Mode Selector for Use in Semiconductor Memory Device

The present invention relates to a semiconductor memory device and more particularly a mode selecting circuit for selecting mode of operation of the semiconductor memory device.

A semiconductor memory device is often manufactured so as to perform a special mode of operation to increase its performance. The Dynamic Random Access Memory (DRAM) is manufactured so as to function optionally through one of the various special modes to improve its read/write performance such as fast page mode, static column mode and nibble mode. Conventionally, a particular operational mode is determined by connecting a metal line with the logic circuit for enabling a selected mode through adding a metal masking process during the manufacturing process. Consequently, the metal masking process requires a different masking process, for each operation mode each different masking process, which further requires each different masking layer. If the number of the masking processes and masking layers is increased, not only the production cost is increased, but also problems inherent in the process occur.

It is an object of the present invention to provide a mode selecting circuit for selecting a single mode of a plurality of operational modes with the simplified design and manufacturing process in a memory device having a plurality of operational modes.

It is another object of the present invention to provide a mode selecting circuit comprising a control clock generator which employs as small of a number of elements and as small of an area as possible.

In order to accomplish the above objects, the present invention provides a circuit for selecting an operational mode of a semiconductor memory in which data may be stored and from which the same data may be retrieved, including a first voltage supplying terminal for receiving the voltage of a power source, a second voltage supplying terminal for receiving the ground voltage, an input terminal for inputting the same voltage as the voltage of said power source after a certain time has passed since supplying said first voltage supplying terminal with the voltage of said power source, a mode selecting pad for supplying a certain amount of voltage, an output terminal, a first node, a second node, a third node, a first inverter for inverting the input signal, said first inverter being connected between the input terminal and the first node, a first switching stage for responding to the input signal of the first node and establishing a conducting channel between the second voltage supplying terminal and the second node, a second switching stage for responding to the input signal of the mode selecting pad and establishing a conducting channel between the second voltage supplying terminal and the third node, a latch for latching a certain voltage state according to the switching of the first switching stage and the second switching stage, the latch being connected between the second and third nodes, an insulating gate field effect transistor having a channel connected between the mode selecting pad and said second voltage supplying terminal and a gate connected with the third node, and a second inverter for inverting the voltage state of the third node, the second inverter being connected between the output terminal and the third node.

FIG. 1 is a schematic circuit diagram of a preferred embodiment of a control clock generator according to the present invention; and

FIG. 2 is a waveform diagram for showing the timing of source supply voltages applied to the control clock generator.

The present invention will now be described with reference to the drawings attached only by way of example. Referring to FIG. 1, the circuit shows a preferred embodiment of a circuit for generating control clock pulses of the present invention, which may be used to select an operation mode of a memory device having two selecting modes together with a basic mode. Between input terminal 10 and first node 1 is connected CMOS inverter 20 consisting of PMOS field effect transistor 22 (hereinafter referred to as PMOS) and NMOS field effect transistor 22 (hereinafter referred to as NMOS). First switching transistors 30a and 30b consist of NMOSs whose channels are connected between second nodes 2 and 4 and a second supply voltage terminal Vss, and whose gates are connected with the first node 1. If the input signal is LOW, the first node becomes HIGH, which triggers ON first switching transistors 30a and 30b, and thereby makes latching circuits 40a and 40b be latched into HIGH state. Latching circuits 40a and 40b each consist of two CMOS inverters (42 and 44; 46 and 48). Second nodes 2 and 4, and third nodes 3 and 5 come to have a different logic states from each other. The state of third nodes 3 and 5 are to affect the output. Second switching transistors 50a and 50b each consist of a NMOS transistor with channels whose channel is connected between the third node and the second voltage supplying terminal, and with gates connected with mode selecting pad 70a, 70b respectively. When the mode selecting pad receives the HIGH state voltage, latching circuits 40a, 40b come to latch into LOW state. NMOS transistors 60a and 60b with channels is connected between mode selecting pad 70a, 70b and second voltage supplying terminal Vss, and with gates connected with the third node, are to assure that second switching transistors 50a, 50b are kept in an OFF state; second switching transistors 50a, 50b go to a floating state when the HIGH signal is not applied to the mode selecting pad because of the mode selecting pad being not selected. Between third nodes 3, 5 and output terminals 90a, 90b are connected CMOS inverters 80a, 80b consisting of PMOS transistors 81, 83 and NMOS 82, 84. Although a CMOS inverter 20 is commonly connected with two circuits for generating control clock pulses to select a mode as shown in FIG. 1, it will be appreciated by those skilled in the art that each separate circuit for generating control clock pulses may be employed to select a single operational mode.

According to the present invention, the mode selecting pads and the control clock generators may be as many as the number of the selecting modes in the memory device to achieve each selection of mode therein. If one of the three modes of fast page mode, static column mode and nibble mode is selected, there are selectively provided a single basic mode and two selecting modes, which require two mode selecting pads and each circuit for generating control signals for each pad, as shown in FIG. 1. If a voltage for operating the second switching transistor is applied to each of the mode selecting pads, the circuit for generating control signals which is connected with the pad receiving the voltage, generates control signals. Therefore, two mode selecting pads are used to select two modes.

First, the case of selecting the basic mode is described with reference to FIGS. 1 and 2. Referring to FIG. 2, Vcc represents the voltage of a power source applied to the first voltage supplying terminal, and Vcc1 the input voltage applied to the input terminal which is obtained by delaying the voltage of the power source for a certain time. If the basic mode is selected, mode selecting pads 70a and 70b of the first and second circuits for generating control clock pulses are not selected. Therefore, second switching transistors 50a and 50b are in the floating state, so that the outputs from output terminals 90a and 90b are controlled only by the input voltage Vcc1. Vcc1 is in LOW state in a the interval b in FIG. 2 so that first node 1 which is the output node of CMOS inverter 20 is in a HIGH state. The output of the node 1 turns ON first switching transistors 30a and 30b, thereby causing second nodes 2 and 4 to be in the LOW state.

The LOW state signal applied to second nodes 2 and 4 makes the third nodes 3 and 5 which are the output nodes of CMOS inverters 80a and 80b change into a HIGH state. The outputs of third nodes 3 and 5 cause CMOS inverters 80a and 80b to be in the LOW state, so that a control clock of operation mode supplied from output terminals 90a and 90b is in the LOW state, thereby disabling the circuit for generating control clock pulses. If Vcc1 goes to a in the HIGH state after a certain time has lapsed, first node 1 goes to a LOW state, and then first switching transistors 30a and 30b keep the floating state. However, latching circuits 40a and 40b consisting of two CMOS inverters make the output of the first switching transistors keep in the LOW state. Insulating gate field effect transistors 60a and 60b are turned ON by the HIGH state once latched onto the latching circuits, and completely turn OFF second switching transistors 50a and 50b. In this way, the basic mode can be performed.

The operation for selecting another mode (`A` mode) will now be described with reference to FIGS. 1 and 2. In order to select A mode, mode selecting pad 70a is bonded to the Vcc supplying voltage terminal, and mode selecting pad 70b not bonded. In interval b as shown in FIG. 2, the state of third node 3 is determined by the ratio between CMOS inverter latches 44 and 42. However, in interval c that the input voltage Vcc1 goes to in the HIGH state, first node 1 goes to a LOW state, causing first switching transistor 30a to be in the floating state so that the circuit works according to the input voltage of mode selecting pad 70a. Consequently, second switching transistor 50a is turned ON, thereby causing third node 3 to be at ground level so that output terminal 90a outputs the HIGH state. Namely, the control clock for selecting A mode is enabled. However as, mode selecting pad 70b in the second circuit for generating control clock pulses is not bonded to the terminal of the power source, second switching transistor 50b is in the floating state. Further, the input voltage Vcc1 is in the LOW state in interval b of FIG. 2, and therefore, first node 1 is in the HIGH state. Second node 4 is in the LOW state, since first switching transistor 30b is turned ON.

Consequently, the output of latching circuit 40b is in the HIGH state, and the final output from output terminal 90b goes to in the LOW state. The operation of the second circuit for generating control clock pulses will be the same as that of selecting the basic mode. Conclusively, in the case of selecting mode A, the A clock from output terminal 90a is in the HIGH state, while the B clock from output terminal 90b is in the LOW state. Thus, all the modes of the chip are controlled.

Alternatively, in the case of selecting the other mode (`B` mode), mode selecting pad 70b of the second circuit for generating control clock pulses is only bonded to the terminal of the power source conversely to the case of selecting mode A. Therefore, the A clock from output terminal 90a of the first circuit for generating control clock pulses is in the LOW state, while the B clock from output terminal 90b of the second circuit for generating control clock pulses is in the HIGH state. Thus, all the modes of the chip are controlled.

As described above, the present invention selects a mode by selective bonding so that an additional mask accompanying selection of a mode by a metal mask need not be used, thereby resulting in the reduction of production cost. Additionally, according to the present invention, one of the mode selecting pads is bonded to the terminal of the power source so as to select a mode in the final assemblying stage of the process, so that the process is simplified and the yield rate is increased. Furthermore, the present invention carries out selection of a mode by the voltage of the power source and the input voltage obtained by delaying the voltage of the power source for a certain time, and uses mininum transistors and minimum area, thereby increasing its usefulness. Also, since each mode requires only a separate pad and a circuit for generating control clock pulses, the construction of logic is simplified.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that modifications in detail may be made without departing from the spirit and scope of the invention.