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## DS1086L Datasheet(HTML) 12 Page - Maxim Integrated Products

 12 / 16 page3.3V Spread-Spectrum EconOscillator12____________________________________________________________________Example Frequency CalculationsExample #1: Calculate the register values needed togenerate a desired output frequency of 11.0592MHz.Since the desired frequency is not within the valid mas-ter oscillator range of 33.3MHz to 66.6MHz, theprescaler must be used. Valid prescaler values are 2xwhere x equals 0 to 8 (and x is the value that is pro-grammed into the P3 to P0 bits of the PRESCALER reg-ister). Equation 1 shows the relationship between thedesired frequency, the master oscillator frequency, andthe prescaler.By trial and error, x is incremented from 0 to 8 inEquation 2, finding values of x that yield master oscillatorfrequencies within the range of 33.3MHz to 66.6MHz.Equation 2 shows that a prescaler of 4 (x = 2) and amaster oscillator frequency of 44.2368MHz generatesour desired frequency. Writing 0080h to thePRESCALER register sets the PRESCALER to 4. Beaware that other settings also reside in the PRESCALERregister.fMASTER OSCILLATOR = fDESIRED x prescaler = fDESIRED x 2XfMASTER OSCILLATOR = 11.0592MHz x 22 = 44.2368MHzOnce the target master oscillator frequency has beencalculated, the value of offset can be determined.Using Table 2, 44.2368MHz falls within both OS - 1 andOS - 2. However, choosing OS - 1 would be a poorchoice since 44.2368MHz is so close to OS - 1’s mini-mum frequency. On the other hand, OS - 2 is idealsince 44.2368MHz is close to the center ofOS - 2’s frequency span. Before the OFFSET registercan be programmed, the default value of offset (OS)must be read from the RANGE register (last five bits). Inthis example, 12h (18 decimal) was read from theRANGE register. OS - 2 for this case is 10h (16 deci-mal). This is the value that is written to the OFFSET reg-ister.Finally, the two-byte DAC value needs to be deter-mined. Since OS - 2 only sets the range of frequencies,the DAC selects one frequency within that range asshown in Equation 3.fMASTER OSCILLATOR = (MIN FREQUENCY OF SELECTED OFFSETRANGE) + (DAC value x 5kHz)Valid values of DAC are 0 to 1023 (decimal) and 5kHzis the step size. Equation 4 is derived from rearrangingEquation 3 and solving for the DAC value.Since the two-byte DAC register is left justified, 647 isconverted to hex (0287h) and bit-wise shifted left sixplaces. The value to be programmed into the DAC reg-ister is A1C0h.In summary, the DS1086L is programmed as follows:PRESCALER = 0080hOFFSET = OS - 2 or 10h (if range was read as 12h)DAC = A1C0hNotice that the DAC value was rounded. Unfortunately,this means that some error is introduced. To calculatehow much error, a combination of Equation 1 andEquation 3 is used to calculate the expected output fre-quency. See Equation 5.fMIN FREQUENCY OF SELECTED OFFSETRANGEDAC VALUE xkHz STEP SIZEprescalerfMHzxkHzMHzMHzOUTPUTOUTPUT()()(.)()..=+=+==541 06475444 235411 05875DAC VALUEfMIN FREQUENCY OF SELECTEDOFFSET RANGEkHz STEP SIZEDAC VALUEMHzMHzkHz STEP SIZEdecimalMASTER OSCILLATOR()(..).()=−=−=≈544 236841 05647 36647ffprescalerfDESIREDMASTER OSCILLATORMASTER OSCILLATORX==2(1)(4)(2)(3)(5)

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