vsc055xkm-01 Maxim Integrated Products, Inc., vsc055xkm-01 Datasheet - Page 123

• Manufacturer Part Number: vsc055xkm-01
• Description: Enhanced I?c Backplane Controller
• Manufacturer: Maxim Integrated Products, Inc.

VSC055-01
Data Sheet
4.6
Figure 7. Oscillator Options
4.7
Figure 8. Optional External Tach Filter
10 MHz
Option A
30 pF
Oscillator Requirements
The VSC055-01 can use an external 3.3-V, 8.0 MHz to 12.5 MHz clock source connected to the OSCI pin,
with CKSEL2 tied to V
OSCI pin with CKSEL2 tied to V
clock source can be connected to the OSCI pin with CKSEL2 tied to V
Alternatively, an 8.0 MHz to 12.5 MHz crystal and several passive components may be used.
The following illustration shows two options when using a crystal. The passive components shown
function properly for all crystal frequencies. Option A requires fewer external components due to the high
input capacitance of the OSCI pin and results in a stable configuration. Option B represents a classic
approach with a higher level of stability.
V
Optional External Tach Filter
The fan tach inputs of the VSC055-01 use Schmitt trigger input buffers and are also internally digitally
filtered. However, excessive external noise on a tach input can result in inaccurate fan speed current count
values. The use of an external low-pass filter, along with the use of the extended tach filter mode (Tach
Filter Extend, bit 7 of register FDh) of the VSC055-01 eliminates inaccurate current count values.
The circuit in the following illustration provides excellent noise rejection at all possible RPM ranges
supported by the VSC055-01.
SS
390 Ω
OSCI
OSCO
VSC055-01
Fan Tach Output
SS
. An external 3.3-V, 32.0 MHz to 50.0 MHz clock source can be connected to the
DD
3.3 kΩ
and CKSEL1 tied to V
V
123 of 133
DD
30 pF
Option B
330 Ω
V
SS
0.1 μF
10 MHz
30 pF
V
SS
SS
. An external 3.3-V 48.0 MHz to 75.0 MHz
V
SS
P2.0 − P2.7
VSC055-01
DD
390 Ω
10 MΩ
and CKSEL1 tied to V
OSCI
OSCO
VSC055-01
January 2008
Revision 4.1
DD
.