CY28510

Rev 1.0, November 20, 2006

Page 7 of 12

Dial-a-Frequency Operation

VCO Frequency = (14.318180 MHz) x (N/ M)

Output1 = VCO/4 = 66.669 MHz

Output2 = VCO/8 = 33.335 MHz

To operate the Dial-a-Frequency feature, you must select the

individual output that is to be modified by selecting the Spread

spectrum control enable bit in the Clock Spread Enable

Registers to multiplex the SS PLL as the input source, which

is the only PLL that can have the “N” register value changed.

Then you must disable spread spectrum by setting MSTRSRD

in the Clock Spread Spectrum Control Register (Byte 2, bit 2)

to 0 so that the spread PLL is not being modulated. It is then

possible to change the N value from it’s default value of 149 to

any value within ±25%. You must also set the DAFEN bit to a

1 in Byte 6, bit 0 to enable the Dial-a-Frequency feature.

Please note that the long-term or accumulated jitter will be

about 3nsec, which does not affect the operation of the device

since only Cycle-to-Cycle jitter can cause system problems.

Crystal Recommendations

The CY28510 requires a Parallel Resonance Crystal.

Substituting a series resonance crystal will cause the

CY28510 to operate at the wrong frequency and violate the

ppm specification. For most applications there is a 300-ppm

frequency shift between series and parallel crystals due to

incorrect loading. See Table 6.

Crystal Loading

Crystal loading plays a critical role in achieving low ppm perfor-

mance. To realize low ppm performance, the total capacitance

the crystal will see must be considered to calculate the appro-

priate capacitive loading (CL). The following diagram shows a

typical crystal configuration using the two trim capacitors. An

important clarification for the following discussion is that the

trim capacitors are in series with the crystal not parallel. It’s a

common misconception that load capacitors are in parallel

with the crystal and should be approximately equal to the load

capacitance of the crystal.

Bit

@Pup

Description

71

N7, MSB

60

N6

50

N5

41

N4

30

N3

21

N2

10

N1

01

N0, LSB

Table 6. Crystal Recommendations

Frequency

(Fund)

Cut

Loading

Load Cap

Drive

(max.)

Shunt Cap

(max.)

Motional

(max.)

Tolerance

(max.)

Stability

(max.)

Aging

(max.)

14.31818 MHz

AT

Parallel

20 pF

0.1 mW

5 pF

0.016 pF

50 ppm

50 ppm

5 ppm

Figure 1. Crystal Capacitive Clarification