Logic synthesis method and semiconductor integrated circuit

Abstract

A semiconductor integrated circuit of registers and combinational logic circuits connected between the registers is generated by a top-down design technique. When performing the logic synthesizing of such a semiconductor integrated circuit by making use of data of register transfer level, a combinational logic circuit with a critical path is driven with power from a high-voltage source, a combinational logic circuit without a critical path is driven with power from a low-voltage source, and a level converter capable of converting an input signal of low voltage level into a high-voltage-level output is arranged in a register located upstream of a combinational logic circuit with a critical path. Compared with a technique in which only critical paths are driven with power from a high-voltage source, it becomes easier to determine where to arrange level converters. Additionally, the number of level converters required can be reduced, thereby facilitating design work. Each combinational logic circuit with a critical path is driven by the high-voltage source, so that the power consumption increases in comparison with a case where only critical paths are driven by the high-voltage source. However, the ratio of the number of combinational logic circuits with a critical path to the total number of combinational logic circuits contained in the whole semiconductor integrated circuit is negligible, and other combinational logic circuits without a critical path are driven by the low-voltage source. The entire power consumption of the semiconductor integrated circuit can be reduced.

The invention claimed is:  
     
       1. A logic synthesis method for synthesizing, based on connection information of logic cells, a semiconductor integrated circuit of a plurality of registers and a plurality of combinational logic circuits connected between said registers, said logic synthesis method comprising: 
       a first step for mapping, when there exists among said plural combinational logic circuits a combinational logic circuit with a signal propagation delay time below a design delay upper limit, such a combinational logic circuit into a combinational logic circuit of a first type driven by a low-voltage source, and for mapping, when there exists among said plural combinational logic circuits a combinational logic circuit with a signal propagation delay time above the design delay upper limit, such a combinational logic circuit into a combinational logic circuit of a second type driven by a high-voltage source;  
       a second step for determining which of said combinational logic circuits of the first type outputs to a combinational logic circuit of the second type and for remapping a combinational logic circuit of the first type, determined to output to a combinational logic circuit of the second type, into the second-type; and  
       a third step for determining which of said registers generates a signal to a combinational logic circuit of the second type, for mapping a register, determined to generate a signal to a combinational logic circuit of the second type, into a register driven by the high-voltage source, and for mapping a register, determined not to generate a signal to a combinational logic circuit of the second type, into a register driven by the low-voltage source.  
     
     
       2. A logic synthesis method according to claim  1 , wherein said first step comprises: 
       evaluating a sum of a signal propagation delay time of a combinational logic circuit and a signal propagation delay time of a register driven by the low-voltage source; and  
       mapping, when there exists a combinational logic circuit with respect to which said evaluation step yields a result below the design delay upper limit, such a combinational logic circuit into the first type, and mapping, when there exists a combinational logic circuit with respect to which said evaluation step yields a result above the design delay upper limit, such a combinational logic circuit into the second type.  
     
     
       3. A logic synthesis method according to claim  1  wherein said first step comprises: 
       mapping each said combinational logic circuit into the first-type;  
       determining which of said combinational logic circuits has a signal propagation delay time above the design delay upper limit, and mapping a combinational logic circuit of the first type, determined to have a signal propagation delay time above the design delay upper limit, into the second type.  
     
     
       4. A logic synthesis method according to claim  1  wherein said second step comprises: 
       determining whether said first-to-second type remapping creates a layout in which a combinational logic circuit of the first type gives its output to a combinational logic circuit of the second type, and repeatedly remapping, if there still exists a combinational logic circuit of the first type that outputs to a combinational logic circuit of the second type, such a combinational logic circuit of the first type into the second type.  
     
     
       5. A logic synthesis method according to claim  1  wherein register-transfer-level design data describing a plurality of registers and a plurality of combinational logic circuits connected between said registers is input, and said logic cell connection information of said first step is generated from said input register-transfer-level design data. 
     
     
       6. A logic synthesis method according to claim  1  wherein a net list describing logic cell connection information is input, and said logic cell connection information of said first step is generated from said input cell connection information. 
     
     
       7. A logic synthesis method according to claim  1  wherein a schematic describing logic cell connection information is input, and said logic cell connection information of said first step is generated from said input logic cell connection information. 
     
     
       8. A logic synthesis method according to any of claims  5 - 7  wherein said logic cell connection information based on said register-transfer-level design data, on said net list, or on said schematic is optimized, and said optimized logic cell connection information serves as said logic cell connection information of said first step. 
     
     
       9. A logic synthesis method according to any of claims  1 - 4  further including a step after said third step for verifying each timing of said registers. 
     
     
       10. A semiconductor integrated circuit having a plurality of registers and a plurality of combinational logic circuits connected between said registers wherein: 
       part of said plural combinational logic circuits are formed by respective combinational logic circuits of a first type driven by a low-voltage source and the remaining part of said plural combinational logic circuits are formed by respective combinational logic circuits of a second type driven by a high-voltage source; and  
       of said plural registers a register that has a combinational logic circuit of the first type on its input side and a combinational logic circuit of the second type on its output side is a circuit that has a temporary data storage driven by the low-voltage source and a level converter which is driven by the high-voltage source and which converts a low-voltage-level output signal, received from said temporary data storage, into a high-voltage-level output.  
     
     
       11. A semiconductor integrated circuit according to claim  10  wherein: 
       of said plural registers a register that has combinational logic circuits of the first type on its input and output sides and a register that has a combinational logic circuit of the second type on its input side and a combinational logic circuit of the first type on its output side are circuits which are driven by the low-voltage source and which do not contain said level converter; and  
       of said plural registers a register that has combinational logic circuits of the second type on its input and output sides is a circuit which is driven by the high-voltage source and which contains said level converter for converting a low-voltage-level output signal, received from said temporary data storage, into a high-voltage-level output.  
     
     
       12. A semiconductor integrated circuit according to either claim  10  or claim  11  further including a clock feed means which is driven by the low-voltage source and which feeds a clock to each said resister. 
     
     
       13. A semiconductor integrated circuit according to claim  10  wherein: 
       of said plural registers a register containing said level converter is formed by a flip-flop;  
       said flip-flop including:  
       a master latch and a slave latch which are driven by the low-voltage source and which are connected in series;  
       an output buffer which is driven by the high-voltage source; and  
       a level converter which is connected between said slave latch and said output buffer and which converts a low-voltage-level signal, received from said slave latch, into a high-voltage signal, to provide same to said output buffer.  
     
     
       14. A semiconductor integrated circuit according to claim  11  wherein: 
       of said plural registers a register not containing said level converter is formed by a flip-flop;  
       said flip-flop including:  
       a master latch and a slave latch which are driven by the low-voltage source and which are connected in series; and  
       an output buffer which is driven by the low-voltage source and which receives an output signal of said slave latch.  
     
     
       15. A semiconductor integrated circuit according to claim  10  wherein: 
       of said plural registers a register containing said level converter is formed by a latch;  
       said latch including:  
       a latch section which is driven by the low-voltage source;  
       an output buffer which is driven by the high-voltage source; and  
       a level converter which is connected between said latch section and said output buffer and which converts a low-voltage-level signal, received from said latch section, into a high-voltage signal, to provide same to said output buffer.  
     
     
       16. A semiconductor integrated circuit according to claim  11  wherein: 
       of said plural registers a register not containing said level converter is formed by a latch;  
       said latch including:  
       a latch section which is driven by the low-voltage source; and  
       an output buffer which is driven by the low-voltage source and which receives an output signal of said latch section.  
     
     
       17. A semiconductor integrated circuit according to any of claims  10  and  11  wherein said plural registers are formed by respective flip-flops for scan testing. 
     
     
       18. A semiconductor integrated circuit according to claim  17  wherein of said scan testing flip-flops a scan testing flip-flop that contains said level converter includes: 
       a multiplexer which is driven by the low-voltage source and which selects one of a plurality of input data items according to an external control signal;  
       a master latch and a slave latch which are driven by the low-voltage source, which receive signals from said multiplexer, and which are connected in series;  
       an output buffer which is driven by the high-voltage source; and  
       a level converter which is connected between said slave latch and said output buffer and which converts a low-voltage-level signal, received from said slave latch, into a high-voltage signal, to provide same to said output buffer.  
     
     
       19. A semiconductor integrated circuit according to claim  17  wherein of said scan testing flip-flops a scan testing flip-flop that contains said level converter includes: 
       a data input selector which is driven by the low-voltage source and which selects one of a plurality of input data items according to a clock signal;  
       a master latch and a slave latch which are driven by the low-voltage source, which receive signals from said data input selector, and which are connected in series;  
       an output buffer which is driven by the high-voltage source; and  
       a level converter which is connected between said slave latch and said output buffer and which converts a low-voltage-level signal, received from said slave latch, into a high-voltage signal, to provide same to said output buffer.  
     
     
       20. A semiconductor integrated circuit according to any of claims  13 ,  15 ,  18 , and  19  wherein: 
       said level converter is formed by two PMOS transistors and two NMOS transistors;  
       one of said two PMOS transistors has a gate and a drain, said gate and said drain respectively being connected with a drain and a gate of the other PMOS transistor, and sources of said two PMOS transistors are connected to the high-voltage source;  
       one of said two NMOS transistors has a gate at which one of a pair of complementary signals from said slave latch is applied, and the other NMOS transistor has a gate at which the other of said pair of complementary signals is applied, and drains of said two NMOS transistors are connected to the drains of said two PMOS transistors, and sources of said two NMOS transistors are connected to ground; and  
       a potential at each of the drains of said two NMOS transistors is provided in the form of a signal.  
     
     
       21. A semiconductor integrated circuit according to any of claims  13 ,  15 ,  18 , and  19  wherein said level converter includes two PMOS transistors and two CMOS inverters; 
       each said CMOS inverter being formed by a PMOS transistor and an NMOS transistor connected in series wherein gates of said PMOS and NMOS transistors together serve as an input terminal and an in-series connection between said PMOS transistor and said NMOS transistor serves as an output terminal;  
       said input terminal of one of said two CMOS inverters being fed one of a pair of complementary signals from said slave latch, and said input terminal of the other CMOS inverter being fed the other of said pair of complementary signals from said slave latch;  
       said two PMOS transistors having drains connected to sources of said PMOS transistors of said CMOS inverters, and said sources being connected to the high-voltage source;  
       sources of said NMOS transistors of said two CMOS inverters being connected to ground;  
       said output terminal of each said CMOS inverter being connected to a gate of a PMOS transistor not connected in series therewith; and  
       a potential at each of said output terminals of said two CMOS inverters being provided in the form of a signal.  
     
     
       22. A semiconductor integrated circuit according to any of claims  10  and  11  wherein low and high voltages are provided from outside said semiconductor integrated circuit. 
     
     
       23. A semiconductor integrated circuit according to any of claims  10  and  11  further including an area for input pads, and an internal core section within which a plurality of registers, a plurality of combinational logic circuits, and a memory cell section are arranged. 
     
     
       24. A method for designing a semiconductor integrated circuit, the semiconductor circuit including a plurality of combinational circuits, each of the plurality of combinational circuits being provided on an associated one of signal propagation paths, said method comprising the steps of: 
         generating a first combinational circuit including a first logic gate driven by a low - voltage source and a second logic gate driven by a high - voltage source, said second logic gate also forming a portion of a first signal propagation path which is a critical path, said first logic gate and said second logic gate also forming a portion of a second signal propagation path which is different from said first signal propagation path;    
       
         determining whether an output of the first logic gate in said first combinational circuit is coupled to an input of said second logic gate in said first combinational circuit; and  
       
         if so, replacing the first logic gate in the first combinational circuit with a logic gate driven by the high - voltage source.   
     
     
       25. The method for designing a semiconductor integrated circuit of claim  24 , wherein said second logic gate is formed such that a signal propagation delay time of the first signal propagation path is equal to or less than a design delay upper limit. 
     
     
       26. A semiconductor integrated circuit comprising a plurality of combinational circuits, each of said combinational circuits having one signal propagation path contained within said combinational circuit, 
         said plurality of combinational circuits including a first combinational circuit utilizing a low - voltage source as a voltage source thereof, and a second combinational circuit utilizing a high - voltage source as a voltage source thereof,    
       
         wherein, the signal propagation paths contained in said second combinational circuit and a predetermined combinational circuit other than said first and second combinational circuits, share a portion, so that said second combinational circuit and said predetermined combinational circuit have at least one logic gate in common,  
       
       
         each of said predetermined combinational circuit and said second combinational circuit includes at least one logic gate other than said common logic gate, which is disposed on a portion of the signal propagation path other than said portion shared by the signal propagation paths of said predetermined combinational circuit and said second combinational circuit, and  
       
         at least said common logic gate and said portion shared by the signal propagation paths of said predetermined and second combinational circuits are driven by the high - voltage source,    
         said semiconductor integrated circuit further comprising a register, said register having an input port coupled to a combinational circuit outputting a low voltage signal and an output port coupled to a combinational circuit operative for receiving a high - voltage signal, said register having a level conversion function,    
         said register operative for receiving and storing a low - voltage output of the combinational circuit which outputs the low - voltage signal, converting a level of the stored low - voltage signal into a level of a high - voltage signal, and then outputting the high - voltage signal to the combinational circuit which receives the high - voltage signal.   
     
     
       27. The semiconductor integrated circuit of claim  26 , wherein said register having a level conversion function comprises: 
         a temporary data storage using a low - voltage source as a voltage source thereof; and    
         a level converter using a high - voltage source as a voltage source thereof.   
     
     
       28. A method for designing a semiconductor integrated circuit comprising a plurality of combinational circuits, each of said combinational circuits having one signal propagation path, and at least one pair of registers which are disposed anterior to and posterior to at least one of the plurality of combinational circuits, said method comprising the steps of: 
         determining if a signal propagation delay time of the signal propagation path of any of the plurality of combinational circuits is equal to or smaller than a design delay upper limit, and if so, defining the combination circuit as a first combinational circuit using a low - voltage source as a voltage source thereof;    
         determining if a signal propagation delay time of the signal propagation of any of the plurality of combinational circuits exceeds the design delay upper limit, and if so, defining the combinational circuit as a second combinational circuit using a high - voltage source as a voltage source thereof; and    
         determining if any of said registers receives a signal from the defined first combinational circuit and outputs a signal to the defined second combinational circuit, and if so, substituting said register with a level shifting register operable for receiving and storing a low - voltage signal output by the first combinational circuit, converting said low - voltage signal into a high - voltage signal, and outputting said high - voltage signal to said second combinational circuit.   
     
     
       29. The method for designing a semiconductor integrated circuit according to claim  28 , further comprising the steps of: 
       
         determining if an output on the signal propagation path of any of said plurality of combinational circuits does not traverse a register but input through an output point on the signal propagation path to an input point on another signal propagation path of another combinational circuit, and if so, determining if there is a mixture in which the former combinational circuit is the first combinational circuit and the latter combinational circuit is the second combination circuit; and  
       
         if there is such a mixture, re - defining another second combinational circuit having one signal propagation path by combining a part of the first combinational circuit located posterior to the output point with a part of the second combinational circuit located anterior to the input point.   
     
     
       30. The method for designing a semiconductor integrated circuit according to claim  29 , further comprising the steps of: 
         determining if any of the registers receives a signal from the defined first combinational circuit and outputs a signal to the re - defined second combinational circuit, and    
         if so, constituting the register by a level shifting register operable for receiving and storing a low - voltage signal output by the first combinational circuit and converting said low - voltage signal into a high - voltage signal, and then outputting said high - voltage signal to said re - defined second combinational circuit.   
     
     
       31. The method for designing a semiconductor integrated circuit according to claim  28 , further comprising the steps of: 
       
         determining if an output on the signal propagation path of any of said plurality of combinational circuits does not traverse a register but input through an output point on the signal propagation path to an input point on another signal propagation path of another combinational circuit, and if so, determining if there is a mixture in which the former combinational circuit is the second combinational circuit and the latter combinational circuit is the first combination circuit, and  
       
         not re - defining another second combinational circuit having one signal propagation path by combining a part of the second combinational circuit located posterior to the output point with a part of the first combinational circuit located anterior to the input point.