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IDT5P49EE602 Datasheet(PDF) 6 Page - Integrated Device Technology |
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IDT5P49EE602 Datasheet(HTML) 6 Page - Integrated Device Technology |
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6 / 25 page  IDT5P49EE602 VERSACLOCK® LOW POWER CLOCK GENERATOR EEPROM CLOCK GENERATOR IDT™ VERSACLOCK® LOW POWER CLOCK GENERATOR 6 IDT5P49EE602 REV C 061110 SPREAD SPECTRUM GENERATION (PLLB) PLLB has spread spectrum generation capability, which users have the option of turning on and off. Spread spectrum profile, frequency, and spread are fully programmable (within limits). The programmable spread spectrum generation parameters are NC[10:0], MOD[12:0], and NSS[10:0] bits. To enable spread spectrum, set SSENB_B=0. The spread spectrum circuitry was specifically developed to accommodate video display applications. The spread modulation frequency can be defined to exactly equal the horizontal line frequency (HSYNC) NC[10:0] These bits are used to determine the number of pulses per spread spectrum cycle. For video applications, NC is the number of pixels on the horizontal display row (or integer multiple of displayed pixels in a row). By matching the spread period to the screen, no tearing or “shimmer” will be apparent. NC must be an even number to insure that the upward spread transition has the same number of steps as the downward spread transition. For non-video applications, this can also be seen as the number of clock cycles for a complete spread spectrum period. MOD[12:0] These bits relate the VCO frequency to the target average spread output frequency (FMID). FMID = (FVCO) / 8 FMAX = FMID + (SS% * FMID) FMIN = FMID - (SS% * FMID) MOD = (FREF* NC) / (2 * FMID) NSS[10:0] These bits control the amplitude of the spread modulation. NSS = (NC / 2) + (NC / 8) * (FMAX - FMIN) / FMID Modulation frequency: FMOD = FMID / NC (Eq. 11) Video Example FREF = 27 MHz, FOUT = 27 MHz, 640 pixels per line, center spread of ±1%. Using FVCO=432MHz, find the necessary spread spectrum register settings. FMID = FVCO/8 NC = 640 (integer number of spread periods/screen) MOD = (25MHz * 640)/(2 * 54MHz) = 160 NSS = (640/2)+(640/8)*(27.27MHz-26.73MHz)/27MHz = 321. FMOD = 27MHz/640 = 11.8kHz. Non-Video Example FREF = 25MHz, FOUT = 27 MHz, 31.25kHz modulation rate, center spread of ±1%. Find the necessary spread spectrum register settings. FMID = FVCO/ 8 FMOD = 31.25kHz = 50.625MHz/NC. NC = 1620 MOD = (25MHz * 1620)/(2 * 50.625MHz) = 400 NSS = (1620/2)+(1620/8)*(27.27MHz-26.73MHz)/27MHz = 814. |
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