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W83977EF-AW Datasheet(PDF) 22 Page - Winbond |
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W83977EF-AW Datasheet(HTML) 22 Page - Winbond |
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22 / 142 page  W83977EF-AW/W83977EG-AW -20- At the start of a command the FIFO is always disabled and command parameters must be sent based upon the RQM and DIO bit settings in the main status register. When the FDC enters the command execution phase, it clears the FIFO of any data to ensure that invalid data are not transferred. An overrun and underrun will terminate the current command and the data transfer. Disk writes will complete the current sector by generating a 00 pattern and valid CRC. Reads require the host to remove the remaining data so that the result phase may be entered. DMA transfers are enabled with the SPECIFY command and are initiated by the FDC by activating the DRQ pin during a data transfer command. The FIFO is enabled directly by asserting DACK# and addresses need not be valid. Note that if the DMA controller is programmed to function in verify mode a pseudo read is performed by the FDC based only onDACK#. This mode is only available when the FDC has been configured into byte mode (FIFO disabled) and is programmed to do a read. With the FIFO enabled the above operation is performed by using the new VERIFY command. No DMA operation is needed. 5.1.3 Data Separator The function of the data separator is to lock onto the incoming serial read data. When a lock is achieved the serial front end logic of the chip is provided with a clock which is synchronized to the read data. The synchronized clock, called the Data Window, is used to internally sample the serial data portion of the bit cell, and the alternate state samples the clock portion. Serial to parallel conversion logic separates the read data into clock and data bytes. The Digital Data Separator (DDS) has three parts: control logic, error adjustment, and speed tracking. The DDS circuit cycles once every 12 clock cycles ideally. Any data pulse input will be synchronized and then adjusted by immediate error adjustment. The control logic will generate RDD and RWD for every pulse input. During any cycle where no data pulse is present, the DDS cycles are based on speed. A digital integrator is used to keep track of the speed changes in the input data stream. 5.1.4 Write Precompensation The write precompensation logic is used to minimize bit shifts in the RDDATA stream from the disk drive. Shifting of bits is a known phenomenon in magnetic media and is dependent on the disk media and the floppy drive. The FDC monitors the bit stream that is being sent to the drive. The data patterns that require precompensation are well known. Depending upon the pattern, the bit is shifted either early or late relative to the surrounding bits. 5.1.5 Perpendicular Recording Mode The FDC is also capable of interfacing directly to perpendicular recording floppy drives. Perpendicular recording differs from the traditional longitudinal method in that the magnetic bits are oriented vertically. This scheme packs more data bits into the same area. FDCs with perpendicular recording drives can read standard 3.5" floppy disks and can read and write perpendicular media. Some manufacturers offer drives that can read and write standard and perpendicular media in a perpendicular media drive. A single command puts the FDC into perpendicular mode. All other commands operate as they normally do. The perpendicular mode requires a 1 Mbps data rate for the FDC. At this data rate the FIFO eases the host interface bottleneck due to the speed of data transfer to or from the disk. |
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