Nonvolatile Memory Precharge Circuit for Write Time Reduction
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Solution Overview
Problem
In nonvolatile semiconductor storage devices with variable resistors, the write time is increased due to the need to prevent non-selected word lines and bit lines from turning ON/OFF at each data set timing, as they require a significant rise-up time due to their larger capacity compared to selected lines.
Innovation Solution
A control circuit precharges non-selected second wires to a standby voltage greater than the reference voltage before the set operation, allowing the program voltage to be applied only to the selected first and second wires, while preventing the rectifying device from turning ON with a control voltage based on the program voltage applied to non-selected second wires.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If non-selected word lines and bit lines are turned ON/OFF at each data set timing to prevent them from turning ON, then data write operation is completed, but data write time increases due to rise-up time of non-selected lines
Solution Approach 1:
The control circuit precharges non-selected bit lines to a standby voltage (e.g., 2.5V) before the set operation begins. This preliminary action ensures that when the set operation starts, the non-selected bit lines are already at the appropriate voltage level, eliminating the need to wait for them to charge up during the set operation. The precharging is performed during the reset operation or beforehand, so the additional time required is concealed within existing operation cycles.
2Ease of operation
If non-selected word lines and bit lines are charged to program voltage during set operation, then they are prepared for next operation, but charge time increases write operation duration
Solution Approach 1:
The control circuit performs precharging of non-selected bit lines during the reset operation or before the set operation. By separating the precharging action from the set operation timing, the set operation duration is not extended. The standby voltage is maintained on non-selected bit lines throughout, so they are ready for the next operation without adding delay.
Solution Approach 2:
The control circuit maintains the standby voltage on non-selected bit lines continuously between operations. Instead of charging and discharging these lines at every set operation, the voltage is held steady, eliminating repeated charging cycles. This continuous maintenance ensures readiness for next operation while avoiding the time penalty of repeated charge/discharge cycles.
3Reliability
If control voltage is applied to non-selected second wires to prevent rectifier from turning ON, then data write accuracy is maintained, but power consumption increases
Solution Approach 1:
The control circuit changes the voltage parameter on non-selected bit lines from 0V to a standby voltage (e.g., 2.5V) during precharging. This parameter change prevents the rectifiers from turning on during set operation without requiring full program voltage (4.5V) to be applied continuously. The standby voltage is sufficient to keep rectifiers in blocking state while consuming less power than applying full program voltage.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach accelerates the rise of the set operation by concealing the charge time for non-selected word lines within the reset operation, reducing power consumption and write time by avoiding unnecessary charging and discharging of non-selected word lines.
Implementation Method 1
the control circuit precharges a non-selected second wire up to a standby voltage larger than a reference voltage prior to a set operation
Implementation Method 2
the variable resistor is changed from a high resistance state to a low resistance state
Implementation Method 3
applying a control voltage which prevents the rectifying device from turning ON based on the program voltage to the non-selected second wire
Data Source
AI summary
A nonvolatile semiconductor storage device includes first and second intersecting wires; a electrically rewritable memory cell disposed at each intersection of the first second wires, including a variable resistor for memorizing a resistance value as data in a nonvolatile manner and a rectifying device are connected in series; and a control circuit which applies a voltage necessary for writing of data to the first and second wires. The control circuit precharges a non-selected second wire to a standby voltage larger than a reference voltage prior to programming a variable resistor connected to selected first and second wires by supplying the reference voltage to a non-selected first wire and the selected second wire, applying to the selected first wire a program voltage for programming of the selected variable resistor and applying to the non-selected second wire a control voltage which prevents the rectifying device from turning ON.