Patent Abstract:
A test mode signal system includes: a test mode block for generating a plurality, N, of test mode signals; a test mode send block, for generating and outputting a pulsed signal according to a command signal, and for multiplexing the N test mode signals in sets according to the pulsed signal and outputting the multiplexed pairs of test mode signals over M signal wires wherein M is less than N, such that each signal wire carries a multiplexed set of the N test mode signals; and a test mode receive block, for receiving the multiplexed sets of N test mode signals and the pulsed signal and demultiplexing each multiplexed set of N test mode signals according to the pulsed signal.

Full Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to memory devices and, more particularly, to memory devices having a reduced number of wires for test mode signals. 
         [0003]    2. Description of the Prior Art 
         [0004]    Typically, various test mode signals are generated for testing the integrity of individual circuits of a memory device during initialization or after resetting of the device. The test mode signals are generated by a test mode (TM) block, wherein a memory device may have a single or many TM blocks. Regardless of the number of TM blocks, these blocks are usually situated near the centre of the chip, thereby enabling the test mode signals to be easily routed to all circuits on the device. As the number of circuits within a memory device increases, the routing becomes more complex. Moreover, it further complicates the routing issue with the reduction in size of a semiconductor device. 
       SUMMARY OF THE INVENTION 
       [0005]    A test mode signal system comprises: a test mode block for generating a plurality, N, of test mode signals; a test mode send block, for generating and outputting a pulsed signal according to a command signal, and for multiplexing the N test mode signals in sets according to the pulsed signal and outputting the multiplexed pairs of test mode signals over M signal wires wherein M is less than N, such that each signal wire carries a multiplexed set of the N test mode signals; and a test mode receive block, for receiving the multiplexed sets of N test mode signals and the pulsed signal and demultiplexing each multiplexed set of N test mode signals according to the pulsed signal. 
         [0006]    A method for sending test mode signals comprises: receiving a command signal; generating and outputting a pulsed signal according to the command signal; generating a plurality, N, of test mode signals; multiplexing the N test mode signals in sets according to the pulsed signal; outputting the multiplexed sets of test mode signals over M signal wires wherein M is less than N, such that each signal wire carries a multiplexed set of the N test mode signals; receiving the multiplexed sets of N test mode signals and the pulsed signal; and demultiplexing each multiplexed set of N test mode signals according to the pulsed signal. 
         [0007]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram of a test mode system in a memory device according to an exemplary embodiment of the present invention. 
           [0009]      FIGS. 2-4  are schematics of the internal circuitry of the TM send block shown in  FIG. 1 . 
           [0010]      FIG. 5  is a timing diagram of the signals generated in  FIGS. 2-4 . 
           [0011]      FIG. 6  is a schematic of the internal circuitry of the TM RCV block shown in  FIG. 1 . 
           [0012]      FIG. 7  is a timing diagram of the signals generated by the circuit shown in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    In order to solve the problems associated with the prior art, the present invention provides a method and apparatus that can reduce the number of wires carrying test mode signals, by carrying more than one test mode signal on each individual wire. 
         [0014]    Please refer to  FIG. 1 , which shows a Test Mode system  100  inside a memory device (not illustrated), according to an exemplary embodiment of the present invention. The Test Mode system  100  comprises a Test Mode (TM) block  110  for generating test mode signals and sending the test mode signals to a TM send block  130 . In  FIG. 1  the TM block  110  and TM send block  130  are shown as separate blocks, but in an alternative embodiment the TM send block  130  may be located inside the TM block  110 . The TM send block  130  is further coupled to a TM RCV block  150 , for receiving the test mode signals. Only one TM RCV block is shown here for simplicity, but the TM block  110  and TM send block  130  may send test mode signals to a plurality of TM RCV blocks, located in different regions of the memory device. Furthermore, as described above, the memory device may have many TM blocks, but only a single set of circuits is shown in  FIG. 1  for simplicity. The memory device may be a DRAM, SRAM, MRAM etc. and with suitable modifications the present invention can also be applied to logic devices. 
         [0015]    The TM block  110  receives a number of signals including a test mode clock tmCLK, as well as signals for address lines and load mode register commands. According to these inputs, the TM block  110  will generate a plurality, N, of TM signals, which are then routed via the TM send block  130  to the TM RCV block  150  as shown in  FIG. 1 . In addition, the TM send block  130  also receives the Load Mode Register (LMR) commands—by means of an inverter (not shown) such that the TM send block  130  receives the inverted LMR command LMRF—and tmCLK, as well as a test mode clear all signal (tmCLRALL). This tmCLRALL signal is for resetting the test mode system  100  by sending default test mode values. Conventionally, the TM send block  130  will output the test mode signals on individual wires. In the system  100  shown in  FIG. 1 , the TM send block  130  will generate a pulsed signal and multiplex at least two signals onto a single wire according to the timing of the pulsed signal. The means and circuitry by which the TM send block  130  multiplexes the signals will be described later and is illustrated in  FIGS. 2 ,  3  and  4 . The multiplexed TM signals are routed to the TM RCV block  150  along with the pulsed signal so that the TM RCV block  150  can latch both test mode signals received on the same wire and decode them. The pulsed signal is shown in  FIG. 1  as TMCLKPULSEF. 
         [0016]    Please refer to  FIGS. 2 ,  3  and  4 , which are schematic diagrams of the internal circuitry of the TM send block  130 , and also refer to  FIG. 1 . The TM system  100  has three states of operation: power up mode, which occurs when the TM system  100  is powered on; TM clear mode, which occurs when default TM values are transmitted, i.e. when the tmCLRALL signal goes high; and regular mode, which occurs when the LMR commands are transmitted according to the tmCLK. At power up, a single pulse will be generated by the TM send block  130 , as shown by the signal output TMCLKPULSEF output from TM send block  130  in  FIG. 1 . When the tmCLRALL signal goes low the regular mode can be entered, in which LMR commands are latched. The tmCLRALL signal will also intermittently go high between periods of regular mode operation in order to clear test mode values. When the tmCLRALL signal goes high a single pulse will be generated by the TM send block  130 , as shown by the signal output TMCLKPULSEF output from TM send block  130  in  FIG. 1 . Therefore, power up mode and TM clear mode can both be referred to as Pulse mode. After the system leaves Pulse Mode and enters Regular Mode, clock pulses will be generated according to the LMR commands. 
         [0017]      FIG. 2  shows internal circuitry of the TM send block  130  for generating the clock pulses in Regular Mode. Not shown is an inverter by means of which the LMR commands are generated from signal LMRF. The circuit  200  comprises a latch  210  which receives the signal for LMR commands and is clocked by a differential tmCLK signal. A power up signal Pwrup 2 F is provided to the reset input of the latch  210 . Latched LMR commands, LMR_LATCHED, are output and sent to a delay block  220  and an inverter  230 , and then input to a NAND gate  240 . The output of the NAND gate  240 , CLKF, is then passed through a second inverter  250  to generate a CLK signal. When LMR_LATCHED transitioning from a logic low to a logic high state is output by the latch  210 , the delay block  220  will delay the signal and the first inverter  230  will invert the signal such that both inputs to the NAND gate  240  will be at a logic low ‘0,0’. Therefore, CLKF will be at a logic high state and a logic low is generated at the CLK. Once LMR_LATCHED is output by the delay block  220 , the inputs of the NAND gate  240  will be ‘1,0’, meaning CLKF will remain at a low state as the two inputs to the NAND gate  240  will be at logic high and logic low respectively. Therefore, during regular mode, clock pulses CLK are generated at the falling edge of each LMR_LATCHED signal as the output of the delay block  220  will remain at a high state while the output of the first inverter  230  will also be at a high state causing the output of the NAND gate  240  to be at a low state and the output clock CLK at a high state. 
         [0018]    Please refer to  FIG. 3 .  FIG. 3  is a schematic diagram of the internal circuitry  300  of the TM send block  130  for generating the clock pulses in pulse mode (i.e. power up or TM clear mode). Please note that the circuit  300  is split into two lines for clarity of illustration. Furthermore, the circuit  300  is able to generate pulses in both power up mode and TM clear mode. No signal is provided to tmCLK and no power is provided to the VCC on the input lines to latch  310  before power up in power up mode. Therefore, tmCLK_ARRIVEDF signal is at a logic high state. Pwrup 2  is low as the system has not yet entered the power up state, so the output of the first NAND gate  330  is logic high. As tmCLRALL is also at a logic high state, Clrtmf output from the second NAND gate  340  will be at a logic low state. When Pwrup 2  first goes to a logic high state, the tmCLK has not yet been generated, so the output of the NAND gate  330  is at a logic low state. As tmCLRALL remains high, output from the second NAND gate  340  Clrtmf will turn to a logic high state. The output of the multiplexer  350  will follow the ‘1’ input, meaning PULSEMODEF is entered. As in the circuit  200 , the signal is simultaneously input to both a delay block  360  and an inverter  365 . The outputs are then sequentially passed through a NAND gate  370 , a delay block  380  and an inverter  390  to generate a pulsed signal, PULSEF. 
         [0019]    As is well known, the tmCLK will be generated at a certain amount of time after the memory device is powered up. The system  100  remains in power up mode while tmCLRALL signal stays high; however, Clrtmf will switch to logic low state when tmCLK_ARRIVED signal switches to logic high state. When tmCLRALL goes to logic low state, Clrtmf switches to logic high state again. At this time, the memory device changes to Regular Mode, and LMR commands are latched according to tmCLK, i.e. pulses are generated by the circuit  200  and Clrtmf follows tmCLRALL. As described above, the system  100  will occasionally reset all test mode values by toggling the tmCLRALL signal, and this is similar to the system  100  entering Pulse mode. Each time the tmCLRALL signal switches to a logic high state, a pulse will be generated at the falling edge of the Clrtmf signal. 
         [0020]    Therefore, as demonstrated by the circuit diagrams in  FIGS. 2 and 3 , clock pulses are generated in each mode. In addition, each circuit generates the inverse of the pulsed signal as well. Through the generation of these pulsed signals, at least two test mode signals can be multiplexed together on a single wire regardless of which mode the system is in. Please refer to  FIGS. 4A ,  4 B and  4 C. Each diagram is a schematic demonstrating how an output signal may be generated through a multiplexer.  FIG. 4A  is a schematic of a multiplexed circuit  415  having a multiplexer that controls selection of a first test mode signal; and  FIG. 4B  is a schematic of a multiplexed circuit  425  having a multiplexer  425  which controls selection of a second test mode signal. For demonstration purposes, the following description refers to test mode signals TM 0  and TM 1 . Multiplexing of all other pairs of signals uses the same methodology as described for TM 0  and TM 1 . 
         [0021]    Multiplexer  415  receives an inverse clock signal CLKF output from circuit  200  and the inverse pulse signal PULSEF output from circuit  300 . Signals PULSEMODE, PULSEMODEF are received as selection inputs. According to these selection signals, a pulsed signal SELECT_TM 0  will be generated by multiplexer  417  wherein that pulsed signal follows CLKF in non-pulse mode, i.e., regular mode, or PULSEF in pulse mode respectively. The pulsed signal is also passed through an inverter  419  and output as TMCLKPULSEF, which is shown in  FIG. 1 . The multiplexed circuit  425  receives the clock signal CLK output from circuit  200  and pulse signal PULSE output from circuit  300 . As in multiplexer  415 , signals PULSEMODE, PULSEMODEF are received as selection inputs. According to these selection signals, a pulsed signal SELECT_TM 1  will be generated by multiplexer  427  wherein the pulsed signal follows CLK in non-pulse mode, i.e., regular mode or PULSE in pulse mode respectively. Therefore, when SELECT_TM 0  signal is at a high state, SELECT_TM 1  will be at a low state.  FIG. 4C  is a schematic illustrating the final circuit of the TM send block  130 , which is a multiplexer  437 . Both original test mode signals, TM 0  and TM 1  are received from the test mode block  110 , and the SELECT_TM 0  and SELECT_TM 1  signals are input as selection signals. Therefore, the multiplexer  437  will multiplex both test mode signals onto a single output, TM 01 . 
         [0022]    For a full illustration of the various signals generated by the internal circuitry of the TM send block, please refer to the timing diagram illustrated on  FIG. 5 , which is a timing diagram of the circuit that controls the generation of clock pulses for the select signals going into the multiplexer of the circuits described above. In particular, please note that PwrupmodeF switches to a logic low state when Pwrup 2  is at a logic high state until tmCLK_ARRIVED is generated. Furthermore, TMCLKPULSEF is the inverse of SELECT_TM 1 ; CLKF is generated at the falling edge of LMR_LATCHED; and PULSEF is generated at the falling edge of Clrtmf, except when PwrupmodeF goes high, when it is generated at the rising edge of Clrtmf. The remaining control signals and their respective timing should be clear to one skilled in the art after referring to  FIGS. 2˜4  and reading the accompanying description. 
         [0023]    As detailed in the above paragraphs, the TM send block  130  of the present invention uses internal circuitry to generate clock pulses during all three operation modes, and uses the timing of these clock pulses to multiplex two test mode signals onto a single wire. A timing/pulsed signal TMCLKPULSEF is output along with the multiplexed signals to the TM RCV block  150 . The decoding and demultiplexing of these signals will be described below. 
         [0024]    Please refer to  FIG. 6 , which is a block diagram of internal circuits of the TM RCV block  150 . The TM RCV block  150  receives both the TM 01  signal (which is the TM 0  and TM 1  signals multiplexed onto a single wire) and the TMCLKPULSEF as shown in  FIG. 1 . The pulsed signal TMCLKPULSEF is first input to an inverter  615  to generate TMCLKPULSE. The multiplexed signal TM 01  (as illustrated in  FIG. 4C ) is then input to two latches,  625  and  635 . In an exemplary embodiment, latches  625  and  635  are edge-sensitive latches wherein  625  latches the signal TM 0 _LATCHED at the rising edge of TMCLKPULSE and  635  latches the signal TM 1 _LATCHED at the falling edge of TMCLKPULSE.  FIG. 7  is a timing diagram of the signals received at the TM RCV block  150 . 
         [0025]    In order to ensure there is no timing issue between the TM send block and the TM RCV block, it is preferable the buffers be constructed with the same materials. The above circuitry does not require any change to the actual test mode program, as pulses are only sent according to power up and Test Clear mode, and when a TM entry occurs. Therefore, no neighbouring wires are affected. Furthermore, as the pulses only toggle according to different modes being entered, no extra power is required for the DRAM. 
         [0026]    As mentioned above, there may be more than one TM RCV block for a single TM send block. Furthermore, in the TM send block, a MUX is required for each pair of TM signals. The control circuit only requires a single set of devices as detailed above—as each pair of signals will be multiplexed separately and decoded separately at the RCV end, the control circuit only needs to generate two selection signals, wherein these selection signals can be input to every MUX for multiplexing two signals on a single wire. 
         [0027]    It should be noted that the internal circuitry of the TM send block as detailed in the figures and the accompanying description is merely an exemplary embodiment of the method for achieving the multiplexing of at least two test signals on a single wire. Other circuitry for achieving the above objective may be realized by those skilled in the art. Moreover, it is possible that more than two test signals are multiplexed on a single wire. 
         [0028]    In summary, by means of multiplexers both in the TM send block and the TM RCV block and a pulsed signal generated between the two TM blocks, it is possible to mux multiple signals on a single wire and utilize the pulsed signal and the multiplexers at the receiving end for independently latching and decoding the muxed signals. In this way, circuit area for a test mode signal system is significantly reduced. 
         [0029]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.

Technology Classification (CPC): 6