Abstract:
An elasticity buffer has a reset input that, when activated, causes the elasticity buffer to temporarily cease operation. When the reset input bit is released, the elasticity buffer may resume operation. During periods when a device on a serial bus may be halted for power saving mode, for example, the serial communication may be reestablished and then the elasticity buffer may be released to continue operation.

Description:
BACKGROUND OF THE INVENTION 
     a. Field of the Invention 
     The present invention pertains to data transmission systems and specifically to serial data transmissions with elasticity buffers. 
     b. Description of the Background 
     Serial communications are commonly used between electronic devices. In cases where two devices must communicate through a continuous serial protocol but the devices do not share a common clock, an elasticity buffer may be provided at one or both ends of the communications link. An elasticity buffer may add or remove null characters from the serial data stream to compensate for slight mismatches between the clock signals of the two devices. 
     During normal start up and initialization of the serial communication, the serial devices and their associated elasticity buffers begin communication in a known state. Since the elasticity buffer operates by having asynchronous read pointers and write pointers, it is possible that when an elasticity buffer is stopped and restarted that the offset between the read and write pointers may shift. Thus, after repeated stopping and restarting of an elasticity buffer, the pointers of the elasticity buffer may shift to the point where the read and write pointers overlap, causing the elasticity buffer to potentially produce noisy and unreliable data. 
     When one of the devices is not transmitting, noise on the transmission line may be picked up by the interface containing the elasticity buffer. The noise may be received and processed by the elasticity buffer, potentially causing downstream problems. 
     In an example of a serial communication between a disk drive and the main processor of a portable computer, a constant serial communication may exist between the disk drive and the main processor. Such a communication may incorporate one or more elasticity buffers. During a period of inactivity, the disk drive may be shut down to conserve electrical power and also may cease transmitting on the serial communications bus. During reestablishment of the serial communication, any problems with the pointers of the elasticity buffer may cause some problems in properly establishing communications. After repeated cessations of transmissions, the chances increase that the elasticity buffer pointers may be misplaced and prone to errors. 
     It would therefore be advantageous to provide a system and method for pausing an elasticity buffer when there is no activity on a serial communications bus and properly restarting the elasticity buffer when serial communication is reestablished. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages and limitations of the prior art by providing a system and method for pausing and restarting an elasticity buffer by providing a signal to the elasticity buffer to pause operation during periods of inactivity. A serial communication detection bit is used to toggle an elasticity buffer pause bit that causes the elasticity buffer to stop. When the serial communications has been reestablished and is operating properly, the elasticity buffer may then be restarted and may continue operating properly. 
     The present invention may therefore comprise a method for communicating on a serial bus comprising: receiving an input data stream through a communications protocol; passing the input data stream through an elasticity buffer, the elasticity buffer having a pause input that causes the elasticity buffer to pause operation when the pause input is activated; detecting that the input data stream has stopped and activating the pause input of the elasticity buffer; reestablishing the communications protocol; detecting that the communications protocol is reestablished; deactivating the pause input of the elasticity buffer; and receiving the input data stream. 
     The present invention may further comprise a device for receiving a serial communications comprising: an input for the serial communications; an elasticity buffer connected to the input and capable of receiving the serial communications, the elasticity buffer having a pause input capable of pausing the elasticity buffer when the pause input is activated; and a monitor capable of detecting that the communications is not functioning and activating the pause input, the monitor further capable of detecting reestablishment of the communications and deactivating the pause input of the elasticity buffer. 
     The present invention may further comprise a communications system comprising: a transmitting device; and a receiving device comprising an input for the serial communications, an elasticity buffer connected to the input and capable of receiving the serial communications, the elasticity buffer having a pause input capable of pausing the elasticity buffer when the pause input is activated, and a monitor capable of detecting that the communications is not functioning and activating the pause input, the monitor further capable of detecting reestablishment of the communications and deactivating the pause input of the elasticity buffer. 
     The present invention may further comprise a device for receiving a serial communications comprising: a first means for receiving the serial communications; a second means for buffering the serial communications, the second means connected to the input and capable of receiving the serial communications; and a third means for pausing the second means when the third means is activated. 
     The advantages of the present invention are that a serial communications bus that uses an elasticity buffer may have one of the devices change to a low power mode, cease communications, resume normal power mode, and correctly restart communications. In this manner, the resumption of communication from power saving mode may happen without overruns or other problems associated with improperly restarting the elasticity buffer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, 
         FIG. 1  is a block diagram illustration of an embodiment of the present invention of a serial communications system. 
         FIG. 2  is a time line illustration of an embodiment of an elasticity buffer operation. 
         FIG. 3  is an illustration of the status lines of an embodiment of a serial communication. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an embodiment  100  of a serial communications system. The device  102  and device  104  are communicating using an asynchronous serial communications system. Device  102  sends out a serial communications stream  106  to an elasticity buffer  108  that sends an asynchronous serial communications stream  110  to device  104 . A monitor  112  may check for communications stream  106  and cause the elasticity buffer  108  to pause when communications are not properly established by actuating the input  114 . 
     The serial communications streams  106  and  110  may be asynchronous. In other words, the serial communications may not necessarily share a common clock line. The elasticity buffer  108  may compensate for the asynchronous nature of the communication by adding or removing special characters to the data stream. For example, if the incoming data stream is slower than the outgoing data stream, the elasticity buffer may add placeholder or null characters to the outgoing data stream so that the outgoing data stream remains at its speed. 
     The elasticity buffer  108  may operate by having a specific number of registers and a read pointer and a write pointer. As data blocks come into the elasticity buffer  108 , each block may be placed in a register. The write pointer may indicate the register for the next incoming block of data. The write pointer will typically fill up half of the registers with data blocks before the read pointer will begin reading data blocks out of the registers and sending them on the outgoing data stream. The elasticity buffer  108  is an effective method for compensating for jitter between the incoming and outgoing data streams. In a typical application, the speeds of the two data streams are very similar. 
     The read pointer and the write pointer are asynchronous, since each pointer tracks a different data stream with a different clock or timing. When the elasticity buffer  108  is stopped for any reason, one of the pointers may stop slightly sooner than the other pointer, causing the difference in location between the pointers to shift. Repeated stopping and starting of the elasticity buffer  108  may cause the pointers to become shifted to the point where the read and write pointers are pointing to the same register or they may even switch positions. Such a case may cause the elasticity buffer  108  to corrupt data or to function incorrectly. 
     The monitor  112  may be able to detect the correct operation of the serial communications stream  106  and may be able to pause the elasticity buffer  108  using the pause input  114 . The monitor  112  may be a portion of logic on an integrated circuit or other device for determining proper communications. 
     The elasticity buffer  108  may have a pause input  114  that causes the elasticity buffer to cease operation while the input is activated. The pause input  114  may cause the read and write pointers to be reset to a known state. In some embodiments, the pause input  114  may cause the elasticity buffer  108  to be cleared so that when restarted, the elasticity buffer  108  may start correctly. 
       FIG. 2  illustrates an embodiment  200  of an elasticity buffer operation. Device  1  begins communication in block  202  simultaneously with device  2  in block  204 . At some later point in time, device  1  may enter a power save mode or otherwise cease communication in block  206 . The communication becomes disabled in block  208  and device  2  detects the inactivity in block  210 . The device  2  then pauses the elasticity buffer in block  212 . At some later point in time, device  1  initiates handshaking in block  214 . Device  2  responds with handshaking in block  216  and communications are established in blocks  218  and  220 . Device  2  then restarts the elasticity buffer in block  222 . 
     When the elasticity buffer is paused in block  212  and restarted in block  222 , the pointers of the elasticity buffer may be reset. In some cases, the elasticity buffer may be cleared and the pointers set to a start up position. In other cases, the pointer positions may be verified and checked so that any problems with overlapping of the pointers may be avoided. Those skilled in the art may use several different methods to pause and restart the elasticity buffer while keeping within the scope and intent of the present invention. 
       FIG. 3  illustrates an embodiment  300  of a serial communication. The serial link activity is illustrated by line  302 . Lines  304  and  306  illustrate serial link ready signal and elasticity buffer reset signal, respectively. During the period  308 , communications are established. At point  309 , the communications are broken and the serial link ready line  304  changes state. Simultaneously, the elasticity buffer reset line  306  changes state. The initial handshaking  310  and re-established communications  312  may cause the serial link ready line  304  and elasticity buffer reset line  306  to change state. 
     The elasticity buffer reset line  306  may cause the elasticity buffer to pause operation and may further cause the pointers of the elasticity buffer to be reset to a proper location so that when the elasticity buffer is restarted, it is restarted in a proper manner. 
     The serial link ready line  304  may be the output line from a hardware state machine or other device that may detect that serial communications are occurring. In some cases, the proper protocol of the communication system may be discriminated against random noise to ensure that the devices are communicating properly. For example, the serial link ready line  304  may ignore transient signals  314  or handshaking  310 . 
     In some embodiments, the elasticity buffer reset line  306  may cause the elasticity buffer to cease any transmission to the receiving device. For example, transient noise signals  314  may be ignored and not transmitted. If the elasticity buffer were functioning when transient signals  314  were received, there is a potential that noise or corrupt data may be received by the elasticity buffer and transmitted to the receiving device. 
     The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.