Abstract:
An apparatus includes a plurality of data lines defining a data bus for communicating data. A controller is operable to communicate a plurality of data transfers over the data bus using a plurality of data time slots, wherein for at least a subset of the data time slots the controller is operable to communicate an associated data bus inversion indicator indicating that bits communicated during the associated data time slot are inverted, the data bus inversion indicators for the subset of the data transfers are grouped into a data bus inversion vector, and the controller is operable to communicate a global data bus inversion indicator indicating an inversion of the data bus inversion vector.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     BACKGROUND 
     The disclosed subject matter relates generally to computing systems and, more particularly, to a method and apparatus for reducing simultaneous switching outputs using data bus inversion signaling. 
     In computing systems, dynamic memory devices are used to store large amounts of data for use by a processor or other computing device during its operation. Data is transferred between the computing device and the memory device over a memory bus. In such electronic systems, it is common that different power requirements exist for driving an electrical “1” versus driving an electrical “0”. For example, in some double data rate (DDR) synchronous dynamic random access memory (SDRAM), more power is consumed driving a “0” than a “1”. 
     Data bus inversion (DBI) is an I/O signaling technique that aims to reduce DC power consumption by selectively inverting the data bus for systems where the power consumed between alternate signaled states is asymmetric. The device communicating the data (i.e., the processor for a write operation or the memory device for a read operation) counts the number of 0s driven on a bus during a bit transfer time, and if more than half the bus is electrical 0, the bus state is inverted. A DBI indicator bit is toggled to indicate that bus inversion has occurred. If the number of 0s and 1s in the bit transfer time are less than or equal to half the bus width, no inversion takes place. When the receiving device processes the data, the DBI indicator bit is used as a trigger to invert the data again to reconstitute the original data pattern. In this manner, the average number of 1s transmitted in a bit transfer time is increased, thereby reducing DC power. Bus inversion may also be used in the case of address lines. Hence, as used herein the term data bus inversion applies generically to any type of bus inversion, such as DQ buses or address buses. 
     DBI also has the property of reducing simultaneous switching outputs (SSO), defined as the absolute value of the number outputs that change to 1 minus the number of outputs that change to 0 in two consecutive bit time transfers. In a system where transmitting 1s is lower power than transmitting 0s, the transmitted DBI bit is defined as 1 for no inversion and 0 for inversion. If all bit transfer times require inversion (e.g., a stream of 0s, which would be inverted to 1s), and the DBI vector is transmitted after the last data transfer time, the system sees a worst case SSO transition where the last data transfer is all 1s and the DBI bit transfer is all 0s. Thus, DBI can introduce new SSO problems and reduce the overall SSO benefit. 
     This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
     BRIEF SUMMARY 
     The following presents a simplified summary of the disclosed subject matter in order to provide a basic understanding of some aspects of the disclosed subject matter. This summary is not an exhaustive overview of the disclosed subject matter. It is not intended to identify key or critical elements of the disclosed subject matter or to delineate the scope of the disclosed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
     One aspect of the disclosed subject matter is seen in an apparatus including a plurality of data lines defining a data bus for communicating data. A controller is operable to communicate a plurality of data transfers over the data bus using a plurality of data time slots, wherein for at least a subset of the data time slots the controller is operable to communicate an associated data bus inversion indicator indicating that bits communicated during the associated data time slot are inverted, the data bus inversion indicators for the subset of the data transfers are grouped into a data bus inversion vector, and the controller is operable to communicate a global data bus inversion indicator indicating an inversion of the data bus inversion vector. 
     Another aspect of the disclosed subject matter is seen in a method including communicating a plurality of data transfers over a plurality of data lines defining a data bus using a plurality of data time slots, communicating a data bus inversion indicator for at least a subset of the data time slots indicating that bits communicated during the associated data time slot are inverted, wherein the data bus inversion indicators for the subset of the data transfers are grouped into a data bus inversion vector, and communicating a global data bus inversion indicator indicating an inversion of the data bus inversion vector. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The disclosed subject matter will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
         FIG. 1  is a simplified block level diagram of a microprocessor interfaced with external memory; 
         FIG. 2  is simplified block diagram illustrating the interface between a memory controller and a memory in the system of  FIG. 1 ; 
         FIGS. 3-5  are diagrams illustrating data transfers using a sideband DBI signaling technique; 
         FIG. 6  is a simplified block diagram of an alternative embodiment of an interface between the memory controller and the memory in the system of  FIG. 1 ; and 
         FIG. 7  is a diagram illustrating data transfers using a global DBI signaling technique 
     
    
    
     While the disclosed subject matter is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the disclosed subject matter to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosed subject matter as defined by the appended claims. 
     DETAILED DESCRIPTION 
     One or more specific embodiments of the disclosed subject matter will be described below. It is specifically intended that the disclosed subject matter not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the disclosed subject matter unless explicitly indicated as being “critical” or “essential.” 
     The disclosed subject matter will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the disclosed subject matter with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the disclosed subject matter. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
     Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIG. 1 , the disclosed subject matter shall be described in the context of a microprocessor  100  coupled with an external memory  105 . Those skilled in the art will recognize that a computer system may be constructed from these and other components. However, to avoid obfuscating the instant invention only those components useful to an understanding of the present invention are included. 
     In one embodiment, the microprocessor  100  employs a pair of substantially similar modules, module A  110  and module B  115 . The modules  110 ,  115  include similar processing capabilities. The modules  110 ,  115  engage in processing under the control of software, and thus access memory, such as external memory  105  and/or caches, such as a shared L3 cache  120  and/or internal caches (not shown). An integrated memory controller  125  is provided to interface the modules  110 ,  115  with the external memory  105  over a memory bus  130 . Those skilled in the art will appreciate that each of the modules  110 ,  115  may include additional circuitry for performing other useful tasks. 
     In general, the memory bus  130  includes data lines (DQ), address lines (AD), and control lines (CTL), such as chip select (CS), write enable (WE), bank select (BS), column access strobe (CAS), row access strobe (RAS), data mask (DM), and clock (CLK). In the illustrated embodiment, the external memory  105  is a double data rate (DDR) memory, where data may be transferred on both rising and falling edges of the clock signal. 
     The integrated memory controller  125  and the external memory  105  communicate using a data bus inversion (DBI) scheme, where the bits driven on the DQ lines and/or address lines may be inverted to reduce the power consumption of the device or reduce noise by limiting the number of simultaneously switching outputs (SSO). For purposes of illustration, the following examples relate to the inversion of the DQ lines, however, the concepts may be applied to any bus, such as an address bus. In general, data transfers occupy n time slots, and the data bus inversion is controlled by an n-bit DBI vector, where each bit in the vector indicates whether the associated bits in the time slot have been inverted. In addition to the conventional DBI vector, a global DBI (DBIG) bit is utilized to indicate whether the DBI vector itself has been inverted. Providing the global DBI control increases the degree of control so that power savings and/or noise performance are not compromised by the DBI vector. 
     In some embodiments, the global DBI bit may be communicated within the data time slots, while in other embodiments, the global DBI bit may be communicated using a sideband signal (i.e., outside the bits of the data transfer). 
     As illustrated in  FIGS. 2-5 , a first topology for communicating a global DBI control bit using a side band signal is illustrated. In the embodiment of  FIG. 2 , it is assumed that the external memory  105  has an 8-bit data bus and that a data transfer is implemented using 8 data time slots, 1 DBI control time slot, and 1 cyclic redundancy check (CRC) time slot. It is also assumed that a bit value of “1” is the low power state for the external memory  105 . Conventionally, a data mask (DM) line  135  is used during write operations to indicate when data on the DQ lines  140  is valid. If the DM bit is asserted for a data slot, the data is ignored. The DM line  135  is typically unused during a read operation. In accordance with the present embodiment, the DM line  135  is used in a bidirectional manner to communicate DBI signaling information, as illustrated in  FIG. 3  for a write operation and as illustrated in  FIG. 4  or  5  for a read operation. 
     As shown in the write operation of  FIG. 3 , data time slots  0 - 7  are implemented conventionally, where the DM bit is used to selectively mask the bytes being written. During time slot  8 , a DBI vector  145  is communicated on the DQ lines  140  indicating whether the bytes in the previous data slots had been inverted. A global DBI bit (DBIG)  150  is communicated using the DM line  135 . Hence, if the DBIG bit  150  is asserted, the external memory  105  is signaled that the DBI vector  145  has itself been inverted. In response to the assertion of the DBIG bit  150 , a controller in the external memory  105  inverts the DBI vector  145  and then uses the inverted values for processing the bytes in the data time slots. 
     As shown in the read operation of  FIG. 4 , data transfer slots  0 - 7  are implemented conventionally, and the DM bit is unused (i.e., held at the low power state of “1”. During time slot  8 , the DBI vector  145  is communicated on the DQ lines  140  to indicate whether the bytes in the previous data time slots have been inverted. The global DBI bit (DBIG)  150  is communicated using the DM line  135  during the DBI time slot  8 . Hence, if the DBIG bit  150  is asserted, the memory controller  125  is signaled that the DBI vector  145  has itself been inverted. In response to the assertion of the DBIG bit  150 , the memory controller  125  inverts the DBI vector  145  and then uses the inverted values for processing the bytes in the data time slots. 
       FIG. 5  illustrates an alternative embodiment of a read operation, where data time slots  0 - 7  are implemented conventionally, but the DM line  135  is used to communicate the DBI vector  145 . In data slot  8 , the CRC data is sent, and the DBIG bit  150  is communicated using the DM line  135 . 
     Turning now to  FIGS. 6 and 7 , another embodiment is described where a sideband signal is not available for communicating the global DBI information. As shown in  FIG. 6  the external memory  105  may include a bank of two 4-bit DDR memories  155 ,  160  grouped an 8-bit arrangement. The memory  155  is designated as an even bank, and the memory  160  is designated as an odd bank through the use of mode registers. The DQ lines  140  of the even bank  155  and those of the odd bank  160  are interleaved by bank. This interleaving pattern repeats for additional banks. In 4-bit implementations, data mask lines are not typically available for the memories  155 ,  160 , so there is no sideband pin for sending a global DBI signal. As illustrated in  FIG. 7 , to enable the use of global DBI, the number of data slots for which DBI is implemented is reduced, and the global DBI bit  150  is sent over the DQ lines  140  along with a reduced DBI vector  165 ,  170  for each memory  155 ,  160 . 
     The nibbles communicated in data slots  0 - 7  are conventional. However, rather than providing a DBI vector having 8 bits corresponding to the 8 data time slots, each DBI vector  165 ,  170  only covers 6 time slots. In the illustrated embodiment, the DBI vector  165  for the even mode memory  155  implements DBI for data slots  0 - 5 , and the odd mode memory implements DBI for data slots  2 - 7 . The nibbles in time slots  6  and  7  for the even mode memory  155  and nibbles in time slots  0  and  1  for the odd mode memory  160  are never inverted. The DBI vectors  165 ,  170  are communicated over control time slots  8  and  9 . A global DBI vector  175 ,  180  is also sent in control time slots  8  and  9 , with a DBIA bit indicating if the time slot  8  portion of the DBI vector  165 ,  170  has been inverted, and a DBIB bit indicating if the time slot  9  portion of the DBI vector  165 ,  170  has been inverted. 
     Using this approach, the maximum 8 SSO over 8 bits between bit transfer slots  7  and  8  and between slots  8  and  9  can be avoided. Albeit, there is a slight reduction in DC power savings because there are four slots where only one nibble of the pair is covered by DBI. The SSO reduction over both banks  155 ,  160  in the pair is better than the SSO characteristic of each bank individually. While the SSO characteristic for a single bank would be worst case of 4, over the two adjacent banks, the SSO is limited to a maximum of 6. 
     The DBI signaling techniques described herein enable DBI with minimum SSO. For x8/x16 devices having a sideband signal to carry the global DBI bit  150 , the SSO is less than 4. For x4 devices without a sideband signal, the SSO is a maximum of 6 over 8 bits. Reducing power consumption has the potential to reduce cooling requirements and extend battery life. Reducing SSO improves noise performance, which may have the potential to increase the maximum frequency at which the memory bus operates. 
     The particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.