTTT - to essentially VDD, or logic "1". In summary, therefore, when the TTL input signal V_ _N is a logic "1", ₀ - _J -- ₁ will be a logic "0" and _ou __ will be a logic "1", where the actual values of _0UT1 and ^V ₀ UT2 ^wil1 be the CMOS voltage levels of ground (Ov) and VDD (5V), respectively. Similarly, when input signal V_ I..N. is a logic "0", transistor 32 of first inverter 30 will turn "on" and transistor 34 will turn "off", thus bringing the voltage at node B up from ground to essentially VDD, the CMOS logic "1" level. In turn, this logic "1" input to second inverter 38 will turn transistor 40 "off" and transistor 42 "on", bringing the output of second inverter 38 down from VDD to ground, the CMOS logic "0" level.

As briefly stated above in association with FIG. 1, a problem arises when the TTL input signal V_.„ is at its lowest logic "1" value of 2V. Under this condition, both transistors 32 and 34 will be "on" and will draw a current from VDD to ground. Under some circumstances, this current may reach a value of 3A, which is not

acceptable for many situations. To avoid this situation, therefore, it is necessary to bring the TTL logic "1" input voltage of 2V appearing at node A up to a level which is sufficient to ensure that p-channel transistor 32 will be completely turned "off" . This solution is provided in accordance with the present invention by including the transition detector 44 and the voltage boosting circuit 50 in the input buffer.

Transition detector 44 comprises a p-channel transistor 46 and an n-channel transistor 48. The source of transistor 46 is connected to VDD, the drain is connected to the drain of transistor 48, and the gate is coupled to receive the output signal ^v _nTIT - from second inverter 38. The source of transistor 48 is connected to ground and the gate of transistor 48 is coupled to receive the TTL input signal _τ „. The output of transition detector 44, denoted V_ _R _, will appear at node D, the interconnected drain terminals of transistors 46 and 48. Voltage boosting circuit 50, as shown in FIG. 3, comprises a pair of p—channel transistors 52 and 54 connected in series between VDD and node A, the input to first inverter 30. In particular, the source of transistor 52 is connected to VDD, the drain of transistor 52 is connected to the source of transistor 54, and the drain of transistor 54 is connected to node A. The gate of transistor 52 is controlled by output signal V-._-_-_ from second inverter 38 and the gate of transistor 54 is controlled by output signal V-.-.-.-,, from transition detector 44. A delay element 56 is included in boosting circuit 50, the purpose of this device being explained below.

As previously discussed, conventional TTL to

CMOS input buffers will draw a static current Ia. when the

TTL input signal makes a logic "0" to a logic "1" transition, where the logic "1" signal is of insufficient magnitude to turn "off" the p-channel transistor of the CMOS inverter. In accordance with the operation of the

present invention, when V_- _N moves from a logic "0" to a logic "1" value, transistor 48 of transition detector 44 will turn "on", bringing the output ^v _---w-_ of detector 44 to ground. This CMOS logic "0" value of ^V _TR is subsequently applied as the gate input of p-channel transistor 54, turning "on" transistor 54. At this point in time, the signal appearing at the gate of p-channel transistor 52 will also be at ground, since the TTL input transition from logic "0" to logic "1" has not yet propagated through delay element 56. Therefore, once transistor 54 is activated, node A will be brought up from the TTL logic "1" value of approximately 2V towards the full VDD CMOS logic "1" level. Thus, with node A sufficiently above the threshold of p—channel transistor 32, transistor 32 will remain "off" and no current will flow through first inverter 30. As can be seen from the above description, delay element 56 (which may simply comprise a pair of CMOS inverters) functions to ensure that transistor 52 will not be turned "off" (by the transition of V _u „ from "0" to "1" in association with the identical transition of input signal V_ I„N) until transistor 54 has been "on" long enough to bring the voltage at node A to a level which will turn transistor 32 "off". It has been determined that a delay of approximately 10-15 nsec is sufficient for this purpose.

Once the logic "1" value of V _IT Γ _O ^nas Propagated through delay element 56 and reaches the gate of transistor 52, transistor 52 will be turned "off", thereby disconnecting node A from VDD. Transistor 52 is turned "off" so that when input signal V _τ I„N makes its next transition from logic "1" to logic "0", node A is not actively held at VDD, where that condition would draw current from the TTL input source, which is not desirable. Also, it would considerably slow down the "1" to "0" transition time and thus decrease the operating speed of input buffer 20.

^•

A problem may be encountered when the input signal V-.. remains at a logic "1" value for a considerable length of time. Under these circumstances, the voltage at node A may decay to a level which would allow p-channel transistor 32 to turn "on" and static current to flow through inverter 30. To alleviate this problem, a p- channel leakage transistor 60 may be included in first inverter 30 of input buffer 20 as shown in FIG. 3. Leakage transistor 60 is connected at its source to VDD, and the drain of transistor 60 is connected to node A. The gate of transistor 60 is connected to node B, thus transistor 60 will turn "on" when output signal ^V _0UT1 is at a logic "0" value. By design, transistor 60 is made to be extremely small, and will therefore not interfere with the operation of voltage boosting circuit 50. However, after transistor 52 has been "off" for a given length of time, transistor 60 will provide a path for leakage current so that static current will not flew from VDD through transistors 32 and 34 to ground. Due to its extremely small size, the current flow through leakage transistor 60 is considered negligible.