WO1986001659A1

WO1986001659A1 - Assist circuit for a data bus in a data processing system

Info

Publication number: WO1986001659A1
Application number: PCT/US1985/001600
Authority: WO
Inventors: Donald Keith Lauffer; Paul Michael Rostek; Mehdi Hamidi Sani
Original assignee: Ncr Corporation
Priority date: 1984-08-27
Filing date: 1985-08-23
Publication date: 1986-03-13
Also published as: EP0191842A1; CA1247201A; JPH084221B2; JPS62500067A; US4598216A; EP0191842B1; DE3577504D1

Abstract

A circuit for assisting transitions of a signal on a bus conductor (28) having a sensing means (44) connected to the bus conductor for determining if the bus conductor is in a first state or a second state, an assist element (30) for assisting transitions of a signal on the bus conductor between its first state and its second state responsive to clock pulses (61), and a logic circuit (32) connected to the sensing means (44) and the assist element (30) for enabling said assist element (30) when the transition of the said signal is from its first state to its second state during said clock pulses (61), and for disabling the assist element (30) when the signal transition is from its second state to its first state during said clock pulses (61).

Description

ASSIST CIRCUIT FOR A DATA BUS IN A DATA PROCESSING SYSTEM

Technical Field

This invention relates to a circuit for assisting transitions of a signal on a bus conductor.

Background Art

Data processing systems contain data transfer buses such as, for instance, a data bus between a processor and a memory. The data processing system is typically timed by a clock circuit which controls the processor and the memory, allowing the processor to place data on the bus, and giving sufficient time for the data signals to stabilize before the memory reads the data from the bus for storage.

Disclosure of the Invention

According to the present invention, there is provided a circuit for assisting transitions of a signal on a bus conductor, characterized by: sensing means connected to said bus conductor and adapted to determine if said bus conductor is in a first state or a second state; assist means adapted to assist transitions of a signal on said bus conductor between said first state and said second state in response to clock pulses; and logic means connected to said sensing means and said assist means and adapted to enable said assist means when said signal transition is from said first state to said second state during said clock pulses and to disable said assist means when said signal transition is from said second state to said first state during said clock pulses.

It will be appreciated that a circuit according to the invention has the advantage of preventing contention between bus signal transitions to an inactive state and desired data signal transitions on said bus to an active state. Thus, the circuit according to the invention is effective to speed up data bus transfers by shortening the time needed to set conductors which are in an active state to an inactive state.

In a preferred embodiment of the invention, the bus assist circuit includes a pull-up network for providing current to a bus conductor, and a logic circuit which looks ahead to the conductor being pulled up for disabling the pull-up network for a conductor which is in an inactive or high state, or when the state of the conductor is changing from a high, inactive state to a low, active state.

Brief Description of the Drawings

One embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which:-

Fig. 1 is a block diagram of a simplified data processing system utilizing the bus assist cir¬ cuit of the present invention;

Fig. 2 is a logic diagram of the bus assist circuit of Fig. 1;

Figs. 3A-3C illustrate waveforms for the bus assist circuit of Fig. 2 when the bus conductor is returned to the inactive state;

Figs. 4A-4C illustrate waveforms for the bus assist circuit of Fig. 2 when the bus conductor re¬ mains in the inactive state; and

Figs. 5A-5C illustrate waveforms for the bus assist circuit of Fig. 2 when the state of the bus conductor is changing from its inactive state to its active state.

Best Mode for Carrying Out the Invention

Fig. 1 is a block diagram of a simplified data processing system having a processor 10, a memory device 12, and a multi-conductor data bus 14 connected between the processor 10 and the memory device 12. As is known, the data bus 14 includes a separate conduc¬ tor for each data bit of the particular data incre¬ ment for which the system is designed.

A system clock 16 is provided for controlling, among other things, data transfers be¬ tween the processor 10 and the memory device 12. The clock 16 may include one or more clock outputs such as outputs 18 and 20 for controlling the data processing system, as is known. A bus assist circuit 22 contains a plurality of modules 24, one for each of the bit conductors in the bus 14. A bus assist module 24 is connected to each conductor in the data bus 14 for assisting the processor 10 and returning the separate conductors of the data bus 14 to their inactive state before a data transfer from the processor 10 to the memory 12 takes place. A common input 26 is connected to one of the clock outputs 20, and the circuit 22 has a plurality of independent outputs 28, one for each module 24 each of which is connected to one of the bit conductors in bus 14. Each bus assist module 24 includes a pull up network 30 and a logic circuit 32, to be discussed later.

Fig. 2 is a logic diagram of the bus assist module 24 of Fig. 1, and includes a network pull-up circuit 30, and a logic circuit 32. The pull-up network 30 includes a voltage source 34, a voltage divider circuit made up of resistors 36 and 38, and an N-channel enhancement MOS transistor 46. The input 42 of the pull-up network 30 is connected to the gate of the transistor 46, and the output 44 of the pull-up network 30 is connected to one of the bit conductors in the data bus 14 by conductor 28 as previously explained. It will thus be understood that a positive pulse at input 42 turns on the transistor 46 allowing current to flow from the voltage source 34 to the output 44 to the connected conductor 28 thus assisting in desired transitions on the bit conductors connected to conductor 28. A feedback conductor 44 is connected between output conductor 28 and the logic circuit 32 for providing a sensing means which senses the state on the output conductor 28 for controlling the logic of the circuit 32 to be described.

One input of a NOR gate 50 is connected to the common input conductor 26, and the other input of the NOR gate 50 is connected to the output of an inverter 52 which is connected to the feedback conduc¬ tor 44. A second NOR gate 54 has one input connected to the common input conductor 26, and the other input connected to the feedback conductor 44.

NOR gates 56 and 58 are connected to form a set-reset flip-flop 59 having a reset terminal R connected to the output of NOR gate 50, and a set terminal S connected to the output of NOR gate 54, as shown. An AND gate 60 has one input connected to the common input conductor 26, and the other input con¬ nected to an output Q of the set-reset flip-flop 59, providing an output gate for the logic circuit 32.

In the described embodiment, a high signal on output conductor 28 is the inactive state, or a 0, on the connected bit conductor of the data bus 14 of Fig. 1. A low signal on output conductor 28, represents the active state, or a 1, on the bit conductor con¬ nected to the output conductor 28.

When the data transfer cycle begins, the clock 16 of Fig. 1 outputs a positive pulse on its output 20, which is input over conductor 26. to the input of NOR gate 50 and the AND gate 60 of the logic circuit 32 shown in Fig. 2. If the state on the output conductor 28 is low, this low is fed back over conductor 44 and inverted by inverter 52 and placed on the other input of NOR gate 50. The output of NOR gate 50 will be low, which will be applied to the reset terminal R of the flip-flop 59 formed by NOR gates 56 and 58. The high on the common input conduc¬ tor 26 will be applied to one input of NOR gate 54, and the low on conductor 44 will be applied to the other input, such that the output of NOR gate 54 will go high, and in turn be applied to the set terminal S of the flip-flop 59. A low on the reset terminal R, and a high on the set terminal S will cause the Q terminal of the flip-flop to go high. Thus, the two inputs on the AND gate 60 will be high. providing a high at the output of AND gate 60 which is applied to the input 42 of the pull-up network turning on tran¬ sistor 46. As previously described, the turning on of transistor 46 will apply current to the output lead 28 which will assist in changing the state on the lead 28 from a low active state to a high inactive state. This condition is shown in Figs. 3A-3C. Fig. 3A is the waveform of the signal placed on the input lead 26, Fig. 3B is the waveform of the signal on the output lead 28, and Fig. 3C shows the waveform of the input signal on pull-up network input 42. When the positive pulse 61 of Fig. 3A arrives on the common input conductor 26 of Fig. 2, and the low 62 of Fig. 3B is present on the output conductor 28 of Fig. 2, the output of AND gate 60 goes high as shown by pulse 64 of Fig. 3C for the duration of the input pulse 61. As previously explained, this turns on transistor 46 of Fig. 2 as shown at 66 of Fig. 3B.

When the state of the output conductor 28 of Fig. 2 is high, this high is inverted by inverter 52 to a low and applied to NOR gate 50 with the positive pulse on conductor 26 from the clock 16. In this case, the outputs of NOR gates 50 and 54 both go low, causing the Q output of the flip-flop 59 to remain low, thereby holding the output of AND gate 60 low. This low is applied to the gate of transistor 46, disabling transistor 46 such that the pull-up network has no effect on the state of conductor 28. This condition is shown in Figs. 4A-4C. Fig. 4A is the waveform for the signal on the input conductor 26 and shows a clock pulse 70 which occurs when the state on the conductor 28 is high as shown at 72 of the Fig. 4B. Fig. 4C shows the waveform on the output 44 of the pull-up network which, in this case, remains low as the AND gate 60 is held in its low or off condition as previously described.

If the state of the output conductor 28 is high, but is being changed to low or active by the processor 10 of Fig. 1, the AND gate 60 is turned off or placed in its low condition as discussed in connec¬ tion with Figs. 4A-4C. The AND gate 60 remains off, holding the transistor 46 off for a sufficient length of time to allow the processor 10 of Fig. 1 to change the state on output conductor 28 of Fig. 2 to its low or active condition. This prevents any contention between the output of the pull-up network 30 and the output of the processor 10, thereby shortening the time necessary to change the state of the output conductor 28 in this case from its high or inactive state to its low or active state. This condition is shown in Fig. 5A-5C, wherein the pulse 80 on Fig. 5A represents a positive pulse from the clock 16 of Fig. 1, and the state on conductor 28 is high as shown at 82 of Fig. 5B. The signal on the input 42 remains low as shown in Fig. 5C throughout the duration of the positive pulse 80. The transition 84 from the high state to the low state is thus controlled by the output of processor 10 of Fig. 1, and is not inter¬ fered with by the pull-up network 30, as previously explained.

The NOR gates 50, 54, 56 and 58 of the logic circuit 32 may be provided by a 74F02 chip available from Fairchild Camera and Instrument Corporation of Mountain View, California, and the inverter 52 and the AND gate 60 may be provided by the proper connections to a 74S38 chip available from Texas Instruments Corporation of Ricardson, Texas, as is known. The pull-up network 30 may be formed by a bus assist chip, or may be formed by an N-channel enhancement MOS transistor available from Siliconix of Santa Clara, California, under the designation of VQ1001. The voltage divider network may be formed by a 2K ohm resistor 36 and a 3K ohm resistor 38.

While a bus assist circuit has been shown being formed from discrete components, it will be understood that an integrated circuit chip may be fabricated for the pull-up network 30, the logic circuit 32, or both together, using standard design and fabrication techniques.

A bus assist circuit for use in a data processing system having a data transfer bus has been described which assists transitions of the states of ^• separate bit conductors in the data transmission bus from an active state to an inactive state, but which is disabled and allows transitions from the inactive state to the active state without contention from the bus assist circuit. It will be understood that data transmission buses other than one from a processor to a memory device may be used, or that data bit states having a low for an inactive state and a high for an active state may be substituted by those skilled in the art.

It will thus be readily appreciated by those skilled in the art that, while the described embodiment relates to a data bus between a processor and a memory where the processor is controlling the state on the conductors of a data bus, the present invention is equally useful for a memory bus wherein the memory is controlling the state of the conductors on the bus, or for a data bus connecting a processor and a peripheral device, or for a data bus connecting individual peripheral devices.

Claims

1. A circuit for assisting transitions of a signal on a bus conductor, characterized by: sensing means (44) connected to said bus conductor and adapted to determine if said bus conductor is in a first state or a second state; assist means (30) adapted to assist transitions of a signal on said bus conductor between said first state and said second state in response to clock pulses (61); and logic means (32) connected to said sensing means (44) and said assist means (30) and adapted to enable said assist means (30) when said signal transition is from said first state to said second state during said clock pulses (61) and to disable said assist means (30) when said signal transition is from said second state to said first state during said clock pulses (61) .

2. A circuit according to claim 1, characterized in that said assist means (30) includes a current source (34), and a switch means (46) adapted to switch said current source (34) on to said bus conductor when in a closed condition, and to prevent current flow from said current source (34) to said bus conductor when in an open condition; said switch means (46) having a control input (42) connected to said logic means (32) adapted to switch said switch means (46) to its closed condition when said logic means (32) enables said assist means, and to switch said switch means (46) to its open condition when said logic means (32) disables said assist means (30) .

3. A circuit according to claim 2, characterized in that said sensing means includes a feedback conductor (44) connected between said bus conductor and said logic means (32) ; and said logic means (32) includes a clock input (26) for receiving said clock pulses (61) , and has an output connected to the control input (42) of said switch means (46) , and a further input connected to said feedback conductor; said logic means (32) having a first enabling condition on its output when the bus conductor is at said first state at the start of a clock pulse (61) on said clock input (26), and having a second disabling condition on its output when the bus conductor is at said second state at the start of a clock pulse (61) on said clock input (26) .

4. A circuit according to claim 3, characterized in that said logic means (32) includes an inverter (52) having an input connected to said feedback conductor (44) ; a first NOR gate (50) having a first input connected to said clock input (26) , and a second input connected to the output of said inverter (52) ; a second NθR gate (54) having a first input connected to said clock input (26) , and a second input connected to said feedback conductor (44) ; a set-reset type flip-flop (59) having a reset input connected to the output of said first NOR gate (50) , and a set input connected to the output of said second NOR gate (54) ; and an AND gate (60) having a first input connected to said clock input (26) , a second input connected to an output of said set-reset type flip-flop (19) , and an output forming the output of said logic means (32) .

5. A circuit according to any one of the preceding claims, characterized in that said logic means (32) enables said assist means (30) when the first state of said bus conductor is a low signal state, and said logic means disables said assist means (30) when the second state of said bus conductor is a high signal state.