Abstract:
A timing signal generator which can operate stably even when, or directly after, a power supply is switched on. The generator is of the type having a first dynamic delay circuit for generating a first timing signal in response to said input control signal and a second dynamic delay circuit for generating a second timing signal in response to the first timing signal, and is featured by a first transistor connected between the output of the first dynamic delay circuit and a voltage terminal with a gate connected to the input of the first dynamic delay circuit and a second transistor connected to the output of the second dynamic delay circuit and the voltage terminal with a gate connected to receive the first timing signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a timing signal generator and more particularly to one suitable for a dynamic memory (DRAM). 
     Dynamic memories have been utilized in various fields due to their large memory capacities. Dynamic memories operate under control of a basic, externally generated control signal. One of the important functions of the basic control signal is to control memory reset operations. 
     In dynamic logic circuits, prior to any logic operation, circuit nodes are reset to establish the standby mode. In the standby mode, the circuit is made ready for the subsequent logic operation. In dynamic memories, a plurality of dynamic type functional blocks such as address buffers and address decoders are subjected to reset operations in predetermined timing sequences. For example, the address buffers are reset first to set the outputs therefrom at a low level and then NOR output nodes of the address decoders are reset to a high precharged level. By keeping the sequence of resets of the respective functional blocks in a predetermined order, the respective functional blocks can operate adequately, without malfunction, and at maximum speed when the memory is shifted to a read-out or write operation. In order to achieve the above sequence of reset operations for the functional blocks, a timing signal generator is employed in the memory. The timing signal generator generates a sequence of timing signals which control the reset operations of the respective blocks in response to a basic signal. However, the above sequence of reset operations is not maintained when, or directly after, a power supply to a memory is switched on, to shift the memory from a non-powered state to a powered state. In this instance, potential states of circuit nodes in the respective functional blocks are likely to be indefinite and unstable. Therefore, such circuit nodes are subjected to an initializing operation in order to set the circuit nodes at predetermined states. Namely, circuit conditions of the functional blocks must be set at predetermined initial states directly after the power voltage is turned on. Otherwise, the circuit operation will become incomplete and an abnormally large current is generated in the circuit. Such abnormal current sometimes causes the circuit to break down or do harm to other elements of the system or circuit board. 
     In the initializing operation, a plurality of timing signals generated from a timing signal generator employed in a memory are also required to occur in a predetermined order. 
     However, it has been difficult to achieve proper initializing operations when circuit node potentials are in the unstable state for the conventional timing signal generators may cause a timing abnormality in signal timing when, or immediately after, the power is switched on. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to present a timing signal generator which can generate a plurality of timing signals in a desired order even when power supply is switched on. 
     It is another object of the present invention to provide a timing signal generator suitable for dynamic memories. 
     The timing signal generator according to the present invention is of the type comprising a first delay circuit adapted to generate a first timing signal in response to an input control signal and a second delay circuit for generating a second timing signal in response to the first timing signal. The timing signal generator of the invention further comprises a first field effect transistor connected to the output terminal of the first delay circuit so as to supply it with a power voltage when the input control signal becomes high and a second field effect transistor connected to the output terminal of the second delay circuit so as to supply it with the power voltage when the first timing signal becomes high. 
     According to the present invention, when the power supply is switched on, the first transistor becomes conducting first in response to the input control signal so that the first timing signal is generated. Then, in response to the generation of the first timing signal, the second transistor becomes conducting so that the second timing signal is generated. Thus, the first and second timing signals are generated in a desired order even when the power supply is switched on. 
     The timing signal generator of the invention is useful not only for dynamic memories but also for dynamic logic circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram showing a dynamic memory; 
     FIG. 2 is a schematic circuit diagram showing a 1-bit unit of an address buffer; 
     FIG. 3 is a schematic circuit diagram showing a 1-bit structure of the address decoder; 
     FIG. 4 is a schematic diagram of a timing signal generator according to the prior art; 
     FIG. 5 is a schematic diagram showing a timing signal generator according to the present invention; 
     FIG. 6 is a schematic circuit diagram showing a detailed example of the timing signal generator according to the present invention; and 
     FIGS. 7A and 7B show waveform diagrams which illustrate the operation of the circuit of FIG. 6. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following explanation, N-channel MOS transistors are employed as transistors, and a power voltage V cc  serves as a high level while ground potential serves as a low level. 
     With reference to FIG. 1, the general structure of a dynamic memory will be briefly explained. 
     The memory is the so-called multi-strobe type in which row address signals and column address signals are incorporated through the same set of address input terminals Ta to An in response to a row strobe signal RAS and a column strobe signal CAS in a time divisional way. A buffer 23 receives RAS and generates an internal signal RASA. In response to RASA, a timing signal generator 20 generates timing signals φ 1  and φ 2  in a predetermined sequence. Namely, in response to a high level of RASA, the generator 20 produces the signal φ 1  first which controls a reset operation of the address buffer 11, and then produces the signal φ 2  for precharging the decoder 12 which is connected to word lines WL of a memory cell array 13. The array includes a plurality of memory cells MC at intersections of the word lines WL and digit lines DL in a known manner. A column timing signal generator 21 receives CAS and an output signal of the timing signal generator 20 and generates timing signals φ 3  and φ 4  for controlling reset operations of a column address buffer 16 and a column decoder 15, respectively, and also generates a signal φ c  for controlling a read-write control signal generator 22. The generator 22 controls a switch circuit 17 which selectively connects a data output circuit 18 and a data input circuit 19 to a column selection circuit 14. 
     In this memory, the signal RASA serves as a basic timing signal to control the whole memory directly and indirectly. The operations based on RASA will be explained hereinbelow. 
     FIG. 2 shows a one bit structure of the buffer 11. A flip-flop F/F receives the address input Ao and generates its true signal ao and complementary signal ao. Here, transistors Q 1  and Q 2  are connected between the outputs ao, ao and a ground potential, respectively. The transistors Q 1  and Q 2  operatively clamp the outputs ao and ao to the ground potential in response to the timing signal φ 1  for resetting. 
     FIG. 3 shows a one bit structure of the decoder 12. The decoder is basically composed of a NOR circuit including transistors Q 4 , Q 5 , Q 6  receiving the outputs from the buffer 11 in a predetermined combination, a precharge transistor Q 3  and a word line drive transistor Q 7 . Here, the time relation between the signal φ 1  and the signal φ 2  is determined such that the signal φ 2  will be at the high potential level after the reset signal φ 1  is at the high potential. If the signal φ 2  is at the high potential when the reset signal φ 1  is at the low potential, at least one of the OR transistors Q 4 , Q 5 , Q 6  conducts because one of the outputs a i  or a i  is necessarily at the high potential. In this circumstance, it sometimes happens that when the signal φ 2  is produced, a large quantity of current flows through the address decoder so that a protection circuit of the power source is switched and the operation of the DRAM is stopped. 
     FIG. 4 is a block diagram of a part of the conventional timing signal generator 20 in the DRAM. Delay circuits 31 and 32 are composed of dynamic logic circuits. The delay circuit 31 generates the signal φ 1  with a time lag of T 1  with respect to the input signal RASA, and the delay circuit 32 generates the signal φ 2  with a time lag of T 2  with respect to the signal φ 1 . Transistors Q 10  and Q 11  are pull-up transistors for initializing the states of φ 1  and φ 2 . 
     The current capacity of transistors Q 8  and Q 9  is made about 100 times that of the Q 13 . In the quiescent state, signals P 1  and P 2  are complementary to each other so that one of the transistors Q 8  and Q 9  conducts while the other is cut off. Therefore, in the quiescent state the level of the signal φ 2  is dependent on the signals P 1  and P 2 , which are, in turn, determined solely by the signal φ 1 . 
     At the time of turning on the power voltage, however, the level of the signal RASA is uncertain and may be either high or low level and the levels of the signals P 1  and P 2  are indefinite. Therefore, the signals φ 1  and φ 2  are set comparatively slowly to a high level by the pull-up transistors Q 10  and Q 13 . Since the gates of the pull-up transistors Q 10  and Q 13  are conncted to the terminals of the power source, the characteristics of the transistors Q 10  and Q 13  and their loads determine which one of the signals φ 1  and φ 2  will rise first in potential when the power voltage is switched on. However, to accurately obtain the necessary manufacturing characteristics relative to the pull-up transistors Q 10  and Q 13 , particularly current capacity, and to conveniently determine the load, are extremely difficult if not impossible. Therefore, an abnormality in timing between the signals may occur with the effect that the signal φ 2  may become high earlier than the signal φ 1 . As described above, this timing abnormality disadvantageously leads to excessive current supply. 
     An embodiment of the invention will now be described with reference to FIG. 5. According to the invention, the gate of the transistor Q 10&#39; , which is analogous to transistor Q 10  of FIG. 4, is connected to receive the signal RASA and the gate of the transistor Q 13&#39; , which is analogous to the transistor Q 13  of FIG. 4, is connected to the output (φ 1 ) of the delay circuit 31. Here, RASA is the signal which takes a high level during at least a part of the stand-by period after the power is switched on. 
     Therefore, the signal φ 1  rises in potential first in response to a high level of RASA applied to the delay circuit 31 and gate of transistor Q 10&#39; . Then, in response to the rise of φ 1 , the transistor Q 13&#39;  becomes conducting to raise φ 2  to a high level directly after the power is switched on. In this instance, the states of the delay circuits 31 and 32 are unstable and cannot properly drive their outputs. 
     Accordingly, the transistor Q 13 , never conducts unless the reset signal φ 1  assumes the high level. Consequently, when initializing immediately after turning on the power, the order of the generation of signals, in which after the signal φ 1  becomes high level, the signal φ 2  should become high level, is steadily preserved. 
     With reference to FIG. 6, a detailed example of the timing signal generator of the invention will be explained with a detailed example of the buffer 23 of FIG. 1. 
     The buffer 23 is composed of three stages of inverters 23-1 to 23-3 connected in cascade. The inverter stage 23-1 generates RAS and RASO having the opposite phase to RAS. The stage 23-2 generates PXO of the opposite phase to RASO in response to RASO and the stage 23-3 generates RASA. 
     The delay circuit 31 is made of transistors Q 53  through Q 60 , Q 8  and Q 9  in which Q 8  and Q 9  form a push-pull type output section. The delay circuit 32 has a structure similar to that of the circuit 31. 
     Next, operations of the circuit of FIG. 6 will be explained with reference to FIGS. 7A and 7B for the cases where RAS is at a high level and a low level when the power voltage is switched on, respectively. 
     With reference to FIG. 7A, the operation when RAS is at a high level will be explained. 
     The power voltage is switched on at Ton. RAS and RASO generated from the stage 23-1 are at a low level because the transistors Q 34  and Q 36  are conducting, and hence PXO and RASA rise in potential in proportion of the rise of the power voltage V cc  through the pull-up transistors Q 44  and Q 52 . Since RASA is high and RAS is low, the gas of the transistor Q&#39; 8  of the delay circuit 31 becomes low. Although RASA is at a high level, a boot capacitor C 1  does not store any charge in this instance, and therefore, the gate potential of Q&#39; 8  is at a low level. Accordingly, the transistors Q&#39; 8  and Q&#39; 9  are non-conducting so that the output of the delay circuit 31 is in a floating state unless the transistor Q&#39; 11  is present. But, in this instance, the transistor Q&#39; 11  is conducting in response to a high level of RASA applied to its gate so that the signal φ 1  gradually rises along with RASA. 
     In the delay circuit 32, the gate potentials of the transistors Q 8  and Q 9  are at a low level. After φ 1  becomes high, φ 2  rises gradually through the transistor Q 13&#39; . Thus, the order in which φ 1  and φ 2  rise is established. 
     With reference to FIG. 7B, the operation when RAS is at a low level when the power is switched on will be explained. After the power voltage is switched on at a time point Ton, RAS and RASO rise in proportion to the rise of V cc  while PXO and RASA becomes low because the transistors Q 40 , Q 43 , Q 44 , Q 49  and Q 51  become conducting. Here, the drivability of the pull-up transistors Q 44  and Q 52  are very small as compared to those of Q 40 , Q 50  and Q 51 . In this instance, the transistor Q 55  is conducting in response to a high level of RAS and the transistors Q 60  and Q 9  become conducting so that the φ 1  is kept at a low level irrespective of Q&#39; 11 . Also, in the delay circuit 32, the transistors Q 63 , Q 68  and Q 9  become conducting so that φ 2  is set low. 
     As long as RAS is at a high level, the memory cannot introduce the access operation. Therefore, RAS is changed from a low level to a high level at a time point T 2 . Then, RAS and RASO become low while PXO and RASA become high. With RASA high, the transistors Q 53  and Q 50  conduct so that the gate protentials of Q 60  and Q&#39; 9  are kept at a low level and also the gate potential of Q 58  is kept at a low level. Consequently, the transistors Q&#39; 8  and Q&#39; 9  become non-conducting so that φ 1  starts to rise through the transistor Q&#39; 11 . 
     While the delay circuit 32, after φ 1  becomes high, the gate potentials of Q 66 , Q 68  and Q 9  are at a low level because the transistor Q 64  is conducting. Consequently, the transistors Q 67 , Q 68 , Q 8  and Q 9  become non-conducting and the delay circuit 32 itself does not drive the signal φ 2 . In response to the rise of φ 1 , the transistor Q&#39; 13  becomes conducting so that φ 2  starts to rise in potential. 
     As described above, the timing signal generator of the invention can generate a plurality of timing signal in a desired order when the power voltage is switched on. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.