DATA BUS DISCHARGING CIRCUIT

Abstract: A data bus discharging circuit capable of enabling the high-­ speed operation of a microprocessor, comprising a control sig­ nal generating circuit which provides control signals, a pre­ charge/enable signal generating circuits which provides a pre­ charge control signal and an enable signal, a discharge detect­ ing circuit which detects the potential of the bit lines of the data bus, and a discharging circuit which sets the bit lines of the data bus selectively at the ground potential.

Description: The present invention relates to a data bus control circuit. It is general practice to initialise all the bit lines of the internal data bus of a MOS circuit device, such as a MOS microprocessor or a MOS microcomputer, at the level of the power source potential V DD (logic "1") immediately before starting data transfer, for the high-speed transfer of the data accumulated in the internal register of the MOS circuit device to the data bus. An improved data bus precharging circuit for the high-speed initialisation of a data bus is disclosed, for example, in Japanese Unexamined Patent Publication No. 58-186827 by the inventors of the present invention. More particularly, it discloses a data bus control circuit comprising: a data bus, comprising a plurality of bit lines; a source of clock signals; controllable precharge means for precharging the bit lines of the data bus to a first logic level V DD ; control signal generating means, responsive to the logic levels on the bit lines of the data bus, to produce a first state at its output K when all said bit lines are at the first logic level V DD and to produce a second state at its output K on receipt thereafter of a clock signal A from said source. However, in transferring the data of a logic "0" stored in the internal register to the bit lines of the data bus set at the level of the power source potential, reducing the potential of the bit lines to the ground potential (logic "0") requires a considerably long time, for example 50 to 60ns, due to the parasitic capacitance of the bit lines. Therefore, it has been extremely difficult to operate a microprocessor IC at a high speed by clock signals of 15 MHz or above. It is possible to enhance the output driving ability of thirty to forty registers. However, such means requires microprocessor IC chips having a large area, which disadvantageously increases the manufacturing costs. DE 3 228 013 discloses a system for discharging the bit lines of a data bus on which a logic "0" has been applied. A comparator detects when the voltage on any bit line falls below a reference and sends a signal to turn on an associated MOSFET to connect the bit line to 0V. However this system is not suited to high speed clock operation. In order to overcome the aforementioned disadvantages, the present invention provides a data bus control circuit characterised by a precharge/enable signal generating circuit, responsive to the control signal generating means to produce a precharge control signal J to produce cessation of precharging of the data bus by said precharge means when the output K of the control signal generating means is in said first state and to produce a signal B enabling data signals of first or second logic levels V DD ,0V to be transferred to the data bus, control means for selectively applying to the or each bit line, to which is applied a data signal at said second logic level 0V, a control signal at said second logic level 0V, said control means being disabled during precharging of the data bus. By employing a circuit according to the present invention, it is possible to enable a microprocessor to operate at a clock frequency of 20MHz or above. Likewise, a microprocessor incorporating a circuit according to the present invention may be capable of enabling one state having a period of 50ns or below to be operated by one clock signal. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a microprocessor including a data bus control circuit embodying the present invention; and Figure 2 is a timing chart showing a series of operation of the microprocessor of Figure 1 in the data bus discharging mode. Figure 1 shows an 8-bit microprocessor including a data bus control circuit according to the present invention. Referring to Figure 1, the microprocessor comprises a quartz-crystal oscillator circuit 10 which generates a reference clock signal A, a timing/control circuit 20 which is controlled by the reference clock signal A and a reset signal, and generates a control timing signal, and a processing unit 30 connected to an internal data bus 190, and which decodes commands on the basis of a timing signal and an enable signal to control the internal circuits. The processing unit 30 comprises an ALU, a RAM and registers and is connected through I/O ports to external circuits. The microprocessor further includes data registers 140 and 160 for the temporary storage of the data of the data bus 190, and gate circuits 120 and 180 for transferring the data stored in the data registers to the data bus 190. When a write signal G (WRITE RA) and an inversion signal C of the clock signal A output from the processing unit 30 to an AND circuit 130, the data of the data bus 190 is stored in the data register 140. When the processing unit 30 sends an output command signal E (OUT RA) and an enable signal B to an AND circuit 110, a gate circuit 120 transfers the data stored in the data register 140 to the data bus 190. When the processing unit 30 outputs a write signal F (WRITE RB) and the inversion signal C of the clock signal A to an AND circuit 150, the data of the data bus 190 is stored in a data register 160. When the processing unit 30 sends an output command signal D (OUT RB) and the enable signal B to an AND circuit 170, the data stored in the register 160 is transferred through the gate circuit 180 to the data bus 190. The microprocessor further includes a data bus precharging circuit for forcibly charging all the bit lines of the data bus 190 at a power source potential V DD at the initial state, and a data bus discharging circuit for forcibly causing bit lines having data of a logic "0" among the data given to the data bus 190, to discharge to the ground potential. The data bus precharging circuit comprises a precharge detection circuit 90 which provides an output signal of a logic "1" when all the bit lines of the data bus 190 become a logic "1", a control signal generating circuit 50 which provides a control signal K of a logic "0" upon the reception of the clock signal A and provides a control signal K of a logic "1" when a precharge detection signal L given by the precharge detection circuit 90 is a logic "1", and precharging means 80 which charges all the bit lines of the data bus 190 at the power source potential V DD upon the reception of a precharge control signal J. The precharge dectection circuit 90 comprises, for example, a NAND circuit 91 and an inverter 92 having Schmitt characteristics. The control signal generating circuit 50 is, for example, a D-type flip flop circuit. The precharging means 80 comprises a plurality of p-channel MOSFETs 81. The data bus discharging circuit comprises a unit 200 including a plurality of discharge detection circuits 210 connected to the bit lines of the data bus 190, respectively, discharging means 260 which forcibly sets the bit lines tending to become a logic "0" at the ground potential, and a precharge/enable signal generating circuit 280 which provides the precharge control signal J or the enable signal B according to the logical level of the control signal K provided by the control signal generating circuit 50. The output signal of the discharge detection circuit 210 becomes a logic "1" when the level of the bit lines of the data bus 190 is a logic "0", or becomes a logic "0" when a reset signal (control signal K) of a logic "0" is given thereto. The discharge detection circuit 210 comprises a detecting circuit 220 which provides a detection signal S of a logic "1" upon the detection of the bit line being a logic "0", a filter circuit 230 which allows only the detection signals Z having a width not less than a predetermined width, for example, 5ns, to pass, and a signal holding circuit 250 which provides a discharge control signal of a logic "1" when the detection signal Z is a logic "0", and provides a discharge control signal of a logic "0" when a reset signal of a logic "0" is given thereto. The detection circuit 220 is, for example, an inverter having the Schmitt characteristics. The filter circuit 230 comprises, for example, inverters 231 and 232, an AND circuit 233 and a NAND circuit 234. The signal holding circuit 250 is, for example, an RS-type flip flop circuit. The charge/enable signal generating circuit 280 comprises a precharge control signal generating circuit 70 which provides the precharge control signal J of a logic "0" for switching on the p-channel MOSFETs 81 of the precharging means 80 when the output signal K of a logic "0" and a delayed enable signal H of a logic "0" are given thereto, an enable signal generating circuit 60 which provides the enable signal B for transferring the contents of the registers to the data bus 190 upon the reception of the precharge control signal J of a logic "1" and the control signal K of a logic "1", and a delay circuit 100 which delays the enable signal B in order to inhibit the simultaneous execution of data bus precharging and data bus discharging. The precharge control signal generating circuit 70 comprises, for example, a NOR circuit 71 and an inverter 72. The enable signal generating circuit 70 comprises, for example, and AND circuit 61 and a buffer circuit 62. The delay circuit 100 comprises, for example, inverters 101, 102, 103 and 104. The operation of the data bus discharging circuit of the present invention will be described in connection with the timing chart of Figure 2. Assuming that the quartz-crystal oscillator circuit 10 provides a reference clock signal A of 20 MHz frequency and 50ns period and one state is established for one clock pulse, 8-bit data 101011100 (ACH) is given to the data bus 190, and 8-bit data 00001111 (OFH) is stored in the register B 160, the register output control signal E (OUT RA) is a logic "1", the register write signal F (WRITE RB) is a logic "1", and 8-bit data 11001010 (CAH) is stored in the regiser A 140. During cycle S1, upon the change of the clock signal A from a logic "0" to a logic "1", the control signal K of the control signal generating circuit 50 changes from a logic "1" to a logic "0" and the enable signal B of the enable signal generating circuit 60 becomes a logic "0" (Figure 2: 1, 2, 4). Since the reset signal of the logic "0" data detecting circuit 200 becomes a logic "0" when the control signal K becomes a logic "0", all the RS-flip flops of the discharge signal generating circuit 210 are reset and the output signal (OUT 0 to 7) thereof become a logic "0" (Figure 2; 3). Simultaneously, the control signal K of a logic "0" is sent to the AND circuit 61, whereby the enable signal B becomes a logic "0", so that the gate circuits 120 and 180 interrupt the output operation of the microprocessor (Figure 2; 4). The control signal K of a logic "0" is delayed, for example, by 10ns, by the delaying cirucit 100, and then the delayed control signal K is given to the NOR circuit 71. When the input signals K and H both become a logic "0", the NOR circuit 71 provides the precharge control signal J of a logic "0" (Figure 2: 6). Consequently, all the p-channel MOSFETs 81 of the precharging means 80 are switched on and all the bit lines of the data bus 190 are charged forcibly and quickly to the power source potential V DD (logic "1"), for example, 5V (Figure 2: PR). During the above-mentioned process, all the output signals OUT 0 to 7 of the discharge control signal generating circuit 210 of the logic "0" data detecting circuit 200 are a logic "0". Then, when all the bit lines of the data bus 190 become a logic "1", the output signal L of the precharge detecting circuit 90 changes from a logic "0" to a logic "1", while the output signal K of the flip flop circuit 50 is changed from a logic "0" to a logic "1" (Figure 2: M, N, RS). Consequently, the output signal J of the control signal generating circuit 70 changes from a logic "0" to a logic "1" and the p-channel MOSFETs 81 are switched off to complete the precharging operation (Figure 2: P). Upon the completion of the data bus precharging operation, both the output signal K of the control signal generating circuit 50 and the output signal J of the precharge control signal generating circuit 70 become a logic "1", therefore, the enable signal B of the enable signal generating circuit 60 changes from a logic "0" to a logic "1" (Figure 2: U). The AND circuit 110 opens the gate circuit 120 to transfer the data 11001010 stored in the register A 140 to the data bus 190. When data of a logic "0" is sent to the data bus 190, the potential of the bit lines starts dropping from the power source potential V DD (logic "1") (Figure 2; OE). The drop of the potential of the bit lines is detected by the logic "0" data detecting circuit 200. Upon the drop of the potential of the bit lines below a predetermined potential, for example, 4V, the output signal of the RS-flip flop circuit 250 changes from a logic "0" to a logic "1" (Figure 2: DS, S). Accordingly, the n-channel MOSFETs 261 connected to the bit lines received the data of a logic "0" are switched on to make the bit lines discharge forcibly to the ground potential (Figure 2: R). Ordinarily, it takes 50 to 60ns to change the bit line from a logic "1" top a logic "0". However, the additional employment of the data bus discharging circuit of the present invention reduces the time necessary for changing the bit line from a logic "1" to a logic "0" to approximately 5 to 10ns. The contents of the register A 140 11001010 (CAH) is given to the data bus 190. The change of the clock signal A from a logic "1" to a logic "0" causes the inverted signal C of the clock signal A to become a logic "1" (Figure 2: WC). The output signal WB of the AND circuitry 150 becomes a logic "1" and the contents 11001010 (CAH) of the data bus 190 are stored in the register B 160. Then, during cycle S2, when the clock signal A changes from a logic "0" to a logic "1", the control signal K becomes a logic "0" to reset the output of the RS-flip flop 250 of the discharge detecting circuit 210 at a logic "0" and makes the enable signal B a logic "0". Then, the data bus precharging operation is restarted. As apparent from the foregoing description, the data bus discharging circuit of the present invention is capable of reducing the time necessary for giving data of a logic "0" to the data but without requiring enhanced register output driving capacity. Furthermore, the data control circuit of the present invention enables the high-speed operation and the reduction of power consumption of microprocessors and microcomputers, particularly, those of the CMOS type.

Claims: A data bus control circuit comprising: a data bus (190), comprising a plurality of bit lines; a source of clock signals (10); controllable precharge means (80) for precharging the bit lines of the data bus (190) to a first logic level (V DD ); control signal generating means (50), responsive to the logic level on the bit lines of the data bus (190), to produce a first state at its output (K) when all said bit lines are at the first logic level (V DD ) and to produce a second state at its output (K) on receipt thereafter of a clock signal (A) from said source (10); and characterised by a precharge/enable signal generating circuit (280), responsive to the control signal generating means (50) to produce a precharge control signal (J) to produce cessation of precharging of the data bus by said precharge means (80) and to produce a signal (B) enabling data signals of first or second logic levels (V DD ,0V) to be transferred to the data bus (190) when the output (K) of the control signal generating means (50) is in said first state; and control means (210,260) for selectively applying to each bit line, to which is applied a data signal at said second logic level (0V), a control signal at said second logic level (0V) in order to force the potential of said bit line to said second logic level (0V), said control means (210,260) being disabled during precharging of the data bus (190). A data bus control circuit according to claim 1 wherein the precharge/enable signal generating circuit (280), comprises a precharge control signal generating circuit (70) which produces said precharge control signal (J), an enable signal generating circuit (60) which produces said data transfer enabling signal (B), and a delay circuit (100) which delays the output of said data transfer enabling signal (B). A data bus control circuit according to claim 2 or 3 wherein said precharge control signal generating circuit (70) comprises a NOR circuit (71) and an inverter (72). A data bus control circuit according to claim 2 or 3 wherein said enable signal generating circuit (60) comprises an AND circuit (61) and a buffer circuit (62). A data bus control circuit according to claim 2, 3 or 4 wherein said delay circuit (100) comprises a plurality of inverters (101, 102, 103, 104). A data bus control circuit according to any preceding claim wherein said control means (210, 260) comprises detection circuit means (210), for each said bit line, for producing an output signal (OUT) when the signal level on the respective bit line is moving towards said second logic level (0V) and means (260,261) responsive to the occurrence of said output signal (OUT) for applying a signal at said second logic level (0V) to said respective bit line. A data bus control circuit according to claim 6 wherein said detection circuit means (210) is disabled during precharging. A data bus control circuit according to any preceding claim wherein said control means (210, 260) is disabled when said output (K) assumes said second state. A data bus control circuit according to any preceding claim including data register means (140, 160) coupled to said data bus (190) by gate circuits (120, 180) wherein transfer of data from said registers (140, 160) to said data bus (190) is enabled by applying said data transfer enabling signal (B) to said gate circuits (120, 180).

Inventors: YOKOUCHI, HIROSHI OKI ELECTRIC IND. CO. LTD.
Applicants: OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1985
Main classification: G06F13
IPC codes: G06F13, G06F3