Abstract:
Methods and systems provide recognition of a device in a daisy chain cascade configuration. Input circuitry at a device receives an input signal that indicates device configuration following a power-up, reset or other operation of the device. A pulse generator generates a pulse in response to the operation, the pulse occurring while the input signal indicates device configuration. A state latch register stores the state of the input signal in response to the received pulse, thereby storing a state indicating configuration of the respective device. Following this operation, the input circuitry may receive signals unrelated to the device configuration, thereby obviating the need for additional pin assignment.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/787,710, filed on Mar. 28, 2006, the entire teachings of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    A typical computer-based system comprises a system board and optionally one or more peripheral devices, such as display units and disk units. The system board often contains one or more processors, a memory subsystem and other circuitry, such as serial device interfaces, network device controllers and hard disk controllers. 
         [0003]    The type of processors that are employed on a particular system board usually depends on the type of tasks performed by the system. For example, a system that performs a limited set of tasks, such as monitor emissions generated by an automobile engine and adjust an air/fuel mixture to ensure the engine is burning fuel completely may employ a simple specialized processor that is tailored to performing these tasks. On the other hand, a system that performs many different tasks, such as managing many users and running many different applications, may employ one or more complex processors that are general purpose in nature, configured to perform high-speed calculations and manipulate data to minimize the response time to servicing the users&#39; requests. 
         [0004]    The memory subsystem is a storage that holds information (e.g., instructions, data values) used by the processors. The memory subsystem typically comprises controller circuitry and one or more memory devices. The controller circuitry is usually configured to interface the memory devices with the processors and enable the processors to store and retrieve information to and from the memory devices. The memory devices hold the actual information. 
         [0005]    Like the processors, the type of devices employed in a memory subsystem is often driven by the type of tasks performed by the computer system. For example, a computer system may have the task of having to boot without the assistance of a disk drive and execute a set of software routines that do not change often. Here, the memory subsystem may employ non-volatile devices, such as Flash memory devices, to store the software routines. Other computer systems may execute very complex tasks that require a large high-speed data store to hold large portions of information. Here, the memory subsystem may employ high-speed high-density Dynamic Random Access Memory (DRAM) devices to store the information. 
         [0006]    Demand for Flash memory devices has continued to grow significantly because these devices are well suited in various embedded applications that require non-volatile storage. For example, Flash is widely used in various consumer devices, such as digital cameras, cell phones, USB Flash drives and portable music players, to store data used by these devices. Market demand for Flash memory has led to tremendous improvements in Flash memory technology over the past several years both in terms of speed and density. These improvements have led to the prediction that Flash memory-based devices may one day replace hard disk drives in applications that continue to use disk drives for mass storage. 
       SUMMARY OF THE INVENTION 
       [0007]    Some Flash devices employ serial interfaces that are used to perform operations, such as read, write and erase operations, on memory contained in the devices. These operations are typically selected on a device using command strings that are serially fed to the devices. The command strings typically contain a command that represents the operation to be selected as well as parameters. For example, a write operation may be selected by serially feeding an information string to the device that contains a write command, the data to be written and an address in the memory where the data is to be written. 
         [0008]    Some memory subsystems employ multiple Flash devices with serial interfaces. Here, a plurality of devices may be configured in a parallel or multi-drop arrangement, wherein each device receives commands through a signal path connected to the controller. Alternatively, the devices may be configured in a daisy chain cascade arrangement, wherein signals are transmitted from each device to the successive device in the chain. Still other configurations may employ both multi-drop and serial configurations, where some signals are received in parallel and others are transmitted through a daisy chain cascade. 
         [0009]    In a daisy chain cascade configuration, a command string may be fed to all of the devices even though the command may only be performed on one of the devices. A first device in the daisy chain cascade receives a command string from the controller. The command string is then transmitted to each successive device by the preceding device in the daisy chain cascade, until the command is received by all devices. Each device also responds to commands addressed to it, transmitting responsive data through the daisy chain cascade to the controller. 
         [0010]    Memory devices utilized in a daisy chain cascade may require an indication of such a configuration. For example, a Flash memory device may be suitable for operation as a single device or as one of a plurality of memory devices in a daisy chain cascade or multi-drop configuration, or may be utilized interchangeably between multiple configurations. Because these configurations require different modes of operation, a memory device operating in a daisy chain cascade must recognize that it is so configured. 
         [0011]    Embodiments of the present invention provide systems and methods of recognizing a daisy chain cascade configuration of devices. Input circuitry receives an input signal at a device, the input signal indicating device configuration following a power-up, reset or other event associated with the device. A signal generator generates an indicator such as a pulse in response to the operation, the indicator occurring while the input signal indicates device configuration. A storage mechanism such as a state latch register stores the state of the input signal in response to the received indicator, thereby storing a state indicating configuration of the respective device. As a result, an embodiment of the present invention recognizes whether the device is connected in a daisy chain cascade configuration and provides an indication that enables the device to operate according to the configuration. 
         [0012]    Following this operation, further embodiments permit the input circuitry to receive signals unrelated to the device configuration, thereby obviating the need for additional pin assignment. For example, the input circuitry may receive control or chip select signals. The respective device may be a memory device configured in a plurality of memory devices, such as Flash memory devices connected serially in a daisy chain cascade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0014]      FIGS. 1A and 1B  are block diagrams of device configurations comprising a plurality of single port devices in which embodiments of the present invention may be implemented. 
           [0015]      FIG. 2  is a block diagram of a plurality of devices configured for communication in a daisy chain cascade arrangement. 
           [0016]      FIG. 3  is a timing diagram of signals occurring during power-up of a device. 
           [0017]      FIG. 4  is a block diagram of a circuit that detects a configuration of a device during power-up. 
           [0018]      FIG. 5  is a timing diagram of signals occurring at the circuit of  FIG. 4 . 
           [0019]      FIG. 6  is a block diagram of a circuit that detects a configuration of a device following a device reset. 
           [0020]      FIG. 7  is a timing diagram of signals occurring at the circuit of  FIG. 6 . 
           [0021]      FIG. 8  is a block diagram of a plurality of devices configured for communication in a daisy chain cascade arrangement. 
           [0022]      FIG. 9  is a block diagram of 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    A description of example embodiments of the invention follows. 
         [0024]      FIG. 1A  is a block diagram of an exemplary device configuration comprising a plurality of single port devices configured in a serial daisy chain cascade arrangement having inputs and outputs for various signals. The devices  110   a - e  are memory devices, each of which contains a memory (not shown) such as Dynamic Random Access Memory (DRAM) cells, Static Random Access Memory (SRAM) cells, or Flash memory cells. 
         [0025]    Each device  110  comprises a serial input (SI), serial output (SO), chip select (CS#), and clock input (CLK). The input SI is used to transfer information (e.g., command, address and data information) into the device  110 . The output SO is used to transfer information from the device  110 . Input CLK receives an external clock signal to the devices  110   a - d  and the CS# input receives a chip select signal, which enables operations at all devices simultaneously. 
         [0026]    The ports SI and SO are connected between devices  110  in the daisy chain cascade arrangement such that the output SO of a device  110  earlier in the daisy chain cascade is coupled to the SI of the next device  110  in the daisy chain cascade. For example, port SO of device  110   a  is coupled to the SI of device  110   b . The CLK input of each device  110  is fed with a clock signal from, e.g., a memory controller (not shown). The clock signal is distributed to each device  110  via a common link. As will be described further below, CLK is used to, among other things, latch information input to the device  110  at various registers contained in the device  110 . 
         [0027]    The CS# input of each device is a conventional chip select that selects the device. This input is coupled to a common link that enables a chip select signal to be asserted to all of the devices  110  concurrently and consequently selects all of the devices  110   a - e  simultaneously. The CS# input and CLK inputs are coupled to separate links that distribute the CS# and CLK signals, respectively, to the devices  110   a - e , as described above. 
         [0028]    Information transmitted to the devices  110   a - e  may be latched at different times of the clock signal fed to the CLK input. For example, in a single data rate (SDR) implementation, information input to the device  110  at the SI may be latched at either the rising or falling edge of the CLK clock signal. Alternatively, in a double data rate (DDR) implementation, both the rising and falling edges of the CLK clock signal may be used to latch information input at the SI. 
         [0029]    The configuration of the devices  110   a - e  in  FIG. 1A  includes both a serial daisy chain cascade (e.g., input SI and output SO) and conventional multi-drop connections (e.g., CLK and CS#). Thus, the configuration may be referred to as a hybrid of daisy chain cascade and multi-drop configurations, where the advantages of each may be realized. Alternatively, embodiments of the techniques described herein may be implemented in serial, parallel, multi-drop or other connections, and combinations thereof, between devices. Refer to U.S. patent application Ser. No. 11/496,278, titled “Daisy Chain Cascading Devices,” the entire teachings of which are incorporated by reference herein, for further information regarding communication and configuration of devices in a daisy chain cascade. 
         [0030]      FIG. 1B  is a block diagram of a plurality of devices  120   a - c  configured in parallel communication with a controller (not shown). Each device  120   a - c  includes a serial input (SI), serial output (SO), chip select (CS#), and clock input (CLK). The input SI is used to transfer information (e.g., command, address and data information) into the device  120 . The output SO is used to transfer information from the device  120 . Input CLK receives an external clock signal to the devices  120   a - c  and the CS# input receives a chip select signal, which enables operations at each device independently of the other devices. 
         [0031]    The devices  120   a - c  are configured in a parallel arrangement, utilizing multi-drop connections at the communication ports SI and SO for communication with a controller. Separate CS inputs at each device  120   a - c  allow the device to be enabled individually. Therefore, a controller connected to the devices  120  in a multi-drop arrangement may control each device  120   a - c  by first transmitting to its corresponding CS input, and then sending and receiving data at the corresponding ports SI and SO. For example, a controller may communicate with device  120   a  by first selecting it with a signal at input CSO. Once the device is enabled, the controller can send commands, data and other signals to the device  120   a  at input SI 0 , and receive responsive communication from the device at output SO 0 . 
         [0032]    The devices of  FIG. 1A and 1B  are illustrated as comprising a serial input SI, serial output SO, clock input CLK and chip select CS. However, as described above, these ports may serve different functions in each configuration. Memory devices comprising memory such as DRAM cells or Flash cells may be implemented as such devices. In some embodiments, an implemented memory device may not be preconfigured for serial or multi-drop operation. For example, a Flash memory device may be suitable for operation in either of the configurations of  FIG. 1A and 1B , and may be utilized interchangeably between these two or other configurations. Because each memory device must be configured for proper operation in a daisy chain cascade or multi-drop setting, the systems of  FIGS. 1A and 1B  must indicate their respective configurations to their respective devices. 
         [0033]      FIG. 2  is a block diagram illustrating a technique for indicating device configuration. Here, devices  210   a - e  are configured in a serial daisy chain cascade comparable to the configuration of  FIG. 1A . However, the inputs CS at all devices  210  are connected to ground, thereby maintaining a constant signal at the CS input at each device  210   a - e . Circuitry at each device  210   a - e  receives the CS input and determines whether the device is configured in a daisy chain cascade arrangement during a power-up sequence of the device. If the received CS input is maintained at ground as shown, the circuitry at each device  210   a - e  determines that the device is enabled and configured in a daisy chain cascade. 
         [0034]    The technique described above indicates a daisy chain cascade configuration by connecting the chip select input at each device to ground. Alternative embodiments of the devices  210   a - e  can include other inputs that are configured in this manner to indicate a daisy chain cascade configuration. However, the configured input is maintained at ground, and therefore cannot be used in other operations. For example, the CS input at each device  210   a - e  could instead receive an enable signal from a controller for enabling operations at each device. Thus, this technique for indicating a daisy chain cascade configuration requires a dedicated input. 
         [0035]      FIG. 3  is a timing diagram illustrating signals of two exemplary devices during a power up sequence. Signal Vdd is a reference voltage signal providing power to the exemplary devices, and ramps up until it reaches a reference voltage at the beginning of time t. The signal  310  is a CS input of an exemplary device in a daisy chain cascaded connection, such as one of the devices  110   a - e  in  FIG. 1A . The signal  320  is a CS input of an exemplary device in a parallel connection, such as one of the devices  120   a - c  in  FIG. 1B . During the time in which Vdd ramps up, both exemplary devices may receive undetermined signals at their respective CS inputs. However, during a time t in which Vdd has reached a reference voltage, the CS input at each device may receive a signal that corresponds to whether the device is configured in a daisy chain cascade or in parallel. Here, the CS signal  310  in a daisy chain cascaded connection is low during time t, while the CS signal  320  in a parallel connection is high. Time t may be a short interval, such as a span greater than 1 microsecond, that occurs between the power ramp-up and when the device receives communication signals from a controller. The CS signals  310 ,  320  may be provided to the devices by a memory controller (not shown) that controls the devices. Such a memory controller may be configured to indicate, by the state of the CS signal  310 ,  320  during time t, whether the devices it controls are connected in a daisy chain cascaded configuration, in parallel, or as a single device. 
         [0036]    Following time t, the CS signal  310 ,  320  at each device may no longer indicate device configuration, as the signal state may change as determined by a controller. Signals  310  and  320  are accompanied by respective state latch signals  315  and  325 . These accompanying signals  315 ,  325  represent the state of respective latches that receive the state of each CS signal  310 ,  320 . The respective latches latch the inverse state of the CS signals during time t, thereby storing the inverse state of the CS signals  310 ,  320  at time t. As a result, the state latch signals  315 ,  325  indicate whether the respective devices are connected in a daisy chain cascade or parallel (or single device) configuration. The devices may respond to the state latch signals  315 ,  325  by operating according to the indicated configuration. For example, a device receiving the “high” state latch signal  315  (indicating a daisy chain cascaded connection) may be enabled at all times and transmit all commands and data through a serial output. Conversely, a device receiving a “low” state latch signal  325  (indicating a parallel or single device connection) may be enabled by the CS signal  320 . Because the state latch signals  315 ,  325  indicate configuration, CS signals may be used following time t to select the devices or perform other functions. 
         [0037]      FIG. 4  is a block diagram of latching circuitry  400  that stores the state of received signal CS during a power up sequence of a device. The circuit may be internal to a device such as one of the devices  110   a - e ,  120   a - c  of  FIG. 1 , the device being a single device controlled by a controller or as one of a plurality of devices in a daisy chain cascade or parallel configuration. The circuit includes an input buffer  410 , power-up circuit  420 , pulse generator  430  and state latch register  440 . 
         [0038]    Input buffer  410  receives a CS signal and outputs a corresponding signal “chip select” to the state latch register  440  and an internal logic block that receives the CS signal. The power-up circuit  420  receives a reference voltage Vdd and outputs a signal “pwr_ok” that is high when Vdd is ramped up. The pulse generator  430  receives the signal “pwr_ok” and outputs a pulse in response to a transition of “pwr_ok” to high. The state latch register  440  (shown in the circuit  400  as well as inset below the circuit  400 ) receives the pulse and the signal “chip select,” and stores the state of signal “chip select” by latching it while the pulse is received. 
         [0039]    The pulse is provided as an indicator, and may be a signal of any duration that accommodates this latching operation. In the present embodiment, the pulse generator  430  generates a pulse signal “pwr_ok_ps” in response to the device completing a power ramp-up. For a time period following the power ramp-up, the CS signal indicates whether the device is configured in a daisy chain cascade arrangement, a parallel connection, or as a single device (e.g., CS being low indicates a daisy chain cascade, while CS being high indicates a parallel or single device connection). In order for the state latch register  440  to store the CS signal providing this indication, the pulse generator  430  must enable the state latch register  440  during the aforementioned time period. 
         [0040]      FIG. 5  is a timing diagram illustrating signals corresponding to the circuit  400  of  FIG. 4 . The signals are shown during a power up sequence of the device receiving a reference voltage Vdd. The signal “pwr_ok” at line  530  is high when Vdd has reached a predetermined voltage. This time coincides with a time t in which the input signal CS  550   a - b  indicates whether the device is in a daisy chain cascade or parallel configuration. Signal “chip_select”  551   a - b  provides a signal inverted from the CS signal  550   a - b . Thus, a pulse generator generates a pulse signal  540  during time t, enabling a state latch register to latch the state of the signal “chip_select”  551   a - b  during this time t, as shown by the signals “state latch”  552   a - b.    
         [0041]    Signals  550   a - 552   a  correspond to a device in a daisy chain cascade arrangement, while signals  550   b - 552   b  correspond to a device in a parallel connection. As a result of latching the respective “chip_select” signals  551   a - b  while they indicate the device configuration, the signals  552   a - b  maintain a state indicating the configuration of the respective devices. 
         [0042]      FIG. 6  is a block diagram of an alternative embodiment of latching circuitry  600  for storing the state of the received CS signal. The circuit may be internal to a device such as one of the devices  110   a - e ,  120   a - c  of  FIG. 1 , the device being a single device controlled by a controller or as one of a plurality of devices in a daisy chain cascade or parallel configuration. The circuit includes an input buffers  610 ,  620 , pulse generator  630  and state latch register  640 , the functions of which may be comparable to the corresponding components of the circuit  400  of  FIG. 4 . Input buffer  620  receives a signal “reset,” which may be transmitted to the device to reset one or more configurations at the device. The pulse generator  630  generates a pulse in response to the signal “reset.” The state latch register  640  latches the state of the signal “chip select” during the pulse, thereby storing a state corresponding to the CS signal following a device reset operation. 
         [0043]      FIG. 7  is a timing diagram illustrating signals corresponding to the circuit  600  of  FIG. 6 . The signals are shown during a power up sequence of the device receiving a reference voltage Vdd  730 . Following the power up sequence, the signal “reset” is asserted. During a time t following the assertion of the signal “reset,” CS signal  750   a - b  indicates the device configuration. In particular, CS signal  750   a  is low, indicating a daisy chain cascade connection; while CS signal  750   b  is high, indicating a parallel connection. The CS signals  750   a - b  are buffered, resulting in respective “chip select” signals  751   a - b . During time t, the state of the “chip_select” signals  751   a - b  are latched by a state latch, providing respective “state_latch” signals  752   a - b  that maintain the latched state beyond time t. Thus, the circuit  600  recognizes whether the device is connected in a daisy chain cascade configuration, stores a state indicating the device configuration, and permits a related input port to receive communications after the configuration is recognized. 
         [0044]      FIG. 8  is a block diagram of an exemplary device configuration comprising a plurality of single port devices configured in a serial daisy chain cascade arrangement having inputs and outputs for various signals. The devices  810   a - d  are memory devices, each of which contains a memory (not shown) such as Dynamic Random Access Memory (DRAM) cells, Static Random Access Memory (SRAM) cells, or flash memory cells. The latching circuitry  400 ,  600  in  FIGS. 4 and 6  may be incorporated into each of the devices  810   a - d . As such, the latching circuitry  400 ,  600  may latch the state of the signal CS# when it indicates that the devices  810   a - d  are configured in a daisy chain cascade. 
         [0045]    Each device  810  comprises a serial input (SI), serial output (SO), chip select (CS#), and clock input (SCLK), described above with reference to  FIG. 1A . In addition, each device  810  comprises an input port enable (IPE) input, output port enable (OPE) input, input port enable output (IPEQ) and output port enable output (OPEQ). The IPE input receives an IPE signal to the device. The IPE signal may indicate to the device to enable the SI such that when IPE is asserted information may be serially input to the device  810  via the SI. Likewise, the OPE input receives an OPE signal to the device. The OPE signal is used by the device to enable the SO such that when OPE is asserted information may be serially output from the device  810  via the SO. The IPEQ and OPEQ are outputs that output the IPE and OPE signals, respectively, from the device. The CS# input and SCLK inputs are coupled to separate links that distribute the CS# and SCLK signals, respectively, to the devices  410   a - d , as described above. 
         [0046]    The SI and SO are coupled from one device to the next in a daisy chain cascade arrangement, as described above. Moreover, the IPEQ and OPEQ of an earlier device  810  in the daisy chain cascade are coupled to the IPE input and OPE input, respectively, of the next device  410  in the daisy chain cascade arrangement. This arrangement allows IPE and OPE signals to be transferred from one device to the next (e.g., device  810   a  to device  810   b ) in a serial daisy chain cascade fashion. 
         [0047]    Information transmitted to the devices  810   a - d  may be latched at different times of the clock signal fed to the SCLK input. For example, in a single data rate (SDR) implementation, information input to the device  810  at the SI may be latched at either the rising or falling edge of the SCLK clock signal. Alternatively, in a double data rate (DDR) implementation, both the rising and falling edges of the SCLK clock signal may be used to latch information input at the SI. 
         [0048]      FIG. 9  is a block diagram of serial output control logic  1100  that may be incorporated into each of the devices  810   a - d  of  FIG. 8 . Logic  1100  comprises an SI input buffer  1104 , an IPE input buffer  1106 , an OPE input buffer  1108 , an SCLK input buffer  1110 , logical AND gates  1112  and  1114 , latches  1116 ,  1118 ,  1120  and  1122 , selectors  1124  and  1130 , logical OR gate  1126  and an SO output buffer  1128 . Buffers  1104 ,  1106 ,  1108  and  1110  may be conventional LVTTL buffers configured to buffer SI, IPE, OPE and SCLK signals, respectively, that are inputted to the device. 
         [0049]    The output control logic  1100  controls input and output signals according to the received control signals. As described above with reference to  FIG. 1 , a device connected in a daisy chain cascade arrangement (e.g. device  110   a ) operates differently from a device connected in parallel (e.g., device  120   a ) or as a single device. Therefore, the control logic  1100  receives a signal indicating whether the device is configured in a daisy chain cascade, signal CASCADE. Latching circuitry  400 ,  600  may be incorporated into a device with serial control logic  1100 . If so, the signal State_latch signal output by the latching circuitry  400 ,  600  would be received by the output control logic  1100  as the signal CASCADE, described below. 
         [0050]    The buffered SI signal is received by AND gate  1112 , which sends the signal to latch  1116  when IPE is asserted. Latch  1116  is configured to latch the information when a clock signal (SCLK) is provided by buffer  1110 . DATA_OUT represents the state of data read from a memory (not shown) contained in the device. AND gate  1114  is configured to output a state of DATA_OUT when OPE is asserted. The output of AND gate  1114  feeds latch  1118  which is configured to latch the state of DATA_OUT when a clock signal is provided by buffer  1110 . Buffer  1106  is configured to buffer the IPE signal fed to the device. The output of buffer  1106  is latched by latch  1120 . Likewise, buffer  1108  is configured to buffer the OPE signal fed to the device. Latch  1122  is configured to latch the state of OPE as output by buffer  1108 . Selectors  1124  and  1130  are conventional 2-to-1 multiplexers each comprising two inputs. The inputs for selector  1124  are selected for output from the selector  1124  by the above-described ID_MATCH signal. One input is fed with the latched state of DATA_OUT as maintained by latch  1118 . This input is selected for output from selector  1124  when ID_MATCH is asserted. The other input is fed with the latched state of SI as maintained by latch  1116 . This input is selected for output from the selector  1124  when ID_MATCH is not asserted. 
         [0051]    The signal CASCADE indicates whether the device is coupled to one or more other devices in a daisy chain cascade arrangement. Illustratively, this signal is asserted if the device is coupled to one or more devices in a daisy chain cascade arrangement. For example, the signal may be asserted by the state latch  440 ,  640  of respective latching circuitry  400 ,  600  when indicating that the device is configured in a daisy chain cascade. Asserting the CASCADE signal causes the latched state of the IPE signal fed to the selector  1130  to be output from the selector  1130 . When CASCADE is not asserted, the logic low condition input to the selector  1130  is output from the selector  1130 . The inputs for selector  1130  are selected for output from the selector  1130  by the CASCADE signal. One input to selector  1130  is fed with the latched state of IPE as maintained by latch  1120  and the other input is tied to a logical zero. The latched state of IPE is selected for output from the selector  1130  when CASCADE is asserted. 
         [0052]    Conversely, if CASCADE is not asserted, logical zero is selected for output from the selector  1130 . The signal IPE therefore cannot be asserted to enable the serial output of the device. As a result, output buffer  1128  is enabled only by the OPE, thereby controlling the serial output as indicated by the output enable signal. This configuration is suitable where the device is not connected in a daisy chain cascade. 
         [0053]    OR gate  1126  provides an enable/disable signal to output buffer  1128 . The gate  1126  receives the output of selector  1130  and the latched state of OPE, as maintained by latch  1122 . Either of these outputs may be used to provide an enable signal to buffer  1128  to enable the buffer&#39;s output. Buffer  1128  is a conventional buffer that buffers output signal SO. As noted above, buffer  1128  is enabled/disabled by the output of OR gate  1126 . 
         [0054]    Operationally, when IPE is asserted, information that is input to the device via SI is fed to latch  1116 . Latch  1116  latches this information illustratively at the first upward transition of SCLK after IPE is asserted. Likewise, latch  1120  latches the state of IPE at this SCLK transition. Providing that ID_MATCH is not asserted (indicating that a command is not addressed to the device), the output of latch  1116  is fed to buffer  1128  via selector  1124 . Likewise, the asserted IPE is transferred from buffer  1106  to latch  1120  where it is also illustratively latched by the first upward transition of SCLK. Assuming CASCADE is asserted, the latched state of IPE is provided at the output of selector  1130  and transferred to OR gate  1126  to provide an enable signal to buffer  1128 . The latched state of SI is then transferred from the device via buffer  1128  as output SO. 
         [0055]    Illustratively, at the next upward transition of SCLK after OPE is asserted, the asserted state of OPE is latched at latch  1122  and the state of DATA_OUT is latched at latch  1118 . Providing that ID_MATCH is asserted, the latched state of DATA_OUT is selected by selector  1124  and applied to the input of buffer  1128 . Simultaneously, the latched asserted state of OPE from latch  1122  passes through OR gate  1126  to enable buffer  1128  which causes the latched state of DATA_OUT to be output from the device as output SO. 
         [0056]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.