FIFO. The data is based upon the following
formula:
DATA REGISTER (FIFO)
Address 3F5 READ/WRITE
Threshold # x
1
x 8
- 1.5 ms = DELAY
All command parameter information, disk data
and result status are transferred between the
host processor and the floppy disk controller
through the Data Register.
DATA RATE
At the start of a command, the FIFO action is
always disabled and command parameters
must be sent based upon the RQM and DIO bit
settings. As the command execution phase is
entered, the FIFO is cleared of any data to
ensure that invalid data is not transferred.
Data transfers are governed by the RQM and
DIO bits in the Main Status Register.
The Data Register defaults to FIFO disabled
mode after any form of reset. This maintains
PC/AT hardware compatibility.
The default
An overrun or underrun will terminate the
current command and the transfer of data. 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.
values can be changed through the Configure
command (enable full FIFO operation with
threshold control). The advantage of the FIFO
is that it allows the system a larger DMA
latency without causing a disk error. Table 14
gives several examples of the delays with a
Table 14 - FIFO Service Delay
FIFO THRESHOLD
EXAMPLES
MAXIMUM DELAY TO SERVICING AT 2
Mbps* DATA RATE
1 byte
2 bytes
8 bytes
15 bytes
1 x 4 ms - 1.5 ms = 2.5 ms
2 x 4 ms - 1.5 ms = 6.5 ms
8 x 4 ms - 1.5 ms = 30.5 ms
15 x 4 ms - 1.5 ms = 58.5 ms
FIFO THRESHOLD
EXAMPLES
MAXIMUM DELAY TO SERVICING AT 1
Mbps DATA RATE
1 byte
2 bytes
8 bytes
15 bytes
1 x 8 ms - 1.5 ms = 6.5 ms
2 x 8 ms - 1.5 ms = 14.5 ms
8 x 8 ms - 1.5 ms = 62.5 ms
15 x 8 ms - 1.5 ms = 118.5 ms
FIFO THRESHOLD
EXAMPLES
MAXIMUM DELAY TO SERVICING AT
500 Kbps DATA RATE
1 byte
2 bytes
8 bytes
15 bytes
1 x 16 ms - 1.5 ms = 14.5 ms
2 x 16 ms - 1.5 ms = 30.5 ms
8 x 16 ms - 1.5 ms = 126.5 ms
15 x 16 ms - 1.5 ms = 238.5 ms
*The 2 Mbps data rate is only available if VCC = 5V.
25
SMSC [ SMSC CORPORATION ]
