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DS1086L Datasheet(PDF) 12 Page - Maxim Integrated Products |
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DS1086L Datasheet(HTML) 12 Page - Maxim Integrated Products |
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12 / 16 page  3.3V Spread-Spectrum EconOscillator 12 ____________________________________________________________________ Example Frequency Calculations Example #1: Calculate the register values needed to generate 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, the prescaler must be used. Valid prescaler values are 2x where 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 the desired frequency, the master oscillator frequency, and the prescaler. By trial and error, x is incremented from 0 to 8 in Equation 2, finding values of x that yield master oscillator frequencies within the range of 33.3MHz to 66.6MHz. Equation 2 shows that a prescaler of 4 (x = 2) and a master oscillator frequency of 44.2368MHz generates our desired frequency. Writing 0080h to the PRESCALER register sets the PRESCALER to 4. Be aware that other settings also reside in the PRESCALER register. fMASTER OSCILLATOR = fDESIRED x prescaler = fDESIRED x 2X fMASTER OSCILLATOR = 11.0592MHz x 22 = 44.2368MHz Once the target master oscillator frequency has been calculated, the value of offset can be determined. Using Table 2, 44.2368MHz falls within both OS - 1 and OS - 2. However, choosing OS - 1 would be a poor choice since 44.2368MHz is so close to OS - 1’s mini- mum frequency. On the other hand, OS - 2 is ideal since 44.2368MHz is close to the center of OS - 2’s frequency span. Before the OFFSET register can be programmed, the default value of offset (OS) must be read from the RANGE register (last five bits). In this example, 12h (18 decimal) was read from the RANGE 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 as shown in Equation 3. fMASTER OSCILLATOR = (MIN FREQUENCY OF SELECTED OFFSET RANGE) + (DAC value x 5kHz) Valid values of DAC are 0 to 1023 (decimal) and 5kHz is the step size. Equation 4 is derived from rearranging Equation 3 and solving for the DAC value. Since the two-byte DAC register is left justified, 647 is converted to hex (0287h) and bit-wise shifted left six places. The value to be programmed into the DAC reg- ister is A1C0h. In summary, the DS1086L is programmed as follows: PRESCALER = 0080h OFFSET = OS - 2 or 10h (if range was read as 12h) DAC = A1C0h Notice that the DAC value was rounded. Unfortunately, this means that some error is introduced. To calculate how much error, a combination of Equation 1 and Equation 3 is used to calculate the expected output fre- quency. See Equation 5. f MIN FREQUENCY OF SELECTED OFFSET RANGE DAC VALUE x kHz STEP SIZE prescaler f MHz x kHz MHz MHz OUTPUT OUTPUT ( ) ( ) (. ) ( ) . . = + = + = = 5 41 0 647 5 4 44 235 4 11 05875 DAC VALUE f MIN FREQUENCY OF SELECTED OFFSET RANGE kHz STEP SIZE DAC VALUE MHz MHz kHz STEP SIZE decimal MASTER OSCILLATOR ( ) (. . ) . ( ) = − = − =≈ 5 44 2368 41 0 5 647 36 647 f f prescaler f DESIRED MASTER OSCILLATOR MASTER OSCILLATOR X = = 2 (1) (4) (2) (3) (5) |
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