Data Control System Allowing Simultaneous Communication Between a Host and a Plurality of Peripherals Over Predetermined Bit Lines

1. Field of the Invention

This invention relates generally to a bus control system suitable for use in a data processing system such as an image processing system, and more particularly to such a bus control system in which a data bus connecting a central processing unit to a plurality of peripheral devices can be efficiently used.

2. Prior Art

It has become common that a data processing system, such as an image processing system, includes a central processing unit (CPU) and a plurality of peripheral devices connected to the CPU through a common data bus. Data processing systems of such a construction are also widely used for other various purposes.

In such data processing systems, an exchange of data between a CPU and a device is performed in such a manner that the CPU first designates the device, with which it exchanges data, by outputting address data or the like, and then transmits to or receives from the designated device data using all bit-lines of the common data bus.

With such arrangement, however, when the CPU transmits to or receives from the designated device data each composed of one bit or several bits at most, that is to say, when the CPU transmits or receives data using only a few bit-lines of the common data bus, the remaining bit-lines of the common data bus become idle, so that the common bus can not efficiently be used. Thus, when the CPU transmits to or receives from a number of devices data each composed of a few bits, the number of transmissions or receptions of data the CPU must effect becomes large in spite of the actual amount of data being small. As a result, the dead time in the transmission and reception of data significantly increases.

It is therefore an object of the present invention to provide a bus control system in which a common data bus connecting a CPU to peripheral devices can efficiently be used even when the CPU transmits to or receives from the devices data each composed of bits smaller in number than bit-lines of the common data bus.

Another object of the invention is to provide a bus control system in which dead time of the common data bus does not increase even when such a transmission or reception of data is performed.

According to an aspect of the present invention, there is provided a bus control system adapted for use in a data processing system including a central processing unit and a plurality of peripheral devices connected to the central processing unit through a common data bus having a plurality of bit-lines comprising address decoding means for decoding address data fed from the central processing unit to selectively output a commonly selecting signal for bringing all the devices into operative states and a plurality of individually selecting signals each for bringing a respective one of the plurality of devices into an operative state; register means provided in each of the plurality of devices and responsive to a corresponding one of the plurality of individually selecting signals for storing data appearing on predetermined ones of the the bit-lines of the common data bus thereinto; and bit selection means provided in each of the plurality of devices and responsive to the commonly selecting signal for selecting predetermined at least one of the plurality of bit-lines of the common data bus in accordance with the data stored in the register means of the each of the plurality of devices to allow each of the plurality of devices to communicate with the central processing unit through the selected at least one bit-line of the common data bus.

FIG. 1 is a block diagram of a data processing system including a bus control system provided in accordance with the present invention;

FIG. 2 is a circuit diagram of one of the devices 102 of the data processing system of FIG. 1; and

FIG. 3 is a timing chart of various signals appearing in the devices 102 of the data processing system of FIG. 1.

Referring now to FIG. 1, there is shown a data processing circuit 100 provided in accordance with the present invention.

The data processing circuit 100 comprises a CPU (central processing unit) 101 for controlling various portions of this data processing circuit 100. The CPU 101 is connected to peripheral devices (or interface circuits) 102.sub.1 to 102.sub.n of an identical construction through a common data bus 103. Each of the devices 102.sub.1 to 102.sub.n has therein memory means for storing data sent from the CPU 101, and is for example a frame buffer memory in an image processing system. The common data bus 103 comprises sixteen bit-lines for respectively sending sixteen bits D.sub.0 to D.sub.15 of each data, and thus, when the CPU 101 transmits to or receives from one of the devices 102.sub.1 to 102.sub.n, the transmission or reception of data is performed on a sixteen-bit basis. Each of the devices 102.sub.1 to 102.sub.n has a pair of chip selection terminals (or device selection terminals) CSC and CSI, and is brought into an operative state when a "0" signal is applied to either of the two chip selection terminals CSC and CSI.

The CPU 101 is so constructed as to output address data AD, a read/write control signal R/W and a data strobe signal DS at required timings as later described. The lower-order two bits A.sub.0 and A.sub.1 of the address data AD are supplied to the devices 102.sub.1 to 102.sub.n, and the remaining bits of the address data AD are supplied to an address decoder 105. The address decoder 105 decodes the remaining bits of the address data AD fed from the CPU 101 to render one of chip selection signals S.sub.c and S.sub.1 to S.sub.n active or "0". The chip selection signal S.sub.c is supplied to all of the chip selection terminals CSC of the devices 102.sub.1 to 102.sub.n, while the chip selection signals S.sub.1 to S.sub.n are supplied to chip selection terminals CSI of the devices 102.sub.1 to 102.sub.n, respectively. As described above, only one of the chip selection signals S.sub.c and S.sub.1 to S.sub.n is rendered active at a time. Thus, when the chip selection signal S.sub.c is rendered "0" all of the devices 102.sub.1 to 102.sub.n are brought into operative states, and when one of the chip selection signals S.sub.1 to S.sub.n is rendered "0" a corresponding one of the devices 102.sub.1 to 102.sub.n is brought into an operative state.

The data strobe signal DS and the read/write control signal R/W are supplied together with the lower-order two bits A.sub.0 and A.sub.1 of the address data AD to all of the devices 102.sub.1 to 102.sub.n.

The circuit of the device 102.sub.i (i is any one of "1" to "n") will now be more fully described with reference to FIG. 2. As described, the devices 102.sub.1 to 102.sub.n are identical in construction.

In FIG. 2, the device 102.sub.i comprises input terminals T.sub.1 and T.sub.2, to which the bits A.sub.0 and A.sub.1 of the address data AD outputted from the CPU 101 (FIG. 1) are supplied, and input terminals T.sub.3 and T.sub.4 to which the data strobe signal DS and the read/write control signal R/W are supplied, respectively. The chip selection terminal CSC is supplied with the signal S.sub.c and the chip selection terminal CSI of this device is supplied with the signal S.sub.i, as described above. The bits A.sub.0 and A.sub.1 fed to the terminals T.sub.1 and T.sub.2 are supplied to input terminals B and A of four decoders DEC2, DEC3 DEC4 and DEC5 each for decoding the supplied bits A.sub.0 and A.sub.1 to output a "0" signal from one of four output terminals Y.sub.0 to Y.sub.3 thereof. The relationship between the input and output signal of each of the decoders DEC2 to DEC5 is shown in Table 1.

Each of the decoders DEC2 to DEC5 is so arranged to output its decode result only when a "0" signal is applied to an enabling input terminal G thereof. The enabling input terminals G of the decoders DEC2 to DEC5 are connected to output terminals of gates 110 to 113, respectively. The read/write control signal R/W applied to an input terminal T.sub.4 of this device 102.sub.i is fed to inverters 114 and 115 which in turn output inverted signals of the signal R/W to second input terminals of the gates 111 and 113, respectively. Signal levels of respective output signals of the gates 110 to 113 are determined in accordance with signal levels of the signals DS, R/W, S.sub.c and S.sub.i, as follows.

The output signal of this gate 110 is rendered "0" when both of the data strobe signal DS and the chip selection signal S.sub.c are "0" and when the read/write control signal R/W is also "0" (write mode). Otherwise the output signal of this gate 110 is rendered "1".

The output signal of this gate 111 is rendered "0" when both of the data strobe signal DS and the chip selection signal S.sub.c are "0" and when the read/write control signal R/W is "1" (read mode). Otherwise the output signal of this gate 111 is rendered "1".

The output signal of this gate 112 is rendered "0" when both of the data strobe signal DS and the chip selection signal S.sub.i are "0" and when the read/write control signal R/W is also "0". In other conditions, the output signal of this gate 112 is rendered "1".

The output signal of this gate 113 is rendered "0" when both of the data strobe signal DS and the chip selection signal S.sub.i are "0" and when the read/write control signal R/W is "1". In other conditions, the output signal of this gate 113 is rendered "1".

This device 102.sub.i also comprises a four-bit register REG1 of which input terminals are connected to predetermined four bit-lines of the common data bus 103, respectively. This register REG1 stores four-bit data supplied to the input terminals thereinto when a signal fed from the output terminal Y.sub.0 of the decoder DEC4 to a clock terminal CK thereof rises from "0" to "1". The data stored in this register REG1 is supplied to a decoder DEC1 which decodes the supplied data (four bits) to render one of sixteen signals BL.sub.0 to BL.sub.15 "1" in accordance with the decode result. The signals BL.sub.0 to BL.sub.15 are supplied respectively to first input terminals of sixteen AND gates AN.sub.0 to AN.sub.15 of a gate circuit 117. The second input terminals of the AND gates AN.sub.0 to AN.sub.15 are supplied respectively with the bits D.sub.0 to D.sub.15 of data on the first to sixteenth bit-lines of the common data bus 103. The gate circuit 117 further comprises an OR gate OR.sub.1 for logically adding all the outputs of the AND gates AN.sub.0 to AN.sub.15. An output of this OR gate OR.sub.1 is supplied to a data input terminal D of a one-bit register REG2 which stores the output of the OR gate OR.sub.1 thereinto when the signal fed from the output terminal Y.sub.0 of the decoder DEC2 to a clock terminal CK thereof rises from "0" to "1". The data which the register REG2 stores thereinto is that bit of the bits D.sub.0 to D.sub.15 on the common bus 103 which corresponds to the "1" signal among the signals BL.sub.0 to BL.sub.15. In other words, the data stored in the register REG2 is that bit of the bits D.sub. 0 to D.sub.15 of which bit order is represented by the content of the register REG1. The data stored in the register REG2 is supplied to other circuit portions (not shown) of this device 102.sub.i in which the data is used thereby.

The signals BL.sub.0 to BL.sub.15 outputted from the decoder DEC1 are also supplied respectively to first input terminals of sixteen NAND gates NA.sub.0 to NA.sub.15 of another gate circuit 118. Second input terminals of the NAND gates NA.sub.0 to NA.sub.15 are supplied from an inverter INV with an inverted signal of the signal fed from the output terminal Y.sub.0 of the decoder DEC3. Output signals of the NAND gates NA.sub.0 to NA.sub.15 are supplied respectively to enable/disable terminals of sixteen buffer gates BF.sub.0 to BF.sub.15. Input terminals of the buffer gates BF.sub.0 to BF.sub.15 are supplied with an output signal of a one-bit register REG3, and output terminals of the buffer gates BF.sub.0 to BF.sub.15 are connected respectively to the first to sixteenth bit-lines of the common data bus 103. Each of the buffer gates BF.sub.0 to BF.sub.15 is of such a type that an inverted signal of its input signal is outputted when the enable/disable terminal thereof is supplied with a "0" signal, and that the output terminal is brought into a high impedance state when the enable/disable terminal is supplied with a "1" signal. Thus, which one of the buffer gates BF.sub.0 to BF.sub.15 outputs an inversion of the output signal of the register REG3 is determined by the "1" signal among the signals BL.sub.0 to BL.sub.15. In other words, the data from the register REG3 is outputted onto the common data bus 103 through that one of the buffer gates BF.sub.0 to BF.sub.15 which is selected in accordance with the data in the register REG1.

A sixteen-bit register 120 is loaded, by a circuit (not shown) provided in this device 102.sub.i, with data of sixteen bits which is to be sent to the CPU 101 (FIG. 1). This register 120 is enabled to output the loaded sixteen-bit data onto the first to sixteenth bit lines of the common data bus 103 when a "0" signal is supplied to an output enabling terminal OE thereof from the output terminal Y.sub.0 of the decoder DEC5. There is provided another sixteen-bit register 121 which supplies data contained therein to another circuit (not shown) provided in this device 102.sub.i. This register 121 is loaded with the bits D.sub.0 to D.sub.15 on the common data bus 103 when a "0" signal is supplied to an input enabling terminal IE thereof from the output terminal Y.sub.0 of the decoder DEC4.

The operation of the circuit of the device 102.sub.i (i is any one of "1" to "n") of FIG. 2 will now be described with reference to a timing chart of FIG. 3.

A transfer of four-bit data from the CPU 101 to the register REG1 of the device 102.sub.i is carried out in the following manner.

The CPU 101 first outputs at time t.sub.1 in FIG. 3 the address data AD indicative of the device 102i, with which the CPU 101 desires to communicate, to the address decoder 105 (FIG. 1). As a result, the address decoder 105 outputs the chip selection signal S.sub.i at a time substantially the same as the time t.sub.1 (see FIG. 3-(d)). The CPU 101 further renders the read/write control signal R/W "0" at the time t.sub.1, and also renders both of the lower-order two bits A.sub.0 and A.sub.1 of the address data AD "0" to allow the decoder DEC4 to output a "0" signal from the output terminal Y.sub.0 thereof (FIGS. 3-(b) and 3-(g)). The CPU 101 then outputs, at time t.sub.2, a four-bit data to be written into the register REG1 onto the predetermined four bit-lines of the common data bus 103. The CPU 101 subsequently renders at time t.sub.3 the data strobe signal DS "0" , as shown in FIG. 3-(a), whereupon the gate 112 outputs a "0" signal. As a result, the decoder DEC4 outputs a "0" signal from its output terminal Y.sub.0 to the clock terminal CK of the register REG1, so that the register REG1 stores the four-bit data on the common data bus 103 thereinto.

The transfer of the four bit data from the CPU 101 to the register REG1 of the device 102.sub.i is performed as described above, and such transfers of data from the CPU 101 to the registers REG1 of the respective devices 102.sub.1 to 102.sub.n are performed in the similar manner. In this case, the four-bit data written respectively into the registers REG1 of the devices 102.sub.1 to 102.sub.n normally differ in value from one another.

When the four-bit data are stored respectively into the registers REG1 of the devices 102.sub.1 to 102.sub.n, the bit of the data on the common data bus 103 which is to be written into the register REG2 and the bit-line onto which the data in the register REG3 is outputted are determined with respect to all of the devices 102.sub.1 to 102.sub.n.

Description will now be given as to the simultaneous storing of data into the registers REG2 of the devices 102.sub.1 to 102.sub.n.

The CPU 101 outputs at time t.sub.1 in FIG. 3 the address data AD of a predetermined value to the address decoder 105 for bringing all of the devices 102.sub.1 to 102.sub.n into operative conditions. In response to the address data AD, the address decoder 105 outputs at a time substantially the same as the time t.sub.1 the chip selection signal S.sub.c to all of the devices 102.sub.1 to 102.sub.n, as shown in FIG. 3-(e). The CPU 101 further renders the read/write control signal R/W "0" at the time t.sub.1, and also renders both of the lower-order two bits A.sub.0 and A.sub.1 of the address data AD "0" to allow the decoder DEC2 to output a "0" signal from the output terminal Y.sub.0 thereof (FIGS. 3-(b) and 3-(g)). Then, the CPU 101 outputs, at time t.sub.2, the data containing the bits to be written respectively into the registers REG2 onto the common data bus 103. In this case, the correspondency of the bit-lines of the common data bus 103 to the devices 102.sub.1 to 102.sub.n is known to the CPU 101 based on the data written into the registers REG1, so that the CPU 101 outputs the bits in the desired states respectively to the bit-lines of the common data bus 103.

Then, the CPU 101 renders the data strobe signal DS "0" at time t.sub.3, as shown in FIG. 3-(a), whereupon the gates 110 of the devices 102.sub.1 to 102.sub.n output "0" signals. As a result, "0" signals are outputted from the output terminals Y.sub.0 of the decoders DEC2 and supplied to the clock terminals CK of the registers REG2, respectively. Consequently, each of the registers REG2 of the devices 102.sub.1 to 102.sub.n stores that bit of the bits D.sub.0 to D.sub.15 on the common data bus 103 which is designated by the data contained in the register REG1. Thus, each of the bits D.sub.0 to D.sub.15 is stored into a respective one of the registers REG2 of the devices 102.sub.1 to 102.sub.n.

Description will now be given as to the simultaneous readout of data by the CPU 101 from the registers REG3 of the devices 102.sub.1 to 102.sub.n.

The CPU 101 first outputs at time t.sub.1 in FIG. 3 such an address data AD to the address decoder 105 that all of the devices 102.sub.1 to 102.sub.n are simultaneously accessed. As a result, the address decoder 105 outputs the chip selection signal S.sub.c to all of the devices 102.sub.1 to 102.sub.n at a time substantially the same as the time t.sub.1, as shown in FIG. 3-(e). The CPU 101 further renders the read/write control signal R/W "1" at the time t.sub.1 (FIG. 3-(f)), and also renders both of the lower-order two bits A.sub.0 and A.sub.1 of the address data AD "0" to allow the decoder DEC3 to output a "0" signal from the output terminal Y.sub.0 thereof.

Then, the CPU 101 renders the data strobe signal DS "0" at time t.sub.3, as shown in FIG. 3-(a), whereupon the gates 111 of the devices 102.sub.1 to 102.sub.n output "0" signals. As a result, "0" signals are outputted from the output terminals Y.sub.0 of the decoders DEC3 and supplied to the inverters INV of the devices 102.sub.1 to 102.sub.n, respectively. Consequently, in each of the devices 102.sub.1 to 102.sub.n, that one of the NAND gates NA.sub.0 to NA.sub.15 which is designated by the data contained in the register REG1 outputs a "0" signal to enable a corresponding one of the buffer gates BF.sub.0 to BF.sub.15 to output an inversion of the data contained in the register REG3 onto the corresponding bit-line of the common data bus 103. Thus, the data contained in the registers REG3 of the devices 102.sub.1 to 102.sub.n are simultaneously outputted onto the respective bit-lines of the common data bus 103.

In this case, the correspondency of the bit-lines of the common data bus 103 to the devices 102.sub.1 to 102.sub.n is known to the CPU 101 based on the data written into the registers REG1, so that the CPU 101 can discriminate the bits appearing on the common data bus 103 from on another.

Description will now be given as to exchange of sixteen-bit data between the CPU 101 and a desired one of the devices 102.sub.1 to 102.sub.n, such exchange being common in data processing systems.

To readout sixteen-bit data from a desired devices 102.sub.i (i is one of "1" to "n"), the CPU 101 first outputs the address data AD indicative of the desired device 102.sub.i to the address decoder 105. In response to the address data AD, the address decoder 105 outputs the chip selection signal S.sub.i to the desired device 102.sub.i. The CPU 101 further renders the read/write control signal R/W "1" and also renders both of the lower-order two bits A.sub.0 and A.sub.1 of the address data AD "0" to allow the decoder DEC5 to output a "0" signal from the output terminal Y.sub.0 thereof. The CPU 101 then renders the data strobe signal DS "0", whereupon a "0" signal is outputted from the output terminal Y.sub.0 of the decoder DEC5 to the output enabling terminal OE of the register 120 of the designated device 102i. As a result, the sixteen bits of the data contained in the register 120 are outputted respectively onto the bit-lines of the common data bus 103.

To write sixteen-bit data into a desired devices 102.sub.i (i is one of "1" to "n"), the CPU 101 first outputs the address data AD indicative of the desired device 102i to the address decoder 105. In response to the address data AD, the address decoder 105 outputs the chip selection signal S.sub.i to the desired device 102.sub.i. The CPU 101 further renders the read/write control signal R/W "0" and also renders the lower-order two bits A.sub.0 and A.sub.1 of the address data AD "1" and "0", respectively, to allow the decoder DEC4 to output a "0" signal from the output terminal Y.sub.1 thereof. The CPU 101 then renders the data strobe signal DS "0", whereupon a "0" signal is outputted from the output terminal Y.sub.1 of the decoder DEC4 to the input enabling terminal IE of the register 121 of the designated device 102.sub.i. As a result, the sixteen-bit data on the common data bus 103 is written into the register 121.

Thus, according to the above-described bus control system, a plurality of data each composed of bits less in number than the bit-lines of the common data bus 103 can be sent to or received from the plurality of devices 102 at a time. As a result, the common data bus 103 can efficiently be used even when a plurality of data each composed of a few bits must be sent through the common data bus.