Table 25 - Example FIFO Service Delays
EXAMPLE DATA RATES
FIFO
THRESHOLD
EXAMPLES
2Mbps
1Mbps
500Kbps
1 byte
1 x 4µs - 1.5µs = 2.5µs
2 x 4µs - 1.5µs = 6.5µs
8 x 4µs - 1.5µs = 30.5µs
1 x 8µs - 1.5µs = 6.5µs
2 x 8µs - 1.5µs = 14.5µs
8 x 8µs - 1.5µs = 62.5µs
1 x 16µs - 1.5µs = 14.5µs
2 x 16µs - 1.5µs = 30.5µs
8 x 16µs - 1.5µs = 126.5µs
15 x 16µs - 1.5µs = 238.5µs
2 bytes
8 bytes
15 bytes
15 x 4µs - 1.5µs = 58.5µs 15 x 8µs - 1.5µs = 118.5µs
DIGITAL INPUT REGISTER (DIR)
The Digital Input Register (Bass Address + 7: Read-only) is read-only in all modes. Table 26 shows the DIR in PC/AT
mode, Table 27 shows the DIR in PS/2 mode, and Table 28 shows the DIR in Model 30 mode.
PC-AT Interface Mode
Table 26 - DIR PC/AT Interface Mode
7
6
5
4
3
2
1
0
DSK CHG
N/A
RESET
N/A
N/A
N/A
N/A
N/A
N/A
N/A
CONDITION
Undefined, Bits 0 - 6
The data bus outputs D0 - 6 will remain in a high impedance state during a read of this register.
DSK CHG, Bit 7
The DSK CHG bit monitors the state of the pin of the same name and reflects the opposite value seen on the disk
cable. The DSK CHG bit also depends upon the Force Disk Change bits in the Force FDD Status Change register
(see section CR17 on page 108).
PS/2 Interface Mode
Table 27 - DIR PS/2 Interface Mode
7
6
1
5
1
4
1
3
1
2
1
0
DSK CHG
DRATE
DRATE
nHIGH
SEL1
SEL0
DENS
RESET
N/A
N/A
N/A
N/A
N/A
N/A
N/A
1
CONDITION
nHIGH DENS, Bit 0
This bit is low whenever the 500 Kbps or 1 Mbps data rates are selected, and high when 250 Kbps and 300 Kbps are
selected.
Data Rate Select, Bits 1 - 2
These bits control the data rate of the floppy controller. See Table 22 for the settings corresponding to the individual
data rates. The data rate select bits are unaffected by a software reset, and are set to 250 Kbps after a hardware
reset.
Undefined, Bits 3 - 6
Always read as a logic “1”
DSK CHG, Bit 7
The DSK CHG bit monitors the pin of the same name and reflects the opposite value seen on the disk cable. The
DSK CHG bit also depends upon the Force Disk Change bits in the Force FDD Status Change register (see section
CR17 on page 108).
SMSC DS – FDC37N769
Page 28 of 137
Rev. 12/21/2000
SMSC [ SMSC CORPORATION ]
