cy28548 SpectraLinear Inc, cy28548 Datasheet - Page 13

• Manufacturer Part Number: cy28548
• Description: Clock Generator For Intel Crestline Chipset
• Manufacturer: SpectraLinear Inc

Rev 1.5 September 16, 2008
Byte 17: Control Register 17 (continued)
Byte 18: Control Register 18
Table 5. Crystal Recommendations
The CY28548 requires a Parallel Resonance Crystal. Substi-
tuting a series resonance crystal causes the CY28548 to
operate at the wrong frequency and violates the ppm specifi-
cation. For most applications there is a 300-ppm frequency
shift between series and parallel crystals due to incorrect
loading
Crystal Loading
Crystal loading plays a critical role in achieving low ppm perfor-
mance. To realize low ppm performance, use the total capac-
itance the crystal sees to calculate the appropriate capacitive
loading (CL).
Figure 1 shows a typical crystal configuration using the two
trim capacitors. It is important that the trim capacitors are in
series with the crystal. It is not true that load capacitors are in
parallel with the crystal and are approximately equal to the
load capacitance of the crystal.
14.31818 MHz
Frequency
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
(Fund)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Cut
AT
PCI/ PCIF_DSC0
SE1/SE2 _DSC0
PCI/PCIF_DSC2
Loading Load Cap
Prog_PCI-E_EN
SE1/SE2_DSC2
Parallel
Prog_CPU_EN
SMSW_SEL
RESERVED
RESERVED
RESERVED
RESERVED
REF_DSC0
USB_DSC0
REF_DSC2
USB_DSC2
SMSW_EN
20 pF
Enable Smooth Switching
0 = Disabled, 1= Enabled
Smooth switch select
0 = CPU_PLL, 1 = SRC_PLL
SE1 and SE2 drive strength Setting 2 of 3(see Byte 18 for more setting)
0 = Low, 1= High
Programmable PCI-E frequency enable
0 = Disabled, 1= Enabled
Programmable CPU frequency enable
0 = Disabled, 1= Enabled
REFdrive strength strength Setting 2 of 3(see Byte 18 for more setting)
0 = Low, 1= High
USB drive strength strength Setting 2 of 3(see Byte 18 for more setting)
0 = Low, 1= High
0 = Low, 1= High
Drive Strength Control
PCI drive strength strength Setting 2 of 3(see Byte 18 for more setting)
0.1 mW
(max.)
Drive
Def ault
Shunt Cap
Calculating Load Capacitors
In addition to the standard external trim capacitors, consider
the trace capacitance and pin capacitance to calculate the
crystal loading correctly. Again, the capacitance on each side
(max.)
5 pF
Figure 1. Crystal Capacitive Clarification
(Byte18)
DSC_2
Motional
0.016 pF
1
1
1
1
0
0
0
0
(max.)
(Vario us B ytes )
Tolerance
DSC_1
35 ppm
(max.)
1
1
0
0
1
1
0
0
(Byte 17)
Stability
DSC_0
30 ppm
(max.)
1
0
1
0
1
0
1
0
CY28548
Page 13 of 30
Strongest
Strength
Weakest
Buf f er
Aging
(max.)
5 ppm