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PC97338 数据表(PDF) 104 Page - National Semiconductor (TI) |
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PC97338 数据表(HTML) 104 Page - National Semiconductor (TI) |
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104 / 221 page  104 www.national.com 3.6.9 The PERPENDICULAR MODE Command The PERPENDICULAR MODE command configures each of the four logical disk drives for perpendicular or conventional mode via the logical drive configuration bits 1,0 or 5-2, depending on the value of bit 7. The default mode is conventional. Therefore, if the drives in the system are conventional, it is not necessary to issue a PER- PENDICULAR MODE command. This command supports the unique FORMAT TRACK and WRITE DATA requirements of perpendicular (verti- cal) recording disk drives with a 4 MB unformatted capacity. Perpendicular recording drives operate in extra high density mode at 1 or 2 Mbps, and are downward compatible with 1.44 MB and 720 KB drives at 500 kbps (high density) and 250 kbps (double density), respectively. If the system includes perpendicular drives, this command should be issued during initialization of the FDC. Then, when a drive is accessed for a FORMAT TRACK or WRITE DATA command, the FDC adjusts the com- mand parameters based on the data rate. See Table 49. Precompensation is set to zero for perpendicular drives at any data rate. Perpendicular recording type disk drives have a pre-erase head that leads the read or write head by 200 µm, which translates to 38 bytes at a 1 Mbps data transfer rate (19 bytes at 500 Kbps). The increased space between the two heads requires a larger gap 2 between the address field and data field of a sector at 1 or 2 Mbps. See Perpendicular Format in Figure 58. A gap 2 length of 41 bytes (at 1 or 2 Mbps) ensures that the preamble in the data field is completely pre-erased by the pre-erase head. Also, during WRITE DATA operations to a perpendicular drive, a portion of gap 2 must be rewritten by the con- troller to guarantee that the data field preamble has been pre-erased. See Table 49. Command Phase TABLE 49. Effect of Drive Mode and Data Rate on FORMAT TRACK and WRITE DATA Commands TABLE 50. Effect of GDC Bits on FORMAT TRACK and WRITE DATA Commands 7654 3210 0001 0010 OW 0 DC3 DC2 DC1 DC0 GDC Data Rates Drive Mode Length of Gap 2 in FORMAT TRACK Command Portion of Gap 2 Rewritten in WRITE DATA Command 250, 300 or 500 Kbps Conventional Perpendicular 22 bytes 22 bytes 0 bytes 19 bytes 1 or 2 Mbps Conventional Perpendicular 22 bytes 41 bytes 0 bytes 38 bytes GDC Bits Drive Mode Length of Gap 2 in FORMAT TRACK Command Portion of Gap 2 Rewritten in WRITE DATA Command 1 0 0 0 Conventional 22 bytes 0 bytes 0 1 Perpendicular ( ≤500 Kbps) 22 bytes 19 bytes 1 0 Conventional 22 bytes 0 bytes 1 1 Perpendicular (1 or 2 Mbps) 41 bytes 38 bytes |
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