ERROR CORRECTION VIA PROGRAMMABLE BIAS CURRENT FOR MR RECORDING HEADS

Abstract: A data error correction procedure is disclosed for use in a disk drive that includes a magneto-resistive head. The error correction procedure increases the DC bias current applied to the read stripe of the magneto-resistive head. The increased value is predetermined for a disk drive having a particular magneto-resistive head. If conventional error correction procedures are not successful, the increased DC bias current is applied to the read stripe to reduce the bit error rate that has occurred during a read operation conducted by the disk drive.

Description: ERROR CORRECTION VIA PROGRAMMABLE BIAS CURRENT FOR MR RECORDING HEADS FIELD OF THE INVENTION The present invention relates to controlling bias current applied to a magneto-resistive recording head and, in particular, increasing such bias current as part of an error correction procedure. BACKGROUND OF THE INVENTION Data read from a disk storage medium is typically read successfully so that an accurate representation of the stored data is obtained. Occasionally, the bit error rate associated with reading data from the disk may degrade with increased noise which may take many forms, such as off track noise, external stray fields and noise and increased flying height of the head due to contamination. Error correction procedures are normally called into play under these conditions. However, error correction involves certain drawbacks such as data or access time overhead. Hence, the degree of correction is limited in practice. It would, therefore, be advantageous to achieve error correction while overcoming or reducing such disadvantageous aspects of error correction. SUMMARY OF THE INVENTION In accordance with the present invention, an apparatus is provided for correction of errors that occur when reading data or other information from a disk using a magneto-resistive (MR) head. In addition to the MR head, the apparatus includes a read channel for signal processing the readback signal obtained by the MR head as it reads data from the disk. The read channel communicates with a controller unit that performs a number of functions including proper positioning of the MR head relative to the disk in order to read data from the disk. The controller typically includes a microprocessor or signal processor for processing the data. With respect to using the MR head for its reading function, it is well known that a DC bias current must be applied to the magneto-resistive read stripe or element of the MR head. Such a bias current is necessary in order to provide a linear response during the readback function. In connection with supplying the DC bias current, in one embodiment the controller unit communicates with a digital- to-analog converter (DAC) that converts the desired or determined digital signal from the controller unit into an analog current signal applied to the magneto-resistive read stripe. When reading data from the disk using a particular MR head, data errors can occur. If the bit error rate for the particular head is outside of a specified or predetermined bit error rate, one or more error correction procedures are invoked including, for example, those that utilize parity or error correction bits. Particularly in situations in which such conventional error correction practices are unsuccessful, involve excessive time and/or require a number of error correction attempts, the present invention is useful in error correction by controllably increasing the DC bias current applied to the magneto-resistive read stripe of the MR head. More specifically, the bit error rate approaches an optimum with increasing bias current, as exhibited by many MR heads. Consequently, the DC bias current is controllably increased using the DAC, which receives its input from the controller unit. With regard to the determination of the increased DC bias current, each MR head is tested to obtain a value related to the increased DC bias current. This value is stored with the disk drive having the particular MR head. The increased DC bias current is useful under certain conditions associated with the operation of the disk drive. In particular, during a disk read operation sought by a user, a determination might be made that data error correction procedures should be invoked because a bit error rate has occurred that is outside the range of acceptable error rates. In one embodiment of the present invention, upon making such a determination, one or more standard or conventional error correction procedures are implemented. If the conventional error correction practices are not successful, the present invention causes a change in the nominal DC bias current that is currently applied to the MR head. Specifically, the previously stored value related to increased DC bias current is used by the controller unit to provide an input to the DAC. The DAC outputs the increased bias current. After the increased bias current is applied by the DAC to the MR read stripe, the disk read operation is continued in which the increased DC bias current is part of an error correction procedure by which the error rate is expected to be eliminated or reduced. At the end of a successful read operation that uses the increased DC bias current, in one embodiment, a record is maintained that indicates the number of times that a particular disk drive has invoked this error correction procedure involving the use of increased DC bias current. If the number of times that such an error correction practice has been used exceeds a threshold number, a flag or other indication of this extent of usage is provided for access by the user to provide this information to the user. Based on the foregoing summary, a number of salient features of the present invention are readily discerned. A data error correction procedure is provided that utilizes the DC bias current applied to a magneto-resistive head in a disk drive. This error correction technique is especially useful in supplementing other error correction procedures that might not be successful in order to reduce the bit error rate to an acceptable level. This error correction procedure involving the use of the increased DC bias current recognizes the potential detrimental effects to the magneto-resistive read stripe by controlling the use of this error correction procedure and, in one embodiment, keeping track of the use thereof so that the user is informed as to the data storage problem areas on the disk. Preferably, the value of the increased DC bias current for use in such an error correction procedure is determined during the manufacturing and testing process of the disk drive. A value related to the increased DC bias current for the particular disk drive is stored in disk memory for later possible use during the disk read operation. Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of the present invention; Fig. 2 is a graph evidencing the benefit of increased DC bias current as such relates to reducing bit error rates in disk drives having magneto-resistive heads; Fig. 3 is a flow diagram illustrating steps associated with the determination of the increased DC bias current for MR heads; and Fig. 4 is a flow diagram illustrating steps related to error correction procedures involving the use of the increased DC bias current for a particular MR head during a disk read operation. DETAILED DESCRIPTION With reference to Fig. 1, a disk drive 10 is represented in a block diagram which illustrates certain basic assemblies or components that are relevant to the discussion of the present invention. The disk drive 10 includes a disk or magnetic recording medium 14 on which data or other information is stored for desired retrieval. The data is stored or written to the disk using a magneto- resistive (MR) recording head 18. In the embodiment of the present invention, the head 18 includes a magneto-resistive (MR) read stripe or read element 22 that, when energized or activated, is useful in reading stored data or other information from the disk 14. In one embodiment, the MR head is connected to an actuator assembly (not shown) of the disk drive 10 that is rotatable to position the head 18 relative to the disk 14. The output of the MR read stripe 22 communicates with a read channel 26 that receives the readback signal generated by the MR read stripe 22 during a read operation. The read channel 26 generally includes a number of signal- processing components or assemblies, such as a pre¬ amplifier, equalizer, filter and/or peak detector, which process the readback signal for subsequent analysis, processing or other use by controller unit 30. For example, the controller unit 30 includes a microprocessor or signal processor that is useful in executing an applications program that utilizes the data read from the disk 14 in the form of the processed readback signal. The controller unit 30 is also substantially involved in positioning the MR head 18 relative to a desired track of the disk 14. During a disk read operation, for example, the controller unit 30 receives and analyzes servo information from the disk 14 to accurately locate the head 18 relative to the desired track by controlling the position of the actuator assembly. The controller unit 30 also electronically communicates with a disk memory 34 that is typically used by the controller unit 30 to store data or information that is readily and rapidly accessible by the controller unit 30 in performing its desired functions. The disk memory 34 is typically a semi-conductor structured memory and may be a random access memory (RAM) . In connection with a disk read operation using a MR read stripe 22 for reading the data on the disk 14, a nominal DC bias current must be applied to the read stripe 22 in order to provide a linear response during the readback function in generating the readback signal. In one embodiment, the DC bias current is applied to the read stripe 22 using a digital - to - analog converter (DAC) 38 or other current control circuit. The DAC 38 communicates with the controller unit 30, as illustrated in Fig. 1. The controller unit 30 applies a digital signal representative of the DC bias current to the DAC 38, which converts it to an analog current signal that is applied to the read stripe 22, to thereby activate it in connection with performing the read operations. It is well-known that the current density within the read stripe 22 is related to its effectiveness or working life. Specifically, relatively high DC bias current levels result in a dangerously high current density within the read stripe 22, which will eventually cause the stripe 22 to fail. This failure mode may occur through electron migration or, as more commonly thought, through thermal failure. On the other hand, a relatively higher DC bias current is useful in providing optimal timing margins or bit error rate, as found in many MR recording heads. This is graphically represented in Fig. 2, which is a plot of timing margins or bit shift distributions for a particular MR head when a complex pattern of data is read using the read stripe of the particular head. As can be seen in Fig. 2, a number of curves are illustrated, which each being associated with a different, particular DC bias current being applied to the read stripe. Five separate curves or graphs are illustrated in Fig. 2, with Graph A representing data in which the DC bias current is 12 mA and Graph E represents data associated with a DC bias current of 5 mA. As can be appreciated, as the bias current increases, the error rate decreases, for a given bit shift, with the bit shift being a function of the complex data pattern. By way of example, for a bit shift of 4.1 nSec, the data error rate when the DC bias current is 12 mA is 10 "6 and, for the same bit shift, the error rate is about 10 "3 when the DC bias current is 5 mA. Consequently, for the same complex data pattern using the same MR head, in conjunction with this example, the error rate is reduced or improved by a factor of 1,000 (10 3 ) . Based on the plot of Fig. 2, in order to reduce the data error rate, it would be advantageous to substantially increase the DC bias current applied to the MR read stripe; however, because this results in an unwanted high current density within the stripe, a trade-off must be sought and realized. This is accomplished in the present invention preferably by using increased DC bias current as part of an error correction procedure. In particular, during a read operation by a disk drive having a particular MR head, under certain circumstances, the DC bias current applied to the read stripe is changed from a nominal magnitude to an increased magnitude in order to enhance or improve the readback function and optimize or reduce the error rate. Preferably, the change to an increased DC bias current is made after one or more standard or conventional error correction procedures have been invoked and are not successful. Preferably also, the increased DC bias current is applied to the read stripe 22 for a relatively short period of time and such a period of time depends on the magnitude of the increased DC bias current. In one embodiment, the increased DC bias current is in the range of 10-20 mA and the range of time that such an increased DC bias current is applied is in the range of one sector or full revolution, e.g. about 0.1-15 mSec. With reference to Fig. 3, a discussion directed to the determination of the increased DC bias current for particular MR heads is now provided. In accordance with block or step 50, a disk drive is tested for determining whether or not an increased DC bias current is appropriate as part of an error correction procedure and, if so, the value of the increased DC bias current. For the disk drive having the particular magneto-resistive head, the head begins reading a complex data pattern that is stored on the disk as part of the testing procedure, with a nominal DC bias current being applied to the read stripe. During the reading of the complex data pattern, a determination is made as to whether or not error correction is required at step 54. In that regard, a determination is made as to whether or not an acceptable data error rate is present during the reading of the complex data pattern. In one embodiment, an acceptable error rate is less than 10 bits in error for 10 7 bits read. In the case in which no error correction is required as determined at step 54, a check is made at step 58 as to whether or not the complex data pattern has been read a sufficient or pre-determined number of disk revolutions or times. Such a pre-determined number of times is about 10° total bits and is typically a sufficient number of times that would result in error correction being required for most magneto-resistive heads. If, however, the complex data pattern has been read a sufficient number of times and the error rate does not exceed the acceptable level, a determination is made that, for this particular magneto- resistive head, no increase over the nominal DC bias current is necessary and the next disk drive with another magneto-resistive head is tested. In a significant number of cases, however, the complex data pattern being read results in an acceptable error rate being exceeded for the particular magneto-resistive head and, at step 62, the present DC bias current is incremented by a predetermined magnitude. Initially, this increment increases the DC bias current from the nominal magnitude. The predetermined magnitude is an amount that is intended to effectively affect the DC bias current being applied so that there is a reasonable expectation that the complex data pattern could then be read without error correction being required. Accordingly, at step 66, the complex data pattern is re-read from the disk using the previously incremented DC bias current. That is, in the embodiment of Fig. 1, the output of the DAC 38 results in a current applied to the read stripe that has been incremented by the pre-determined magnitude over the nominal DC bias current. During the re-reading of the complex data pattern, a determination is made as to whether or not an acceptable error rate for reading data from the disk is now present, in accordance with step 70. If a determination is made that the error rate is not acceptable and error correction is required, at block 62, the present DC bias current is again incremented and the complex data pattern is again re¬ read at block 66. If, on the other hand, no error correction is required, a check is made at step 74 as to whether or not the complex data pattern has been re-read a predetermined or sufficient number of times for the purpose of attempting to create an unacceptable error rate during the read operation. If such a predetermined number of times has not been exceeded, the complex data pattern continues to be re-read from the disk in accordance with step 66. If, however, the complex data pattern has been read a sufficient number of times, step 78 is executed. That is, the present magnitude of the DC bias current is incremented by an adjustment factor to obtain the increased DC bias current. The adjustment factor increases the previously determined DC bias current by a predetermined magnitude or percentage. In one embodiment, the adjustment factor increases the determined DC bias current by about 10 percent. This percentage increase provides a safety factor or margin to ensure that the previously incremented DC bias current will effectively meet the desired or acceptable error rate when this increased DC bias current is applied to the read stripe during a read operation that has invoked the increased DC bias current as part of the error correction procedure. Upon arriving at the resulting increased DC bias current based on a combination of the adjustment factor and the previously determined DC bias current, it is stored in disk memory, at step 82, for potential access by the disk controller during an error correction procedure. It should be appreciated that other methods or steps can be utilized to determine the increased DC bias current. In another embodiment, instead of only incrementing the present DC bias current by a predetermined magnitude, the present DC bias current could be decremented. In such a case, it may be worthwhile to increment the present DC bias current by a relatively greater amount and, if that present DC bias current results in an acceptable error rate, the DC bias current could then be decremented. This decreased DC bias current could then be applied to the read stripe to determine whether or not that particular DC bias current resulted in an acceptable error rate. Depending upon that determination, further increments or decrements of the present DC bias current could then be made until the present DC bias current achieves the acceptable error rate, while not being any greater magnitude than is necessary. It should also be understood that, in reading the complex data pattern a predetermined number of times or disk revolutions, such a pattern could be stored continuously on a disk track so that the head essentially does not move while the disk moves relative to the head. Alternatively, the complex data pattern could be stored in such a way that requires movement of the head relative to the disk during the reading of the complex data pattern; moving the head off track would stress the system SNR while adapting the bias current. With an increased DC bias current having been stored in disk memory for a certain disk drive having a particular magneto-resistive head, with reference to Fig. 4, the steps associated with use of the increased DC bias current as part of a disk read operation is next described. In accordance with block or step 86, data or other information is read from the disk using the previously tested magneto- resistive head. During this read operation, nominal DC bias current is applied to the read stripe 22. At step 90, a check is made as to whether or not any error correction procedure should be invoked due to an unacceptable error rate that has occurred during the read operation. If no such error correction procedure is required, a determination is made at step 94 as to whether or not the particular read operation has been completed. If so, an end-of-read operation occurs at block 98. If, however, the read operation has not been completed, data continues to be read from the disk in accordance with step 86 until the read operation is completed or there is an unacceptable error rate and error correction is required. When error correction is required, in one embodiment, standard or conventional error correction procedures are invoked at step 102 and a check is made as to whether or not one or more of these error correction procedures are successful in achieving a desired error rate. If so, again at step 94, a determination is made as to whether or not the read operation has been completed and, if not, reading from the disk continues until the particular read operation is finished. On the other hand, if the standard or conventional error correction procedures are not successful, at block 106, the nominal DC bias current applied to the read stripe 22 is changed to the increased DC bias current. This is typically accomplished by the controller unit 30 accessing the disk memory 34 to retrieve this value and apply it to the DAC 38. The DAC 38 outputs a value related to the increased DC bias current, which is applied to the read stripe 22, thereby generating in the read stripe 22 the increased DC bias current that is intended to correct the data errors resulting from the particular read operation. In that regard, at block 110, the read operation is continued until the end thereof at block 114. At the end of the read operation, a determination is also made, at step 118, as to whether or not use of the increased DC bias current has exceeded a predetermined number of times for the particular MR head. The purpose of this determination is to provide the user of this particular disk drive with information concerning the number of times that the increased DC bias current has been invoked since such increased DC bias current affects the working life of the read stripe and, therefore, the proper functioning of the disk drive. In one embodiment, a flag is set at step 122 to provide this information, which can be accessed or provided to the user. As can be appreciated, in the event that the increased DC bias current does not result in an acceptable error rate, further and other error recovery procedures can be invoked, such as informing the user that the increased DC bias current error correction procedure has not been successful and/or providing some indication that the data just read has not met an acceptable error rate. In another embodiment or variation of the present invention, the increased DC bias current is varied. Instead of a single value stored for a disk drive having a particular MR head, the DC bias current may be increased or changed more than once during the error correction procedure. By way of example, if one increased DC bias current does not achieve the desired error rate results, another increased DC bias current is applied, which is a predetermined increment greater than the previously used increased DC bias current. Accordingly, in addition to, or in lieu of, the manufacturing or assembly determination of a single increased DC bias current value, such an embodiment affords the potential for more than one such value. The foregoing discussion has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Variation and modification commensurate with the above teachings, within the skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain the best modes presently known of practicing the invention and to enable others skilled in the art to utilize the invention as presented, or in other embodiments, and with the various modifications required by their particular applications or uses of the invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.

Claims: What is claimed is: 1. A method for correcting data read from a disk, comprising: providing a first magneto-resistive head for reading data from the disk with said first magneto-resistive head having a read stripe; concluding that error correction should be conducted; and invoking one or more error correction procedures including using increased DC bias current applied to said read stripe. 2. A method, as claimed in Claim 1, wherein: said providing step includes testing said first magneto-resistive head using a data pattern that generates read errors during a disk read operation. 3. A method, as claimed in Claim 1, wherein: said providing step includes determining a value related to an increased DC bias current for said first magneto-resistive head. 4. A method, as claimed in Claim 3, wherein said determining step includes: ascertaining that data being read from the disk is not being read at an acceptable level; changing the magnitude of DC bias current applied to said read stripe of said first magneto-resistive head; re-reading data from the disk using said first magneto-resistive head; continuing the changing of said DC bias current applied to said first magneto-resistive head when an acceptable level of error rate is not achieved; and discontinuing said changing of said DC bias current when an acceptable error rate is achieved. 5. A method, as claimed in Claim 3, wherein: said determining step includes adjusting a previously determined value related to DC bias current using an adjustment factor. 6. A method, as claimed in Claim 3, wherein: said determining step includes storing said value related to increased DC bias current using memory of the disk drive having said first magneto-resistive head. 7. A method, as claimed in Claim 1, wherein: said concluding step includes deciding that a nominal DC bias current applied to said read stripe should be increased. 8. A method, as claimed in Claim 1, wherein: said invoking step includes initiating a first error correction procedure and subsequently changing said DC bias current applied to said read stripe to said increased DC bias current when said first error correction procedure fails to reduce an error rate associated with reading data from the disk using said first magneto-resistive head to an acceptable level. 9. A method, as claimed in Claim 8, wherein: said changing step includes accessing a value related to said increased DC bias current from disk memory. 10. A method, as claimed in Claim 1, wherein: said invoking step includes indicating that said DC bias current was utilized in connection with an error correction procedure. 11. A method, as claimed in Claim 1, wherein: said invoking step includes continuing reading data from the disk using said first magneto-resistive head after said DC bias current has been changed to said increased DC bias current from a nominal DC bias current. 12. An apparatus for error correction during a read operation in a disk drive having a magneto-resistive head, comprising: a disk for storing data; a magneto-resistive head including a magneto-resistive read stripe; controller means communicating with said magneto- resistive stripe for determining that one or more error correction procedures should be invoked; and a control circuit communicating with said controller means for applying an increased DC bias current to said magneto-resistive read stripe when it has determined that said one or more correction procedures should be invoked, said increased DC bias current being different from a nominal DC bias current that is applied by said control circuit to said magneto-resistive read stripe when said one or more error correction procedures are not invoked. 13. An apparatus, as claimed in Claim 12, wherein: said increased DC bias current is predetermined for said first magneto-resistive head. 14. An apparatus, as claimed in Claim 12, wherein: said increased DC bias current is determined using a magnitude based on reading data from a disk by said first magneto-resistive head after successful reading of data from the disk and also using an adjustment factor that increases said magnitude. 15. An apparatus, as claimed in Claim 12, wherein: said controller means includes memory means for storing a value related to said increased DC bias current. 16. An apparatus, as claimed in Claim 12, further comprising: a pattern for storage on a disk for requiring a DC bias current different from said nominal DC bias current in order to acceptably read said pattern. 17. An apparatus, as claimed in Claim 12, wherein: said one or more correction procedures includes using a first error correction procedure different from said increased DC bias current and wherein said first error correction procedure is conducted separately from and prior to said control circuit applying said increased DC bias current to said read stripe.

Inventors: MC NEIL MICHAEL, MC NEIL, MICHAEL
Applicants: MAXTOR CORP, MAXTOR CORPORATION
Application year: 1996
Main classification: G11B27
IPC codes: G11B5, G11B19, G11B20, G11B27, G11B33