SM5168 series

SEIKO NPC CORPORATION —7

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.

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.

FV or FR

TR

Decoder output

TR = LOW frequency divider

Counter set signal

TR = HIGH divider operation starts

TR = HIGH frequency divider

TR = LOW frequency

divider counter set

TR = HIGH frequency

divider counter set