DS1086L Datasheet, PDF (12/16 Pages) Maxim Integrated Products – 3.3V Spread-Spectrum EconOscillator

3.3V Spread-Spectrum EconOscillator
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 2^x
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.
f_{DESIRED =} ^{fMASTER OSCILLATOR} =
prescaler
(1)
f_{MASTER OSCILLATOR}
2^X
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 = f_DESIRED x 2^X
fMASTER OSCILLATOR = 11.0592MHz x 22 = 44.2368MHz ⁽²⁾
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)
(3)
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.
(f_{MASTER OSCILLATOR} −
(4)
MIN FREQUENCY OF SELECTED
DAC VALUE =
OFFSET RANGE)
5kHz STEP SIZE
DAC VALUE = ^{(44.2368MHz − 41.0MHz)}
5kHz STEP SIZE
= 647.36 ≈ 647 (decimal)
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.
(MIN FREQUENCY OF SELECTED OFFSET ₍₅₎
f_OUTPUT
=
RANGE) + (DAC
VALUE x 5kHz STEP SIZE)
prescaler
f_OUTPUT
= ^{(41.0MHz) + (647 x 5kHz)} =
4
44.235MHz _{= 11.05875MHz}
4
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