This invention relates, in general, to an incrementer/decre enter circuit, and more particularly to an incrementer/decrementer circuit having field effect transistors and useful in digital data processing systems.

In many applications such as watch chips, timing circuits, and program counters used within a microprocessor it is desirable to provide incrementation or decrementation of certain digital information. As an example, in a data processing system such as a microprocessor or micro¬ computer, program instructions which are to be executed are normally stored in a memory in sequential order, i.e., as a program instruction is executed the following instruction to be executed is in adjacent memory cells. It is customary to have a program counter which is incremented as each instruction is executed so that the following instruction can then be retrieved from the memory system. In some microcomputer systems it may be desirable to increment an address storage register, a program counter register, and a stack pointer register. An incrementer circuit would increase by one the digital data stored in each of the respective registers serially. Previously, typical incrementer circuits consisted essentially of exclusive OR circuits and AND gate circuits. In the past, some incrementer circuits were merely incrementers and did not have the capability to decrement as well as increment. In addition, a versatile increment or decrement circuit would contain a large number of transistors. Accordingly, it is an object of the present invention to provide an incrementer/decrementer circuit which has a reduced number of circuit elements and yet has the capability of both incrementing or decrementing.

Another object of the present invention is to provide an incrementer/decrementer circuit which is compact in layout and requires a reduced number of interconnect lines

between bit storage locations of a register having a plurality of incrementer/decrementer circuits.

Summary of the Invention

In carrying out the above and other objects of the present invention, there is provided, in one form ^' , an incrementer/decrementer circuit having a carry/borrow generator section and an increment/decrement output ^" section. The carry/borrow generator receives a carry input and provides a carry output and also controllably receives a data bit or its complement. The data bit and its complement are gated to the carry/borrow generator by a first and a second controllable means. The carry input to the carry/borrow generator is received by qate electrodes of series connected P channel and N channel transistors. An output is obtained from a node formed between the P and N channel transistors and is inverted by an inverter which provides the carry output. The output of the inverter is also used to control a feedback means to form a latch around the inverter when the carry out is of a predeter¬ mined logic level. The data bit and its complement along with the carry input signal are also coupled to the increment/decrement output portion of the circuit. The output portion of the circuit uses six field effect transistors to generate the increment/decrement output.

Brief Description of the Drawings

FIG. 1 is a schematic diagram of the present invention in one form thereof; and

FIG. 2 illustrates the circuit of FIG. 1 in combina¬ tion with portions of a microprocessor.

Detailed Description of the Drawings

Referring first to FIG. 1, there is illustrated an incrementer/decrementer circuit 10 receiving a control signal at input 11. The control signal is an increment/ decrement command which controls N channel transistors 13 and 14. The increment/decrement command is coupled from input 11 to a gate electrode of transistor 14, and is inverted by inverter 12 and coupled to a gate electrode of transistor 13. Transistors 13 and 14 serve as controllable means for coupling inputs from inputs 26 and 27, respec¬ tively, to a source electrode of N channel transistor 18. N channel transistor 18 has its gate electrode connected to a gate electrode of a P channel transistor 17. The gate electrodes of transistors 17 and 18 are coupled to an input 16 which is a carry input. Transistors 17 and 18 are connected in series between a voltage terminal V _DD and a node formed between transistors 13 and 14. A no ^' de which is formed between transistors 17 and 18 is coupled to an input of inverter 21. Inverter 21 provides an output on output line 20 which is the carry/borrow output. The output of inverter 21 is also connected to a gate electrode of a P channel transistor 19. P channel transistor 19 is coupled from voltage terminal V _DD to the input of inverter 21 and forms a feedback means around inverter 21. Transistor 19 is only made conductive when the output of inverter 21 is a logic level low. A logic level low on the gate electrode of transistor 19 causes transistor 19 to conduct and maintain a logic level high at the input of inverter 21 thereby latching inverter 21 in a state to produce a logic level low output. Transistors 17, 18, 19, and 21 form a carry/borrow generator having controllable means 13 and 14 for coupling data bits or their complements from lines 26 and 27. A P channel transistor 28 is connected in series with an N channel transistor 23 between voltage terminal V _DD

OMPI

and data input line 26. An output node is formed between transistors 23 and 28 and is connected to increment/ decrement output 32. An N channel transistor 22 is coupled between output 32 and carry input 16. Transistor 22 has its gate electrode coupled to data input 26. A P channel transistor 29 is coupled in series with an N channel transistor 24 between voltage terminal V _DD and data input 27. An output node is formed between transistors 24 and 29 and is coupled to increment/decrement output 31. An N channel transistor 25 is coupled between output 31 and carry input 16. Transistor 25 has its gate electrode coupled to input 27. In a preferred embodiment, transistors 28 and 29 are P channel transistors, however, it will be noted that other suitable load devices could be substituted for transistors 28 and 29.

Data input 27 is a complement of data input 26. Increment/decrement outputs 31 and 32 are also complements of each other. Increment/decrement output 31 will be a logic level high when both inputs 16 and 27 are logic levels low or logic levels high. When output 31 is a logic level high then output 32 will be a logic level low. Increment/decrement output 32 will be a logic level high when both inputs 16 and 26 are both logic level lows or are both logic level highs. Circuit 10 is an incrementer when increment/decrement input 11 is held at a logic level low and is a decrementer when input 11 is held at a logic level high.

Increment/decrement circuit 10 can be used in watch circuits or timing control circuits wherever an increment/ decrement circuit is required. In FIG. 2 a specific example of the use of circuit 10 in a microprocessor is illustrated. Typically, an 8-bit microprocessor would contain at least eight increment/decrement circuits 10. The circuits 10 would be interconnected so that the carry input 16 of one would receive the carry output 20 of the preceeding circuit. The outputs 31 and 32 would each go to

a respective storage cell of registers which are to be incremented or decremented. Examples of such registers could be an address storage register, a program counter register, and a stack pointer. Referring now to FIG. 2 the same reference numerals used in FIG. 1 are used in FIG. 2 for the same elements. Output 31 is shown coupled to a pair of back-to-back inverters 48 and 49 by an N channel transistor 51. Inverters 48 and 49 form a latch or storage location of a typical static register. Transistor 51 has its gate electrode connected to line 56 which could carry a control signal such as an address store load command. Inverters 48 and 49 are also coupled to a bus 46 by an N channel transistor 52. Bus 46 could be an address bus within a microprocessor. Transistor 52 has its gate electrode connected to a line 57 which would carry a control signal such as a bus load command. Output 32 is coupled to inverters 48 and 49 by an N channel transistor 53 which has its gate electrode connected to control line 56. Inverters 48 and 49 are also coupled to a bus 47 by an N channel transistor 54 which is controlled by a control signal on line 57. It should be noted that buses 46 and 47 will carry complementary signals. Transistors 52 and 54 are enabled by the same command signal carried on line 57 and will accordingly couple the true and its complement output to buses 46 and 47. The true and its complement are stored in the latch formed by inverters 48 and 49. Buses 46 and 47 are coupled to an output latch formed by a cross coupled pair of NAND gates 41 and 42. The output latch formed by logic gates 41 and 42 couples buses 46 and 47 to buses 43 and 44. This arrangement will cause buses 43 and 44 to be static buses. NAND gate 41 has a first input connected to bus 46 and a second input connected to the output from NAND gate 42, which is bus 44. Line 26 is also connected to output bus 44, and as discussed hereinbefore, line 26 serves as an input for increment/decrement circuit 10.

NAND gate 41 has its first input connected to bus 47 and its second input connected to the output of NAND gate 41 which is also output bus 43. Line 27 is also connected to output bus 43. By now it should be appreciated that there has been provided an increment/decrement circuit which has a reduced number of interconnect lines between other stages.of a plurality of increment/decrement circuits. In addition, a carry/borrow generator is provided having three transistors plus an inverter and two transistors for coupling input data. The output section of the increment/decrement circuit has two P channel transistors and four N channel transistors.