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IDT5V49EE503

EEPROM PROGRAMMABLE CLOCK GENERATOR

CLOCK SYNTHESIZER

IDT® EEPROM PROGRAMMABLE CLOCK GENERATOR

IDT5V49EE503

REV Q 071015

SS_OFFSET[5:0]

These bits are used to program the fractional offset with

respect to the nominal M integer value. For center spread,

the SS_OFFSET is set to '0' so that the spread spectrum

waveform is centered about the nominal M (Mnom) value.

For down spread, the SS_OFFSET > '0' such the spread

spectrum waveform is centered about the (Mideal -1

+SS_Offset) value. The downspread percentage can be

thought of in terms of center spread. For example, a

downspread of -1% can also be considered as a center

spread of ±0.5% but with Mnom shifted down by one and

offset. The SS_OFFSET has integer values ranging from 0

to 63.

SD[3:0]

These bits are used to shape the profile of the spread

spectrum waveform. These are delta-encoded samples of

the waveform. There are twelve sets of SD samples. The

NSSC bits determine how many of these samples are used

for the waveform. The sum of these delta-encoded samples

(sigma delta- encoded samples) determine the amount of

spread and should not exceed (63 - SS_OFFSET). The

maximum spread is inversely proportional to the nominal M

integer value.

DITH

This bit is used for dithering the sigma-delta-encoded

samples. This will randomize the least-significant bit of the

input to the spread spectrum modulator. Set the bit to '1' to

enable dithering.

This bit will double the total value of the

sigma-delta-encoded-samples which will increase the

amplitude of the spread spectrum waveform by a factor of

two. When X2 is '0', the amplitude remains nominal but if set

to '1', the amplitude is increased by x2. The following

equations govern how the spread spectrum is set:

TSSC = TSSC[3:0] + 2 (Eq. 2)

NSSC = NSSC[2:0] * 2 (Eq. 3)

SD[3:0]K = SJ+1(unencoded) - SJ(unencoded) (Eq. 4)

where SJ is the unencoded sample out of a possible 12 and

SDK is the delta-encoded sample out of a possible 12.

Amplitude = ((2*N[11:0] + A[3:0] + 1) * Spread% / 100) /2

(Eq. 5)

if 1 < Amplitude < 2, then set X2 bit to '1'.

Modulation frequency:

FPFD = FIN / D (Eq. 6)

FVCO = FPFD * MNOM (Eq. 7)

FSSC = FPFD / (4 * Nssc * Tssc) (Eq. 8)

Spread:

= SD0 + SD1 + SD2 + …+ SD11

the number of samples used depends on the NSSC value

 63 - SS_OFFSET

±Spread% = (* 100)/(64 * (2*N[11:0] + A[3:0] + 1) (Eq. 9)

±Max Spread% / 100 = 1 / MNOM or 2 / MNOM (X2=1)

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