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5V49EE704NDGI8 数据表(PDF) 8 Page - Integrated Device Technology |
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5V49EE704NDGI8 数据表(HTML) 8 Page - Integrated Device Technology |
8 / 30 page IDT5V49EE704 EEPROM PROGRAMMABLE CLOCK GENERATOR CLOCK SYNTHESIZER IDT® EEPROM PROGRAMMABLE CLOCK GENERATOR 8 IDT5V49EE704 REV F 022310 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. X2 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: Σ∆ = SD0 + SD1 + SD2 + …+ SD11 the number of samples used depends on the NSSC value Σ∆< 63 - SS_OFFSET ±Spread% = ( Σ∆ * 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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