Signal line pulse enhancing circuit for integrated circuits

Abstract: According to the present invention, a first circuit is used to detect a positive voltage transition on a signal line. A second circuit is used to detect a negative voltage transition on the signal line. Upon detecting a transition, the first circuit will drive the signal line high. The second circuit pulls the line to ground after sensing a negative voltage transition. A delay circuit determines the time period in which the first or second circuit is connected to the signal line.

Description: The present invention relates generally to integrated circuit devices, and more specifically to enhancing a signal on a long line or distributed line within the integrated circuit. In the drive to increase miniaturization on integrated circuits, signal lines are made longer and thinner. This change in signal line geometry has increased the resistance and capacitance on these signal lines. This increase in resistance and capacitance results in the slowing down and degradation of signals sent on the lines. Consequently, sending a signal from one end of the chip to the other has become a limitation on the performance of the circuits. To improve the signals sent on long or distributed lines in an integrated circuit, a series of inverters are typically added at the midpoint of the line. These series of inverters are known in the art as repeaters. The use of repeaters generally results in reducing the degradation of a signal sent on a long or a distributed line. The repeaters, however, also have the undesirable affect of significantly delaying the signal. EP-A-116762 (Fijitsu Limited) discloses circuitry for reforming digital signals in integrated circuits. One embodiment described in this document enables independent timing selection for the reformation of both rising and falling portions of a signal. A first p-type MOS transistor is connected via a second p-type MOS transistor to the voltage supply. The first p-type MOS transistor is also connected to the input of a first inverter. The output of that inverter is connected to the gate of that first transistor. The output of the first inverter is also connected to the input of a second inverter, the output of which is connected to the gate of the second p-type MOS transistor. A first n-type MOS transistor is provided which is connected via a second n-type MOS transistor to a negative voltage. The first n-type transistor is also connected to the input of a third inverter, the output of which is connected to the gate of the first n-type MOS transistor. The output of the third inverter is connected to the input of the fourth inverter, the output of which is connected to the gate of the second n-type transistor. The circuit defined by the first and second p-type transistors and the first and second inverters is connected in parallel to the circuit defined by the two n-type transistors and the third and fourth inverters. It is desirable to provide a circuit for eliminating the degradation of a signal sent on a long or a distributed line, without significantly delaying the signal. It is therefore an object of the present invention to provide a circuit which can remove any degradation of a signal sent down a long or distributed line in an integrated circuit. It is another object of the present invention to provide such a circuit which does not significantly delaying a signal sent down a long or distributed line. It is another object of the present invention to provide such a circuit which is compatible with standard process flows, and which add minimal additional complexity to the fabrication of a typical integrated circuit. Therefore, according to the present invention there is provided a circuit arranged to be connected to a signal line, the signal line having a near end and a far end, the circuit comprising a p-channel transistor having a source connectable to a first voltage supply, and a drain arranged to be connected to the signal line far end; a first logic gate having an output connected to a gate of the p-channel transistor, and having a first input arranged to be connected to the signal line far end; a first delay circuit having an input arranged to be connected to the signal line far end; an n-channel transistor having a source connectable to a second voltage supply, and a drain arranged to be connected to the signal line far end; a second logic gate having an output connected to a gate of the n-channel transistor, and having a first input arranged to be connected to the signal line far end; and a second delay circuit having an input arranged to be connected to the signal line far end; characterised in that said first delay circuit has an output connected to a second input of the first logic gate; and said second delay circuit has an output connected to a second input of the second logic gate; wherein the first logic gate operates to turn on the p-channel transistor when a voltage at the signal line far end rises above a first threshold voltage, for a time period determined by the first delay circuit; and wherein the second logic gate operates to turn on the n-channel transistor when a voltage at the signal line far end falls below a second threshold voltage, for a time period determined by the second delay circuit. The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: Figure 1a is a simplified circuit diagram of a long or distributed line in an integrated circuit; Figure 1b is a graph depicting a signal before and after it is sent on a long or distributed line in an integrated circuit; Figure 2 is a block diagram of a logic circuit for enhancing a signal on a long line in an integrated circuit according to the present invention; Figure 3 is a circuit diagram of a logic circuit for enhancing a signal on a long or distributed line in an integrated circuit according to the present invention; Figure 4 is a graph depicting a signal before and after it is sent on a long or distributed line according to the present invention; and Figure 5 is a block diagram of a logic circuit for enhancing a signal on a distributed line according to the present invention. Figure 1a presents a simplified circuit diagram which illustrates the problem with a long signal line in an integrated circuit. The near end (NE) 10 of the signal line 12 is the driver end of a long line. The far end (FE) 14 of the signal line 12 lies at a point near the end of a long line. Resistor 16 and capacitor 18 represent the resistance and capacitance which result from the length of the signal line 12 . The graph in Figure 1b depicts the waveform of a signal send down the signal line 12 . The waveform 20 marked NE represents a pulse signal at the near end 10 of signal line 12 . The waveform 22 marked FE represents the signal at the far end 14 of signal line 12 . As can be seen, waveform 22 representing the signal at the far end 14 of the signal line 12 is delayed in time and is substantially degraded. Referring to Figure 2 , a block diagram of a logic circuit for enhancing a signal on a long signal line is shown. In this circuit, the NAND gate 24 detects a positive voltage transition in the signal and generates a negative pulse at the gate of transistor 26 . This pulls the signal line 12 to high at the far end 14 . A series of inverters 28 is used to set the width of the negative pulse. Once the pulse passes through the inverters, transistor 26 is turned off. The NOR gate 30 detects a negative voltage transition in the signal and generates a positive pulse at the gate of transistor 32 . This pulls the signal line 12 to ground at the far end 14 . Again, a series of inverters 34 is used to set the width of the positive pulse, and once the pulse passes through the inverters, transistor 32 is turned off. Figure 3 illustrates a detailed circuit diagram implementing the present invention. The trigger voltage levels of input 36 of transistor 38 in NAND gate 24 and input 40 of transistor 42 of NOR gate 30 can be set to detect the early portions of their respective transitions. The trigger voltages are determined by the size ratios of transistors 38 , 42 , 46 , 48 , 50 and 54 used in the circuit. The graph in Figure 4 depicts the waveforms of an enhanced signal sent down the signal line 12 . Again, the waveform 56 marked NE represents the signal at the near end 10 of the signal line 12 . Waveform 58 marked FE represents the signal at the far end 14 of the signal line 12 . As can be seen, waveform 58 representing the signal at the far end 14 of the signal line 12 mirrors waveform 56 . Waveform 58 does not show any degradation and the pulse attains the voltage level of waveform 56 . Furthermore, waveform 58 reaches the voltage level of waveform 56 faster than it would without the pulse enhancing circuit. Although the description of the invention has focused on using the pulse enhancing circuit at the far end of a long line, the circuit can be used for other reasons and at other locations than described above. For example, the circuit could be placed at the middle of a long signal line. It is also possible to use several pulse enhancing circuits placed at multiple locations along a long signal line. In some situations a signal line needs to run the length of an integrated circuit and drive a distributed load. The distributed load has the undesirable effect of creating a severe resistance and capacitance loading problem on the signal line. The signal on the distributed load line can be enhanced by using the circuitry of the present invention combined with a parallel signal line with lower resistance and capacitance. This arrangement is illustrated in Figure 5. The circuitry of the pulse enhancing circuit is as described in conjunction with Figure 2 , with the addition of transistors 60 , 62 . The drains 64 , 66 of transistors 60 , 62 are connected to the distributed load line 68 . The parallel signal line 70 is connected to the series of inverters 28 , 34 and the input to the NAND gate 24 and NOR gate 30 . When the NAND gate 24 generates a negative pulse, transistor 60 pulls the distributed load line 68 high. When the NOR gate 30 generates a positive pulse, transistor 62 pulls the distributed load line 68 to ground. As described earlier, the series of inverters 28 determine the width of the negative pulse. The series of inverters 34 determine the width of the positive pulse. Using the pulse enhancing circuit in combination with the parallel signal line 70 boosts the signal on the distributed load line 68 . This causes the signal on the distributed load line 68 to reach its peak faster and without any degradation. The present invention may provide a circuit wherein a negative edge detector comprises a combination of logic gates and transistors. The present invention may provide a circuit wherein said transistors have sizes which are selected to cause the negative edge detector to trigger at a predetermined value. The present invention may provide a circuit wherein a positive edge detector comprises a combination of logic gates and transistors. The present invention may provide a circuit wherein the transistors have sizes which are selected to cause the positive edge detector to trigger at a predetermined value. The present invention may provide a method wherein said pulse enhancing circuit comprises a combination of logic gates and transistors. The present invention may provide a method wherein the transistors have sizes which are selected to cause the positive edge detector to trigger at a predetermined value. The present invention may provide a method wherein the transistors have sizes which are selected to cause the negative edge detector to trigger at a predetermined value. While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention.

Claims: A circuit arranged to be connected to a signal line (12), the signal line having a near end (NE) and a far end (FE), the circuit comprising: a p-channel transistor (26) having a source connectable to a first voltage supply, and a drain arranged to be connected to the signal line far end; a first logic gate (24) having an output connected to a gate of the p-channel transistor, and having a first input arranged to be connected to the signal line far end; a first delay circuit (28) having an input arranged to be connected to the signal line far end; an n-channel transistor (32) having a source connectable to a second voltage supply, and a drain arranged to be connected to the signal line far end; a second logic gate (30) having an output connected to a gate of the n-channel transistor, and having a first input arranged to be connected to the signal line far end; and a second delay circuit (34) having an input arranged to be connected to the signal line far end; characterised in that said first delay circuit has an output connected to a second input of the first logic gate; and said second delay circuit has an output connected to a second input of the second logic gate; wherein the first logic gate (24) operates to turn on the p-channel transistor (26) when a voltage at the signal line far end rises above a first threshold voltage, for a time period determined by the first delay circuit; and wherein the second logic gate (30) operates to turn on the n-channel transistor (32) when a voltage at the signal line far end falls below a second threshold voltage, for a time period determined by the second delay circuit. The circuit of claim 1, arranged to be connected to a second signal line (68) having a first end adjacent the near end and a second end adjacent the far end; said circuit further comprising: a second p-channel transistor (60) having a source connectable to the first voltage supply, a drain arranged to be connected to the second end, and a gate connected to the first logic gate (24) output; and a second n-channel transistor (62) having a source connectable to the second voltage supply, a drain arranged to be connected to the second end, and a gate connected to the second logic gate (30) output. The circuit of claim 1 or 2 wherein said first voltage supply is high and said second voltage supply is ground. The circuit of any of claims 1 to 3, wherein the first logic gate comprises a NAND gate, and the second logic gate comprises a NOR gate. The circuit of any of claims 1 to 4, wherein the first and second delay circuits each comprise an odd number of inverters in series. The circuit of any of claims 1 to 5, wherein the first and second delay circuits each comprise three inverters in series. The circuit of any of the preceding claims, wherein the first and second delay circuits each have the same number of inverter stages. The circuit of any of the preceding claims, wherein the first input of the first logic gate has a threshold voltage which is lower than a threshold of the first input of the second logic gate. The circuit of claim 2 or any one of claims 3 to 8 when dependent on claim 2, wherein the circuit operates to pull the second signal line high or to ground when the voltage at the signal line far end rises above the first threshold voltage or falls below the second threshold voltage respectively.

Inventors: SLEMMER WILLIAM CARL, SLEMMER, WILLIAM CARL
Applicants: SGS THOMSON MICROELECTRONICS, STMICROELECTRONICS, INC.
Application year: 1993
Main classification: G11C5
IPC codes: H03K19, G11C5, H03K5, H04L25