Three-state content addressable memory

Abstract

The present invention discloses a ternary content addressable memory, which includes a search line and a match line connected to a TCAM cell; the match line is equipped with a charge sharing circuit; the charge sharing circuit includes: a metal line disposed on one side of the match line, and a charge sharing control unit for controlling the connection and disconnection between the metal line and the match line; the charge sharing control unit connects the metal line and the match line after the match line is precharged to a high level and before the search line is activated, so that charge sharing occurs between the match line and the metal line, and the voltage of the match line is reduced through charge sharing. The present invention can reduce the voltage of the match line through charge sharing before the search line is activated. After the voltage of the match line is reduced, the evaluation speed of the match line can be increased, the efficiency of the search operation can be improved, and thus the working frequency and performance of the ternary content addressable memory can be improved.

Description

Technical Field

[0001] The present invention relates to a ternary content addressable memory. Background Art

[0002] The ternary content addressable memory (TCAM) is developed from the content addressable memory (CAM). A storage cell (CELL) of the TCAM can store one of the states 0, 1, or X.

[0003] A keyword is connected to an entry stored in the TCAM through a search line (SL, search line).

[0004] After the input keyword, each entry in the entire column stored in the TCAM is simultaneously matched with the keyword. This operation is called a search and is also referred to as the evaluation of the ML.

[0005] If the keyword matches an entry, the corresponding match line (ML, match line) remains at a high level, and the corresponding sense amplifier (SA, Sense Amplifier) outputs a high level "1", indicating that the entry matches the keyword.

[0006] If the keyword does not match the entry, the corresponding match line (ML, match line) is discharged by the non-matching TCAM bit, and the ML voltage is discharged to be lower than the threshold voltage of the sense amplifier. The SA outputs a low level "0", indicating that the entry does not match the keyword.

[0007] The operating frequency of the TCAM depends on the speed of the search operation, and core depends on the speed of the discharge of the match line ML (i.e., evaluation). Summary of the Invention

[0008] An object of the present invention is to provide a ternary content addressable memory, including a search line and a match line connected to a TCAM unit; the match line is provided with a charge sharing circuit;

[0009] The charge sharing circuit includes: a metal line disposed on one side of the match line, and a charge sharing control unit for controlling the connection and disconnection of the metal line and the match line;

[0010] The charge sharing control unit, after the match line is pre-charged to a high level and before the search line is activated, connects the metal line and the match line, so that charge sharing occurs between the match line and the metal line, and the voltage of the match line at the high level is reduced through charge sharing.

[0011] Preferably, the charge sharing control unit presets the metal line to a low level before connecting the metal line and the match line.

[0012] Preferably, the matching line is also connected to a sense amplifier; after charge sharing is completed, there is a certain margin between the voltage of the matching line and the threshold voltage of the sense amplifier.

[0013] Preferably, the metal line is parallel to the matching line and has the same length as the matching line.

[0014] For the specific structure of the charge sharing control unit and the specific control method of the charge sharing circuit, refer to the embodiments.

[0015] The advantages and beneficial effects of the present invention are as follows: A ternary content addressable memory is provided, which can reduce the voltage of the matching line through charge sharing before the search line is activated. After the voltage of the matching line is reduced, the evaluation speed of the matching line can be accelerated, the efficiency of the search operation can be improved, and thus the working frequency and performance of the ternary content addressable memory can be improved.

[0016] The metal line is parallel to the matching line and has the same length as the matching line, that is, the length of the metal line changes with the matching line. Therefore, the voltage of the matching line can be maintained at a fixed value after charge sharing.

[0017] The metal line does not increase the area additionally.

[0018] The voltage division value of the matching line can be accurately adjusted by changing the width of the metal line.

[0019] The solution of the present invention is applicable to all NOR-type CAMs. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is the circuit diagram of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] The following combines the drawings and embodiments to further describe the specific embodiments of the present invention. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and cannot be used to limit the protection scope of the present invention.

[0022] The present invention provides a ternary content addressable memory, including: a matching line and a search line connected to the TCAM unit, and a sense amplifier connected to the matching line;

[0023] The matching line is also equipped with a charge sharing circuit; the charge sharing circuit includes: a metal line arranged on one side of the matching line, and a charge sharing control unit for controlling the connection and disconnection between the metal line and the matching line;

[0024] Before connecting the metal line and the matching line, the charge sharing control unit presets the metal line to a low level;

[0025] The charge sharing control unit connects the metal line to the match line after the match line is pre-charged to a high level and before the search line is activated, causing charge sharing between the match line at a high level and the metal line at a low level, and reducing the voltage of the match line through charge sharing. After charge sharing is completed, the voltage of the match line maintains a certain margin from the threshold voltage of the sense amplifier; and after charge sharing is completed, the search line is activated.

[0026] The specific embodiments of the present invention are as follows:

[0027] As Figure 1 shown, a ternary content addressable memory includes: match lines ML connected to TCAM units, search lines SL0 to SLn and SLB0 to SLBn, a sense amplifier SA connected to the match line ML, and a first PMOS transistor P1 controlled by an evaluation enable signal CMPEN; the source of the first PMOS transistor P1 is connected to a power supply, the drain of the first PMOS transistor P1 is connected to the match line ML, and the gate of the first PMOS transistor P1 is controlled by the evaluation enable signal CMPEN;

[0028] The match line ML is also equipped with a charge sharing circuit; the charge sharing circuit includes: a metal line MT provided on one side of the match line ML, and a charge sharing control unit for controlling the connection and disconnection between the metal line MT and the match line ML; the charge sharing control unit includes: a charge sharing control signal line CMPENB, a first NMOS transistor N1, and a second PMOS transistor P2; the source of the second PMOS transistor P2 is connected to the match line ML, the drain of the second PMOS transistor P2 is connected to the drain of the first NMOS transistor N1, and the source of the first NMOS transistor N1 is grounded; the charge sharing control signal line CMPENB is connected to the gate of the first NMOS transistor N1, and the charge sharing control signal line CMPENB is also connected to the gate of the second PMOS transistor P2.

[0029] Preferably, the metal line MT is parallel to the match line ML and the metal line MT is of the same length as the match line ML.

[0030] The control method of the charge sharing circuit is as follows:

[0031] In the initial state, the evaluation enable signal CMPEN is at a low level and the charge sharing control signal line CMPENB is at a high level; at this time, the first PMOS transistor P1 and the first NMOS transistor N1 are both turned on, and the second PMOS transistor P2 is turned off; the match line ML is connected to the power supply through the first PMOS transistor P1, and the voltage of the match line ML is pre-charged to a high level; the metal line MT is grounded through the first NMOS transistor N1, and the metal line MT is preset to a low level;

[0032] When a search is required, before the search lines SL0 to SLn and SLB0 to SLBn are started, first flip the evaluation enable signal CMPEN to a high level and flip the charge sharing control signal line CMPENB to a low level; at this time, the first PMOS transistor P1 and the first NMOS transistor N1 are both cut off, and the second PMOS transistor P2 is turned on; the metal line MT is connected to the matching line ML through the second PMOS transistor P2, and the matching line ML at a high level and the metal line MT at a low level undergo charge sharing, and the charge sharing reduces the voltage of the matching line ML; the charge sharing reduces the voltage of the matching line ML to approximately 75% of the power supply voltage, and there is a certain margin from the threshold voltage of the sense amplifier SA;

[0033] After the charge sharing is completed, the search lines SL0 to SLn and SLB0 to SLBn are started, and the matching line ML enters the evaluation process. Since the voltage of the matching line ML has been reduced, the unmatched matching lines ML can complete the evaluation process faster;

[0034] After the search is completed, the initial state is restored.

[0035] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art of this technology, without departing from the technical principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A ternary content addressable memory, comprising search lines and match lines connected to TCAM cells; characterized in that, The matching line is equipped with a charge sharing circuit; The charge sharing circuit includes: a metal line, and a charge sharing control unit for controlling the connection and disconnection between the metal line and the matching line; The charge sharing control unit, after the matching line is pre-charged to a high level and before the search line is started, connects the metal line to the matching line, enables charge sharing between the matching line and the metal line, and reduces the voltage of the matching line through charge sharing; The matching line is also connected to a sense amplifier; after charge sharing is completed, the voltage of the matching line maintains a certain margin from the threshold voltage of the sense amplifier; The metal line is parallel to the matching line and has the same length as the matching line. The metal line changes its length along with the matching line, and the voltage of the matching line remains fixed after charge sharing; The voltage division value of the matching line is precisely adjusted by changing the width of the metal line.

2. The ternary content addressable memory according to claim 1, characterized in that, The charge sharing control unit presets the metal line to a low level before connecting the metal line to the matching line.

3. The ternary content addressable memory according to claim 2, characterized in that, The charge sharing control unit includes: a charge sharing control signal line, a first NMOS transistor, and a second PMOS transistor; The source of the second PMOS transistor is connected to the matching line, the drain of the second PMOS transistor is connected to the drain of the first NMOS transistor, and the source of the first NMOS transistor is grounded; The charge sharing control signal line is connected to the gate of the first NMOS transistor, and the charge sharing control signal line is also connected to the gate of the second PMOS transistor.

4. The ternary content addressable memory according to claim 3, characterized in that, The matching line is also equipped with a first PMOS transistor controlled by an evaluation enable signal; the source of the first PMOS transistor is connected to the power supply, the drain of the first PMOS transistor is connected to the matching line, and the gate of the first PMOS transistor is controlled by the evaluation enable signal.

5. The ternary content addressable memory according to claim 4, characterized in that, The control method of the charge sharing circuit is as follows: In the initial state, the evaluation enable signal is at a low level and the charge sharing control signal line is at a high level; at this time, both the first PMOS transistor and the first NMOS transistor are conducting, and the second PMOS transistor is cut off; the matching line is connected to the power supply through the first PMOS transistor, and the voltage of the matching line is pre-charged to a high level; the metal line is grounded through the first NMOS transistor, and the metal line is preset to a low level; When a search is required, before the search line is started, first invert the evaluation enable signal to a high level and invert the charge sharing control signal line to a low level; at this time, both the first PMOS transistor and the first NMOS transistor are cut off, and the second PMOS transistor is conducting; the metal line is connected to the matching line through the second PMOS transistor, and charge sharing occurs between the matching line at a high level and the metal line at a low level, and the charge sharing reduces the voltage of the matching line; After charge sharing is completed, start the search line, and restore the initial state after the search is completed.

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