SM5168 Просмотр технического описания (PDF) - Nippon Precision Circuits
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SM5168 Datasheet PDF : 9 Pages
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SM5168 series
SM5168C× series
Frequency Dividers
The comparator frequency divider (N-counter) and reference frequency divider (R-counter), with 2 sets of
comparator frequency divider and reference frequency divider ratios, are set in master-slice. The frequency
divider set selected is determined by the state of TR (pin 5). The ratio ranges are the same when TR is HIGH or
LOW, and are:
I Comparator frequency divider (N-counter) = 272 to 65535
I Reference frequency divider (R-counter) = 5 to 65535
Frequency Divider Switching
When switching the frequency dividers using TR, the dividers switch in sync with the R-counter divider signal
(FR) and the N-counter divider signal (FV) to minimize any disturbance in the PLL loop.
t1
FV or FR
TR
TR = HIGH divider operation starts
t2
t2
t3
Decoder output TR = LOW frequency divider
TR = HIGH frequency divider
Counter set signal
TR = LOW frequency
divider counter set
TR = HIGH frequency
divider counter set
If tF represents the FIN cycle time:
t1 = 48tF
t2 = (divider ratio when TR = LOW) × tF
t3 = (divider ratio when TR = HIGH) × tF
If tX represents the XIN cycle time:
t1 = 3tX
t2 = (divider ratio when TR = LOW) × tX
t3 = (divider ratio when TR = HIGH) × tX
Both the R-counter and N-counter are configured with presettable counters. The divider outputs (FR and FV)
are then input to the phase comparator which performs phase comparison on the falling edge of each signal.
The FR/FV signals also function as the R/N-counter preset strobe signals, respectively. Consequently, when the
TR signal level switches, the decoder output changes on the first FR/FV (R/N-counter preset strobe) signal and
the counters are set in the new frequency dividers on the second FR/FV (R/N-counter preset strobe) signal. Fre-
quency division with the new frequency dividers starts on the falling edge of the second FR/FV (R/N-counter
preset strobe) signal.
The timing in the diagram shows an example when TR goes from LOW to HIGH only, but the timing operation
is identical under the reverse transition. The R/N-counters operate with the same timing, although the N-
counter has a dual modulus prescaler in the initial-stage which means the HIGH-level pulsewidth of the FV and
FR signals is different.
SEIKO NPC CORPORATION —7
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