Content Addressable Memory Match Line Precharge Circuit
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Solution Overview
Problem
Content addressable memories (CAMs) face challenges in reducing current consumption and peak current during search operations, especially when dealing with large bit widths and numerous entries, leading to high power consumption and switching noise issues.
Innovation Solution
The implementation of an isolation gate to isolate the match line from the amplifier circuit, precharging the match line to an intermediate voltage level, and using a restricted current value for pull-up operations, along with a capacitance element for charge accumulation, reduces charge/discharge currents and signal amplitudes, enabling faster and more efficient search operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If match lines are precharged to power supply voltage level and discharged to ground level during search operations, then search operation speed is maintained, but current dissipation and power consumption become large
Solution Approach 1:
The patent changes the voltage parameter by precharging match lines to an intermediate voltage level (e.g., VDD/2) instead of full power supply voltage VDD. This parameter change reduces the voltage swing amplitude during search operations, thereby reducing the charge/discharge currents and power consumption while maintaining adequate signal detection capability through the amplifier circuit.
2Productivity
If search lines and match lines are charged and discharged between power supply voltage level and ground level in each search cycle, then search operation is completed, but charge and discharge currents become large resulting in high current dissipation
Solution Approach 1:
The patent applies parameter change by setting the precharge voltage of match lines to an intermediate level rather than full swing between VDD and GND. This reduces the delta V in each charge/discharge cycle, directly reducing the current I = C*dV/dt and thereby reducing energy loss in the form of heat dissipation during frequent search operations.
3Measurement precision
If full swing voltage levels are used for match lines during search operations, then signal detection capability is maintained, but switching noise increases
Solution Approach 1:
The patent changes the voltage parameter to an intermediate level, which inherently reduces the amplitude of voltage transitions and associated switching noise. The amplifier circuit compensates for the reduced signal amplitude, maintaining detection precision while the reduced swing minimizes noise generation from parasitic capacitances and inductances in the interconnects.
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 significantly reduces current consumption, minimizes signal amplitudes, and achieves fast search operations with low power usage, while also reducing peak current and mitigating switching noise.
Implementation Method 1
a precharge circuit for precharging a corresponding match line to a precharge voltage level equal to or lower than an intermediate value between a power supply voltage and a ground voltage
Implementation Method 2
a capacitance element for supplying accumulated charges to the corresponding match line in the operation of comparing the search data on the search data bus with the stored data of the entry
Implementation Method 3
an amplifier circuit for comparing the voltage on the corresponding match line with a reference voltage to produce a signal indicating a result of the comparison
Implementation Method 4
an isolation gate for isolating the amplifier circuit from the corresponding match line before activation of the amplifier circuit
Data Source
AI summary
An entry including multiple bits of unit cells each storing data bit is coupled to a match line. The match line is supplied with a charging current having a restricted current value smaller than a match line current flowing in a one-bit miss state in one entry, but larger than a match line current flowing in an all-bit match state in one entry. A precharge voltage level of a match line is restricted to a voltage level of half a power supply voltage or smaller. Power consumption in a search cycle of a content addressable memory can be reduced, and a search operation speed can be increased.