Circuit and Method to Externally Adjust Internal Circuit Timing

The present invention generally relates to testing integrated circuits (ICs) such as dynamic random access memories (DRAMs) and, more particularly, to circuits which allow external adjustment of internal circuit timing by a tester in order to arbitrarily adjust such timings to facilitate circuit design and characterization.

Timing in a circuit, especially delay circuits, often requires adjustment based on studies on hardware. For example, in a dynamic random access memory (DRAM), the word line (WL) turn-on to the sense amplifier (SA) set timing signal controls the signal level at which the SA starts sensing. The design of timing circuits is based on simulation of detailed circuit models. However, the actual delay time required is often unclear. In some cases, test modes are implemented to add a pre-set delay or remove a pre-set delay for adjustment. Once the circuit is designed, the range of adjustment is limited and inflexible. It is therefore advantageous to provide for arbitrarily adjusting such internal timings externally with a tester.

The present invention is related to the use of a test mode for controlling the timing of an internal signal of an integrated circuit through an external control to facilitate efficient and effective circuit design.

According to the invention, the integrated circuit is provided with different modes of operation. In one embodiment, the integrated circuit has two modes of operation, normal and test mode. A control circuit is used to determine the mode of operation of the integrated circuit. A test mode signal is provided to determine in which mode the integrated circuit operates. For example, the integrated circuit operates in the normal mode when the test mode signal is at a logic low and in the test mode when the test mode signal is at a logic high. When the integrated circuit device is in the normal mode of operation, the timing of the internal control signal is derived from an internal signal. When the integrated circuit device is in the test mode, the internal control signal is derived from an external signal provided at an external pin of the integrated circuit. In this way, the timing of the internal control signal can be externally adjusted via the test mode.

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of the invention with reference to the drawings, in which:

FIG. 1 is block diagram of a control circuit providing for an external control of an internal signal;

FIG. 2 is block diagram of a control circuit which provides for external control of the sense amplifier and column open timings; and

FIGS. 3A and 3B are timing diagrams illustrating the operation of the circuit shown in FIG. 2.

As previously stated, the invention is related to defining test modes for controlling the timing of internal signals using external controls. The ability to control the timing of internal signals facilitates the design of circuits. For purposes of illustration, one embodiment of the invention is described in the context of providing test modes for externally controlling the signals that operate the sense amplifier and column in a DRAM integrated circuit.

In FIG. 1, an illustrative a block diagram of a control circuit 1 in accordance with the invention. Control circuit 1 has two modes of operation, normal and test. In the normal mode, a normal signal path is used to control the timing of an internal signal 40. The normal signal path comprises a subcircuit 5 that is activated by an internal signal 21 and generates an output signal 31. Subcircuit 5 is, for example, a delay circuit such as a timer for producing a delayed output with respect to the input. In test mode, a test mode signal path is used for controlling the timing of internal signal 40. The test signal path comprises a subcircuit 10.

To switch the control circuit to test mode, a test mode signal is provided to subcircuit 10 for activating the test mode signal path. The test mode signal, which is also coupled to subcircuit 5, deactivates the normal mode signal path by disabling subcircuit 5. By activating the test mode signal path, output signal 31 of subcircuit 10 is derived from an external signal 26. The external signal is provided at an external pin of the integrated circuit. The external pin used to provide the external signal may serve a different function in normal mode. Alternatively, the external pin may be defined for test mode functions only. Of course any external pin of the integrated circuit may be used for inputting the external signal provided that its use does not conflict with the operation of the integrated circuit. For example, if an external pin is used to provide the power source to operate the integrated circuit, it obviously cannot be used to input the external test signal.

A subcircuit 15 receives signals 31 and 36 as input signals and outputs internal signal 40. Subcircuit 15 functions to OR signals 31 and 36 together, resulting in signal 40 to be derived from signal 31 in normal mode and signal 36 in test mode. Alternatively, subcircuit 15 may be a decoder for selecting between signal 31 and 36. Using the test mode signal as the select signal, the decoder outputs signal 31 as output signal 40 in normal mode and signal 36 as the output signal in test mode. Since the test mode signal is used as the select signal of the decoder, it is not necessary to use the test mode signal to control the operation of subcircuits 5 and 10. Thus, the control circuit enables the timing of signal 40 to be derived by an external control in test mode.

FIG. 2 shows a control circuit 2 for externally controlling the timing of internal signals used for operating the SA and column in a DRAM. In general, the internal signals for operating the SA and column are SA.sub.-- Enable and Col.sub.-- Enable. Furthermore, the timing of the Col.sub.-- Enable signal is typically dependent upon the SA.sub.-- Enable signal. The timing of the SA.sub.-- Enable signal is typically dependent on the WL.sub.-- Enable signal, which is the internal signal used to control the WL. As shown, the control circuit comprises subcontrol (SC) circuits 3 and 4, which control the timing of SA.sub.-- Enable and Col.sub.-- Enable signals, respectively.

Referring to SC circuit 3, normal and test mode signal paths are provided. In the normal mode, the timing of SA.sub.-- Enable is generated via the normal mode signal path. The normal test signal path includes a WL timer 11. WL timer 11 receives WL.sub.-- Enable for activating and causing the WL timer to generate a delayed output signal 30. As shown, a logic high (1) WL.sub.-- Enable signal activates the WL timer, causing it to generate a logic low (0) delayed output signal. The test mode signal path comprises a test mode circuit 19. A test mode signal path is coupled to the test mode circuit 19. The test mode signal, when generated, switches the DRAM to test mode by enabling the test mode circuit. In addition, the test mode signal is coupled to WL timer 11 for disabling it during test mode. As such, the test mode signal activates the test mode signal path and deactivates the normal mode signal path during test mode. Deactivation of the normal signal path results in output signal 30 of WL timer to go high. Typically, a mode register (not shown) in the DRAM generates the test mode signal. In one embodiment, the DRAM is switched to test mode by generating the test mode signal during, for example, the WCBR (WE and CAS before RAS) with address for mode selection. Generation of the test mode signal during WCBR is described in Kalter et al, A 50 ns 16 Mb DRAM with a 10 ns Data Rate, Digest of Technical Papers, ISSCC90 (1990), which is herein incorporated by reference for all purposes. Test mode circuit 19 also receives an external test signal 27 and produces an output 22. When enabled, the test circuit effectively conducts the test signal through the test signal path.

Illustratively, test mode circuit 19 comprises an NAND gate 13 and inverter 14. The external test signal is provided at the G pin of the DRAM. The external test signal (G) in the example is active low. The inverter switches the external signal to the opposite signal level. NAND gate 13, when enabled by the test mode signal, derives its output signal 31 from the input coupled to the inverter. It is clear that the inverter is used to convert the input signal to the desired signal level before the NAND gate. Of course, the external signal may be conducted through additional circuits (not shown) prior to the NAND gate. However, the example shows that the signal generated by the test mode signal path circuit 19 is, in effect, derived from the external test signal when in test mode. Since the external pin used is the G pin, the test mode signal is labeled TM.sub.-- GSAE (test mode G pin SA enable). When both TM.sub.-- GSAE and G signals are active, output signal 30 of NAND gate 13 becomes low.

Output signals 30 and 22 from the signal paths are coupled to select circuit 15. The select circuit outputs SA.sub.-- Enable signal 31, which is derived either from signal 30 in normal mode or signal 22 in test mode. As shown, select circuit 15 comprises, for example, a NAND gate. The NAND gate generates a high output when either one of its inputs is high, but not both. Since the activated signal path generates a low signal, the output of NAND gate 15 is derived from the activated signal path. As previously mentioned, the SA.sub.-- Enable is an internal signal that controls the operation of the SA. Thus, SC circuit 3 provides a test signal path from which the timing of the SA.sub.-- Enable signal is controlled via an external signal during test mode.

The SA.sub.-- Enable signal, which also controls the Col.sub.-- Enable signal in normal mode, is coupled to SC circuit 4. Similar to SC circuit 3, SC circuit 4 comprises a normal and test signal path. The normal signal path, which is used in mode, comprises a SA timer 12. The SA timer is started by SA.sub.-- Enable signal and produces an output signal 40 in response. The test mode signal path comprises a test mode signal circuit 20 similar to circuit 19 of SC circuit 3. A test mode signal 25 is generated by, for example, the on-chip mode register (not shown). Test mode signal 25 deactivates the SA timer and activates a test mode circuit 20, allowing an output 23 of the test mode circuit 20 to be effectively derived from an external test signal. The external test signal is provided at an external pin such as the CAS. As the CAS pin is used, the test signal is labeled TM.sub.-- CCSLE (test mode column CAS-pin select line enable). Although, illustratively, the external pin used in SC circuit 4 is different from SC circuit 3, it is not necessary that they are different. However, the use of different external control pins advantageously provides for independent control of the test modes. Test mode signal circuit 20, as shown, comprises inverter 17 and NAND gate 16. Since the test mode signal is an active low signal, the inverter is employed to switch the signal level to high. Thus, when both TM.sub.-- CCSLE and test mode signal is active, output of NAND gate 16 is low.

The output signals from the test mode and normal mode signal paths 30 and 22 are fed into select circuit 18 which, for example, is a NAND gate. Select circuit 18 produces a Col.sub.-- Enable signal 41 that controls the operation of the SA. The Col.sub.-- Enable signal is derived from the signal path that is activated at the time. As such, SC circuit 4 enables the timing of the Col.sub.-- Enable signal to be controlled by external signal.

In an alternative embodiment, select circuit 15 and/or 18 comprise a decoding circuit for selecting the output signal to be derived from the test or normal mode signal path using the test mode signal as the select signal. Implementing the select circuit with a decoding circuit eliminates the need control the timer circuits and test mode circuits with the test mode signal.

The operation of the circuit of FIG. 2 will be described with reference to FIGS. 3A and 3B which are respectively timing diagrams for normal operation and test mode operation. First, with reference to FIG. 3A, the normal signal path generating the column enable (Col.sub.-- Enable) signal will be described. Typically, there are control signals, both external and internal, that generate the Col.sub.-- Enable signal. The external control signals are the RAS (the row address strobe, active low), CAS, G, and data signals. The internal signals are addresses, WL.sub.-- Enable (wordline enable), SA.sub.-- Enable (sense amplifier enable), and Col.sub.-- Enable (Column enable). The external strobe RAS initiates the generation of addresses which, in turn, generate the WL.sub.-- Enable signal. After the timer 11 times out, the SA.sub.-- Enable signal is generated, and after the timer 12 times out, the Col.sub.-- Enable signal is generated.

Activating the test mode signals (TM.sub.-- CSAE and TM.sub.-- CCSLE) disable timers 11 and 12, causing the DRAM integrated circuit to be in test mode. Referring to FIG. 3B, timing diagram of the internal and external signals are shown. The external strobe RAS initiates the generation of addresses which, in turn, generate the WL.sub.-- Enable signal, as before, but this time the SA.sub.-- Enable signal is not generated by the timer 11. Instead the SA.sub.-- Enable signal is generated externally by the G signal passed by NAND gate 13. This SA.sub.-- Enable signal, however, does not result in the generation of the Col.sub.-- Enable signal; rather, the Col.sub.-- Enable signal is generated by the CAS passed by NAND gate 16.

As evident by the timing diagram, the timing of internal signals Col.sub.-- Enable and SA.sub.-- Enable is controlled by external signal G and CAS, respectively, in test mode. The ability to control the timing of internal signals gives circuit designers essentially unlimited range of adjustments of timing delays to determine the actual delay time required in the design implementation.

The circuit shown in FIG. 1 or 2 has numerous applications. For example, the circuit can be implemented in a DRAM to adjust the timing of the WL-SA Timing by providing for a different signal path upon activation of a test mode. The different signal path, for example, causes the SA to be set with the falling edge of G and reset with the rising edge of G.

The test mode can be used to study, for example,:

a) Minimum WL-SA delay due to WL rise time and transfer gate delay.

b) Screening of bit lines (BLs) with weak leakage by prolonging the WL-SA timing. Leaky BLs will fail if the pause time is too long.

c) Storage mode series resistance. If the storage mode series resistance is too high, a long WL-SA delay is required. If delay is independent of the distance between the cells and WL drivers, high trench resistance is suspect.

The invention can also be used to define a test mode which adjusts the SA-CSL timing, as depicted by the circuit of FIG. 2. Invocation of this test mode is achieved by using the mode register to generate the TM.sub.-- CCSLE signal. The SA-CSL timing can be adjusted using the CAS (Column Address Strobe) pin. The use of separate external control signals (G and CAS) allows both WL-SA timing and SA-CSL timing to be independently adjusted in one row address strobe (RAS) cycle. Choosing the CAS pin instead of the G pin allows both WL-SA timing and SA-CSL timing to be independently adjusted in one row address strobe (RAS) cycle.

A test mode for adjusting Column Address--DQ sensing timing (Query data from SA) can also be defined. Similarly, this timing can be controlled through the CAS pin. By adjusting the timing difference between addresses and the CAS edge, the column address--memory data query (DQ) sensing delay can be adjusted.

A test mode for controlling the internal clock is also useful. For example, an extra pad is added to be used to provide internal timing. The purpose of the clock is controlled by the test modes. Since this clock pad is a non-functional pad, no conflict of purpose can occur, as with the CAS or G pins. If this extra pad is connected to a non-connect pin of a package, a test mode can be used to convert the non-connect pin to a general purpose clock pin for internal timing control.

The invention can be used in either integrated circuit devices to control internal signals, such as to shift a local clock in a central processing unit (CPU) or adjust WL reset timer in a static RAM (SRAM) with self-reset. Those skilled in the art will understand that the various test modes can be used alone or in combination to achieve effective design of integrated circuit.

While the invention has been particularly shown and described with reference to various embodiments, it will be recognized by those skilled in the art that modifications and changes may be made to the present invention without departing from the scope of the invention. Therefore, the scope of the invention should be determined not with reference to the above description but, instead, with reference to the appended claims along with their full scope of equivalents.