am79c974 Advanced Micro Devices, am79c974 Datasheet - Page 126

• Manufacturer Part Number: am79c974
• Description: Pcnettm-scsi Combination Ethernet And Scsi Controller For Pci Systems
• Manufacturer: Advanced Micro Devices

Motherboard designs which place an internal and exter-
nal connector on either side of the Am79C974 are dis-
couraged since:
NOISE CONSIDERATIONS
Some areas of a PCI motherboard or host adapter de-
sign (which include the Am79C974) are more suscepti-
ble to noise. They are:
126
Peripheral
External
Two SCSI connectors are required on the mothe-
rboard—one more than what the other two ap-
proaches call for.
When the number of internal and external bus
components increase, the on-board terminators
would have to be turned off either through software
or hardware, which may be undesirable to a board
designer.
The possibility for creating stubs exists if the con-
nectors and Am79C974 are not routed correctly.
See Figure E-1 for the ideal routing scheme.
the 40 MHz crystal oscillator
the SCSI cables
the DC power planes
pins on the ICs, specifically the Am79C974.
AMD
High-Density
Termination
Keyboard
Connector
High-Density
Connectors
SCSI-2
SCSI-2
External SCSI
Bus Cable
To SCSI CLK1
and CLK2
High-Density
Connector
Mounted
SCSI-2
25
Figure E-4. Motherboard Layout — Approach #2
to PCI Bus
Am79C974
To DC Power
Ferrite Bead
P R E L I M I N A R Y
Am79C974
This is optional. Use
accordingly to terminate both
ends of the SCSI Bus.
Electromagnetic Interference (EMI)
There are several ways to reduce the amount of noise
present in these areas:
A 40 MHz crystal oscillator must be used in order
to have a 10 MB/s SCSI data rate. Use of this
40 MHz crystal oscillator, which drives the
SCSI CLK pin on the Am79C974, introduces the
possibility of unwanted high frequency compo-
nents, including harmonics, coupling with
Am79C974 signals. To help prevent this, a resistor
should be placed in series with the oscillator to
form a low pass filter with the input capacitance
(10 pF) of the SCSI CLK pin. This filter reduces the
edge rate of the clock waveform, increasing both
rise and fall times by 2 ns. This removes higher
frequencies, specifically harmonics from the crystal
oscillator waveform and thus reduces the amount
of noise introduced into the Am79C974.
A ferrite bead, which is essentially an inductor,
may be used with the oscillator to prevent coupling
of higher frequencies with DC power supply sig-
nals. The ferrite bead blocks high frequency noise
while acting like a short to the DC components.
Riser Card
Memory
(Terminated)
Internal
Drive
CPU
18681A-66