APPARATUS FOR READING DATA FROM MEMORY

Abstract: An apparatus for reading data from a memory in a computer system includes an address register (ADD-REG) for holding an address to be supplied to the memory; a read data register (RD-REG) for holding data read out from the memory; a device ( ) for generating a gated clock signal (GCLK) from a free-run clock signal (FCLK) having a predetermined constant period of time, where the gated clock signal is free-running with the constant period of time in a normal clock mode but is generated by a single pulse with an interval longer than the period of time of the free-run clock signal in a single clock mode; and a device (RDGC), having serially connected plural registers (SR₁, SR₂, SR₃) for shifting a trigger signal (TRi) in accordance with the free-run clock signal (FCLK), for generating a read data clock signal (RCLK), where the trigger signal has a same timing synchronized with a specific phase of the gated clock signal at which a phase the address register is switched to hold a new address to be supplied to the memory, and the shifted trigger signal is fed to the read data register for holding the data read out from the memory. Such apparatus can shorten the read cycle time of the data, and thus can raise the throughput of the computer system.

Description: The present invention relates to apparatus for reading data from a memory. In computer systems, as is well known, data stored in the memory (RAM) is read in response to an address signal and a chip selection signal transmitted from a central processing unit (CPU). In this case, the arrival of these signals at the RAM is delayed due to the long wiring pattern between the CPU and the RAM on the printed circuit board. Further, output of the read data is also delayed for a certain access time due to the transmission delay time caused within each RAM. In this case, it is difficult to shorten the delay time caused for the above reasons because of the structural factor when designing a large scale integrated circuit (LSI). Therefore, for the same reasons, it is also difficult to shorten the read cycle time for improving the throughput of the computer system by reducing the delay time. However, it is relatively easy to shorten the read cycle time by reducing a number of clock cycles between address setting and read out data receiving, but in this case, a problem occurs regarding the timing between the clock signal and the read data clock signal when the computer system has a single clock mode for diagnostic purposes. This problem is explained in detail hereinafter. It is desirable to provide an apparatus for reading data from a memory, which enables a shortening of a read cycle time, and thus an improvement of the throughput of the computer system. It is also desirable to provide a read data clock signal generation circuit for generating a read data clock signal operable in both a normal clock mode and the single clock mode. EP-A-0 161 639 by IBM Corporation discloses an on-chip timing control apparatus for generating timing signals for an LSI or memory array, which uses an off-chip trigger pulse input and can be used both during production testing and normal operation. According to the present invention, there is provided an apparatus for reading data from a memory in a computer system, wherein said apparatus comprises:- an address register for holding an address to be supplied to the memory; a read data register for holding data read out from the memory; and read data clock signal generating means for supplying a signal to said read data register for holding the data read out from the memory; characterized by:- means for generating a gated clock signal from a free-run clock signal having a predetermined constant cycle time, wherein in a normal clock mode, the gated clock signal is free-running with said constant cycle time and in a single clock mode, the gated clock signal is a manually generated single pulse with an interval longer than said constant cycle time; and in that said read data clock signal generating means comprises serially connected plural registers for shifting a trigger signal in accordance with the free-run clock signal, wherein said trigger signal is synchronized with a specific phase of the gated clock signal at which phase the address register is switched to hold a new address to be supplied to the memory, the shifted trigger signal being fed to the read data register for holding the data read out from the memory. Apparatus embodying the present invention can shorten a read cycle time of the data, and thus can raise the throughput of a computer system. Reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a schematic block diagram of a prior art system; Figs. 2 and 3 are signal timing charts of a prior art method; Fig. 4 is a read data clock signal generation circuit embodying the present invention; Fig. 5 is a signal timing chart of the circuit shown in Fig. 4; Fig. 6 is another read data clock signal generation circuit embodying the present invention; and Fig. 7 is a schematic block diagram of a memory access controller including a timing generator embodying the present invention. Before describing the preferred embodiments, an explanation will be given of a prior art method for reading data from the memory, and the problems thereof, with reference to Figs. 1 to 3. In Fig. 1, ADD-REG represents an address register, CS-REG a chip selection register, and OUT-BUF an output buffer. These elements are provided in a large-scale integrated circuit (LSI). IN-BUF represents an input buffer, and RAM a random access memory. These elements are provided in an array card. RD-REG represents a read data register. ADD represents an address signal generated from the register ADD-REG through the buffer OUT-BUF. CS represents a chip selection signal generated from the register CS-REG through the buffer OUT-BUF. The output buffer OUT-BUF and the input buffer IN-BUF are provided for matching a voltage level between an ECL level and a TTL level. A plurality of RAM's are provided in each array card 12. Usually, one memory bank is constituted by a plurality of the array cards and these array cards are extended in accordance with the increase of the memory size. Any one of the array cards is selected by a card selection signal not shown in Fig. 1. RAM-ADD represents a RAM address signal after selection, and RAM-CS a RAM selection signal after selection. RAM-RD represents read data from the selected array card after taking a "wired OR logic" among outputs from other array cards. The data RAM-RD is input to the register RD-REG through the buffer IN-BUF in the LSI 13. The register RD-REG is provided for temporarily storing the data read from the RAM. In this structure, the signals ADD and CS are transferred to the RAM through a relatively long wiring pattern between the LSI 11 and the array card 12 on a printed circuit board. The data RAM-RD is transferred to the register RD-REG also through a relatively long wiring pattern between the array card 12 and the LSI 13. Therefore, these signals are considerably delayed because of the length of the long wiring pattern. In Fig. 2, a first clock signal 1 is input to the registers ADD-REG and CS-REG. This clock is a gated clock generated from the CPU. The signals RAM-ADD and RAM-CS are input to the RAM after a delay D₁ caused by the long wiring pattern. The data RAM-RD is output from the RAM after the time T AA , which is the maximum access delay time of the RAM. This time T AA cannot be shortened because of the delay peculiar to the RAM. The data RAM-RD is input to the register RD-REG after the delay D₂ , also caused by the long wiring pattern, and set in the register RD-REG in response to a read data clock signal RCLK. The signal RCLK is generated from the delay line DL in a single clock mode, or generated from a clock signal in a normal clock mode. As is obvious from the timing chart, the register ADD-REG is switched to the next address (N + 1) when the ninth clock signal 9 is input and the signal RAM-ADD is also switched to the next address (N + 1) after the delay D₁. When the signal RAM-CS is finished at the same timing for switching to the next address (N + 1), the data RAM-RD is also finished after the delay time T LZ . In this case, the data RAM-RD is valid during three cycles (3τ) from the clock 9 to 12, as shown by the slant portion at the RD-REG-IN. However, in practice, the time necessary for taking the data into the register RD-REG is very short. Therefore, the above three-cycle time (3τ) is too long and almost all of this time is unnecessary. Further, the read cycle time from the register RD-REG is "8τ" and this read cycle time (8τ) is very large compared with the delay time T AA . This large read cycle (8τ) is dependent on the delay D₁, D₂ caused by the long wiring patterns and on the three cycles (3τ) of holding the data in the register RD-REG. In this case, as explained above, the delay time T AA , D₁ and D₂ are dependent on structural factors, and therefore, it is difficult to shorten these delay times, but it is possible to reduce the number of clock cycles defining a single read cycle time. One method shown in Fig. 3 has been proposed in order to solve the above long read cycle time. In this method, the register ADD-REG is switched when the sixth clock signal 6 is input thereto. Therefore, since the signal RAM-CS is finished at the same timing for switching to the next address (N + 1), the data RAM-RD is also finished after the delay time T LZ . In this case, the data RAM-RD is valid during approximately one cycle time, as shown by the slant portion at the RD-REG-IN. Therefore, the read cycle time can be shortened to the five cycle time (5τ), as shown by the RD-REG-OUT in Fig. 3. The above method is suitable for the normal clock mode, but unsuitable for the single clock mode for the reasons explained hereinafter. The single clock mode is manually set by an operator when testing the latching state of the data in the registers within the computer system. Therefore, the clock signal is generated in response to the push-button operation by the operator so that one cycle time between clock signals usually becomes very long, for example, a few seconds to a few minutes. In this case, there is no problem in the case of Fig. 2 because the input timing of the read data clock signal RCLK corresponds to that of the address switching. This means that the signal RCLK is input at the same timing as the ninth clock 9 in the single clock mode. Therefore, even in the single clock mode, the read out data is fully latched in the register RD-REG before the new address affected the read out data. However, a problem arises in the case of Fig. 3 because the input timing of the signal RCLK does not correspond to the timing of the address switching. That is, the registers ADD-REG and CS-REG are switched at the sixth clock 6 and the signal RCLK is input at the ninth clock 9. Therefore, the data RAM-RD has been switched before the signal RCLK is input. To resolve the above problem, it has been proposed that the delay line DL is used for obtaining the input timing of the signal RCLK. That is, the input timing of the signal RCLK is obtained by delaying the sixth clock 6 for three clock cycles so as to be apparently generated in the timing of the ninth clock 9 even if in the single clock mode. Therefore, this largely delayed clock is made from the delay line DL. However, it is difficult to obtain a precise largely delayed clock signal from the delay line DL because wide dispersions of the delay time occur in the delay line DL. These wide dispersions are caused by, for example, a change of temperature and the precision of manufacture of the delay line. Apparatus, for reading data from a memory, embodying the present invention will be explained in detail hereinafter. In Fig. 4, a read data clock signal generation circuit RDGC embodying the present invention is provided in a timing generator, (see Fig. 7) for generating the read data clock signal RCLK. Previously the signal RCLK was obtained from the delay line DL as shown in Fig. 1, but here is obtained from the circuit RDGC based on the free-run clock FCLK and the gated clock GCLK. SR1 to SR3 represent shift registers, A1 to A4 AND gates, and DEC a decoder. The register RD-REG is the same register as shown in Fig. 1. RD-REG-WAY-ADD is an address signal for selecting the register RD-REG when the memory is constituted by a plurality of memory banks. In this case, one memory bank is constituted by a plurality of the array cards shown in Fig. 1. The decoder DEC is provided for selecting the array card and outputting a selection signal SEL. Therefore, the circuit RDGC is provided at every memory bank. The operation of this circuit is explained with reference to Fig. 5. The free-run clock signal FCLK is generated from the CPU and the gated clock GCLK is generated based on the signal FCLK and a stop signal STOP through a negative logic AND gate A , as shown in Fig. 7. This stop signal STOP is kept at a logical "0" level during the normal clock mode, and is kept at a logical "1" level and made a logical "0" level for a single clock period only by a single push-button operation during the single clock mode. Tim 6 is the sixth signal of the signal GCLK. In the normal clock mode, the clock cycle time is the same phase between the FCLK and the GCLK. Therefore, the signal timing chart is not shown in Fig. 5 because no problem arises in the normal clock mode. An explanation will be given of the single clock mode shown in Fig. 5, because of the problem that arises in the generation timing of the signal RCLK. In Fig. 5, the upper half and the lower half do not synchronise, except at the sixth clock pulse. The time interval of each GCLK pulse in the upper portion may be a few seconds or a few minutes, but that of the FCLK is a constant period of, for example, a few nano-seconds. The register ADD-REG is switched, in the single clock mode, at the timing of the sixth clock signal Tim 6 of the GCLK. In this case, a long interval occurs until the next clock signal GCLK 7 is input. The trigger signal TRi is generated from the AND gate A1 based on the signal Tim 6 and the selection signal SEL₀. The selection signal SEL₀ is obtained from the signal RD-REG-WAY-ADD. The first free-run clock FCLK(labeled as "6") is input to the shift register SR1 at the same timing as the GCLK 6. The first shifted signal S IN is generated from the AND gate A2 based on the trigger signal TRi and the second signal TRo during one cycle (1τ). That is, the signal S IN is obtained by a differentiating the signal TRi by in the AND gate A2 and the shift register SR1. The signal S IN is shifted by the next clock FCLK 7 and the second shifted signal S OUT0 is output from the shift register SR2. Further, the signal S OUT0 is input to the shift register SR3 and the third shifted signal S OUT1 is output from the shift register SR3 in response to the third clock FCLK 8. The read data clock signal RCLK is obtained from the AND gate A3 based on the clock signal FCLK 9 and the signal S OUT1 after the three cycle time (3τ). The output data RD-REG-OUT from the register RD-REG is input to the AND gate A4 and output in response to the read-out control signal RD-OUT-CONT generated from the GCLK 9. The output data RD is transferred to the register of the next stage. In Fig. 6, SLC represents a selection circuit. The selection circuit SLC comprises three AND gates A5 to A7 and an OR gate. The NORM-CYC, 2τ-MODE and LOW-CYC represent control signals manually applied by the operator when testing the read data clock signal generation circuit RDGC in response to the change of period of the free-run clock signal FCLK. The signal NORM-CYC is input when the circuit RDGC should be operated by the normal clock signal FCLK having a normal period. The signal 2τ-MODE is input when the period of the clock signal FCLK is doubled. The signal LOW-CYC is input when the period of the clock signal FCLK is tripled. Therefore, the input timing of the signal RCLK can be held to a constant timing by selecting these control signals. In Fig. 7, CPU/MCU represents a central processing unit including a memory control unit. MAC represents a memory access controller, and I/F an interface latch circuit. TG represents a timing generator including the read data clock signal generation circuit RDGC shown in Figs. 4 and 6. S A represents an address set timing signal, S W a write set timing signal, and S R a read set timing signal. The signal SR corresponds to the read data clock signal RCLK. Therefore, the register RD-REG corresponds to the register RD-REG shown in Figs. 4 and 6. An explanation of the signals S A and S W is omitted, as these signals are not related to the present invention. A represents an AND gate having negative logic function. The gated clock GCLK is generated based on the free-run clock FCLK and the stop signal STOP. The signal GCLK is also input to the interface I/F to synchronize with the operation of the circuit RDGC. The signal RD-OUT-CONT is the same signal as shown in Figs. 4 and 6, and the AND gate A4 is also the same AND gate as shown in Figs. 4 and 6. CONT represents a control signal for the timing generator TG and output from the memory control unit MCU in response to the gated signal GCLK.

Claims: An apparatus for reading data from a memory in a computer system, wherein said apparatus comprises:- an address register (ADD-REG) for holding an address to be supplied to the memory; a read data register (RD-REG) for holding data read out from the memory; and read data clock signal generating means (RDGC) for supplying a signal to said read data register (RD-REG) for holding the data read out from the memory; characterized by:- means for generating a gated clock signal (GCLK) from a free-run clock signal (FCLK) having a predetermined constant cycle time, wherein in a normal clock mode, the gated clock signal (GCLK) is free-running with said constant cycle time and in a single clock mode, the gated clock signal is a manually generated single pulse with an interval longer than said constant cycle time; and in that said read data clock signal generating means (RDGC) comprises serially connected plural registers (SR1, SR2, SR3) for shifting a trigger signal (TR i ) in accordance with the free-run clock signal (FCLK), wherein said trigger signal is synchronized with a specific phase of the gated clock signal (GCLK) at which phase the address register (ADD-REG) is switched to hold a new address to be supplied to the memory, the shifted trigger signal (S OUT1 ) being fed to the read data register (RD-REG) for holding the data read out from the memory. An apparatus according to claim 1, further comprising:- a first AND gate (A2), having two inputs connected to the input and output of the first-connected shift register (SR1) respectively and an output connected to the input of the second-connected shift register (SR2), for differentiating the trigger signal (TR i ); and a second AND gate (A3), having a first input connected to said free-run clock signal (FCLK), a second input connected to the output of the last-connected shift register (SR3), and an output connected to said read data register (RD-REG), for synchronizing the shifted trigger signal (S OUT1 ) with the free-run clock signal (FCLK). An apparatus according to claim 2, further comprising:- selecting means (SLC) having a plurality of inputs respectively connected to the shift register outputs, and an output connected to the second input of the second AND gate (A3), for selecting one of the shift register outputs in accordance with selecting signals (NORM-CYC, 2τMODE, LOW-CYC). An apparatus according to claim 1, 2 or 3, wherein said means for generating the gated clock signal (GCLK) includes a gate means ( A ) for gating the free-run clock signal (FCLK) with a stop signal (STOP), the stop signal (STOP) being kept OFF during the normal clock mode, and kept ON during the single clock mode except when manually designated for a single period of time of the free-run clock signal (FCLK).

Inventors: IBI, TAKASHI
Applicants: FUJITSU LIMITED
Application year: 1987
Main classification: G11C7
IPC codes: G06F12, G11C7, G11C8