X3T9.2/88-113 Proposal to clean up ESDI timing parameters: Thomas Wicklund Bret Weber Ciprico, Inc. NCR Corp. Version of 9-7-88 There are currently several problems with the inter-sector gap timing parameters of ESDI. These can cause uncertainty when designing controllers and result in the use of less than optimal timing parameters. This document outlines the current text, the problems, and a proposed solution. CURRENT DEFINITION: Currently, the inter-sector gap (ISG) parameters are: 1. Minimum bytes per (ISG) (7.7.1.6). 2. ISG bytes after index/sector pulse to WSplice (ISG post index) (7.7.1.6). 3. Write to read recovery time, equal to ISG post index (8.2.5). 4. Head switch time (read initialization time), 0.7*ISG (8.1.1). There also may be one missing soft sector parameter, minimum AME to address mark time (AME turnon before physical AM on the drive). If a drive defines write to read recovery time as equal to minimum ISG time, a controller can format the drive such that AME will turn on coincident with the physical address mark. It isn't clear that a drive will successfully detect this address mark. PROBLEMS: The problems with the current text include: position of the ISG with respect to index, excess gaps to allow for head switch time, and AME turn-on time uncertainties. - Position of the ISG with respect to index: Some ESDI drives exist which set total ISG equal to ISG post index. This is because the write to read recovery time is the only drive critical time. The format of a drive with these parameters places the end of the data field flush with the index / sector pulse for the next sector. However, most controllers need time after the end of the data field to make next sector decisions. Thus, the controller may need to pad the ISG to include this extra time or be content with sector skewing. - Excess gaps to allow for head switch time: The head switch time (read initialization time) is defined as 0.7*ISG. If a drive has an actual head switch which is a different ratio to the ISG (e.g. equals minimum ISG otherwise required), the drive must report a larger than required ISG to meet the standard. For example, if both head switch time and write to read recovery time equal 18 bytes and these are the only ISG factors, a 26 byte ISG must still be reported. If head switch time is relatively long, the controller's implementation of head switch may change the optimal ISG. Some controllers lose one or more sector times switching heads. Others switch heads during the ISG time but only at index, while others can switch heads during any ISG time. At present, if the factor which controls ISG is the head switch time, controllers which don't need to allow for head switch time must still format a longer than needed ISG. - AME timing uncertainties: If total ISG equals ISG post index, a controller using minimal ISG will turn AME on nearly coincident with the physical address mark. It isn't clear that drives can handle this. PROPOSED SOLUTION: To solve the above uncertainties, explicitly return all parameters from the drive using new configuration words. Adding the following words is recommended. They are used in conjunction with the existing bytes per ISG parameter and controller timing parameters to compute actual gap size and position. Add to section 7.7.1.1, Table 7-11B: ---+-----+----------------------------------------------------------+ d |Subs-| | er |cript| Configuration Response | 1-8| 7-0 | | ---+-----+----------------------------------------------------------+ 0 | x | Values for Configuration of Drive and Format | ---+-----+----------------------------------------------------------+ . . . . #| 17 | Inter sector gap positioning (Hard Sector Only) | | | Bits 15-8 Minimum ISG Before Index / Sector in bytes| | | Bits 7-0 Minimum ISG After Index / Sector in bytes | #| 18 | ISG Related Timing | | | Bits 15-8 Write to Read Recovery Time in bytes | | | Bits 7-0 Head Switch Time in bytes | #| 19 | ISG Related Timing | | | Bits 15-8 Reserved = 0 | | | Bits 7-0 Minimum AME Assert Before Address Mark | | | in bits | +-----+----------------------------------------------------------+ Add section 7.7.1.8: 7.7.1.8 Inter Sector Gap Values If present, these values allow a controller to tune the size and position of the ISG field. 7.7.1.8.1 Determining ISG size and position If subscripts 17 through 19 are supported, the controller may adjust the ISG size and position to meet its own needs. The parameters controlling the ISG are then: - Minimum Bytes per ISG. - Minimum ISG before Index / Sector. - Minimum ISG after Index / Sector. - Write to Read Recovery Time. - Head Switch Time. - Minimum AME Assert Before Address Mark. The "ISG Bytes after Index / Sector" (configuration word 7, bits 15-8) field is not used if subscripts 17 and 18 are implemented. A drive should still implement this byte to support controllers which don't implement subscripts 17 and 18. The Minimum Bytes per ISG field specifies the minimum ISG size. It will often equal the greater of Write to Read Recovery Time or Head Switch Time. The controller must provide an ISG at least this size, and controllers which don't implement subscripts 17 and 18 depend on this value. The Minimum ISG before and after Index / Sector (subscript 17) specify any drive requirements for the position of the ISG. Drives using an embedded servo may use these parameters to ensure that the servo area is completely within the ISG. These parameters should be chosen as small as is practical. The Write to Read Recovery Time specifies the minimum time in bytes from WRITE GATE off to READ GATE on or ADDRESS MARK ENABLE on. The Head Switch Time specifies the minimum delay in bytes after head switch to READ GATE on or ADDRESS MARK ENABLE on. The Minimum AME Assert Before Address Mark defines the minimum time in bits that AME must be on before the start of the physical address mark on the disk. Change Section 8.1.1: 8.1.1 Read Initialization Time The time lapse before READ GATE or ADDRESS MARK ENABLE can be asserted after a head switch is (0.7*ISG) or according to the subscripted value provided in the Request General Configuration subscripts. Drives not able to meet 15 usec head switching time shall drop COMMAND COMPLETE upon head switch. 8.2.5 Write-to-Read Recovery Time The time lapse before READ GATE or ADDRESS MARK ENABLE can be asserted after negating the WRITE GATE is defined by the subscripted value provided in Request General Configuration subscript 18 and the "ISG Bytes after Index/Sector" in Configuration word 7. Add to Appendix A: o The defect list will be written using an ISG bytes after Index as reported in configuration word 7.