CY28548 datasheet(16/31 Pages) SILABS

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CY28548

......................Document #: 001-08400 Rev ** Page 16 of 30

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

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.

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

is in series with the crystal. The total capacitance on both side

is twice the specified crystal load capacitance (CL). Trim

capacitors are calculated to provide equal capacitive loading

on both sides.

Use the following formulas to calculate the trim capacitor

values for Ce1 and Ce2.

SMSW_EN

Enable Smooth Switching

0 = Disabled, 1= Enabled

SMSW_SEL

Smooth switch select

0 = CPU_PLL, 1 = SRC_PLL

SE1/SE2 drive strength

2 of 2

SE1 and SE2 drive strength Setting 2 of 2

Prog_PCI-E_EN

Programmable PCI-E frequency enable

0 = Disabled, 1= Enabled

Prog_CPU_EN

Programmable CPU frequency enable

0 = Disabled, 1= Enabled

REF drive strength

2 of 2

REFdrive strength strength Setting 2 of 2

USB drive strength

2 of 2

USB drive strength strength Setting 2 of 2

PCI/ PCIF drive strength

2of 2

PCI drive strength strength Setting 2 of 2

Byte 17: Control Register 17 (continued)

Table 5. Crystal Recommendations

Frequency

(Fund)

Cut

Load Cap

Drive

(max.)

Shunt Cap

(max.)

Motional

(max.)

Tolerance

(max.)

Stability

(max.)

Aging

(max.)

14.31818 MHz

Parallel

20 pF

0.1 mW

5 pF

0.016 pF

35 ppm

30 ppm

5 ppm

Figure 1. Crystal Capacitive Clarification

XTAL

Ce 2

Ce 1

Cs 1

Cs 2

Ci1

Ci2

Clo c k Ch ip

Tr a c e

2. 8pF

Tr im

33pF

3 t o 6p

Figure 2. Crystal Loading Example

Load Capacitance (each side)

Total Capacitance (as seen by the crystal)

Ce = 2 * CL – (Cs + Ci)

Ce1 + Cs1 + Ci1

Ce2 + Cs2 + Ci2

(

)

CLe

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