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DOC E10523 Rev.D
Interactive Circuits And Systems Ltd. 2000
47
The result:
Fout = 12.8 = (2*14.31818*(P+3)/(Q+2))/2
M
where M = 0, 1, 2, 3, 4, 5, 6, 7
since M = 2: (P+3)/(Q+2) = 1.787936735
The two choices of P and Q giving the nearest to the required frequency are:
P
Q
f
VCO
Error (PPM)
56
31
51.19834
32
115
64
51.19834
32
90
50
51.21503
293
Taking the first set of values, i.e. (P,Q) = (56,31):
P = 56 decimal = 0111000 binary = 0(0)111000
Q = 31 decimal = 0011111 binary = 0011(0)111
NOTE: The presence of three ones in a row in both P and Q values causes zero bit-stuff
values to be inserted in each. However, it is necessary to examine the values in the
previous (I) field before inserting a zero in the least significant bits of the Q value. If the I
field had contained a 1 in the most significant position the inserted zero would have been in
a different position.
The full programming word, W is therefore:
W = P, D, M, Q, I = 00111000, 0, 010, 00110111, 0000 =
001110000010001101110000 (382370 Hex)
6.2.7 Oscillator Programming Example
The oscillator requires three control words plus the programming word in order to program
a new output frequency (see programming sequence given in section 6.2.2 above). The
following paragraphs provide an overview of how the control words are build along side the
programming word.
All data is written to the oscillator serially through the ADC Clock Frequency or FPDP Clock
Frequency register bit 0. The data is written least significant bit first. An example of
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