MC12429

TIMING SOLUTIONS

5

MOTOROLA

Let N = 2, M = 133.3333

÷ 1.0416 x 2 = 256

The value for M falls within the constraints set for PLL

stability, therefore N[1:0] = 01 and M[8:0} = 10000000. If the

value for M fell outside of the valid range a different N value

would be selected to try to move M in the appropriate

direction.

The M and N counters can be loaded either through a

parallel or serial interface. The parallel interface is controlled

via the P_LOAD signal such that a LOW to HIGH transition

will latch the information present on the M[8:0] and N[1:0]

inputs into the M and N counters. When the P_LOAD signal is

LOW the input latches will be transparent and any changes

on the M[8:0] and N[1:0] inputs will affect the FOUT output

pair. To use the serial port the S_CLOCK signal samples the

information on the S_DATA line and loads it into a 14 bit shift

register. Note that the P_LOAD signal must be HIGH for the

serial load operation to function. The Test register is loaded

with the first three bits, the N register with the next two and

the M register with the final eight bits of the data stream on

the S_DATA input. For each register the most significant bit is

loaded first (T2, N1 and M8). A pulse on the S_LOAD pin

after the shift register is fully loaded will transfer the divide

values into the counters. The HIGH to LOW transition on the

S_LOAD input will latch the new divide values into the

counters. Figure 4 illustrates the timing diagram for both a

parallel and a serial load of the MC12429 synthesizer.

M[8:0] and N[1:0] are normally specified once at power–up

through the parallel interface, and then possibly again

through the serial interface. This approach allows the

application to come up at one frequency and then change or

fine–tune the clock as the ability to control the serial interface

becomes available.

The TEST output provides visibility for one of the several

internal nodes as determined by the T[2:0] bits in the serial

configuration stream. It is not configurable through the

parallel interface. Although it is possible to select the node

that represents FOUT, the CMOS output may not be able to

toggle fast enough for some of the higher output frequencies.

The T2, T1 and T0 control bits are preset to ‘000’ when

P_LOAD is LOW so that the PECL FOUT outputs are as

jitter–free as possible. Any active signal on the TEST output

pin will have detrimental affects on the jitter of the PECL

output pair. In normal operations, jitter specifications are only

guaranteed if the TEST output is static. The serial

configuration port can be used to select one of the alternate

functions for this pin.

Most of the signals available on the TEST output pin are

useful only for performance verification of the MC12429

itself. However the PLL bypass mode may be of interest at

the board level for functional debug. When T[2:0] is set to 110

the MC12429 is placed in PLL bypass mode. In this mode the

S_CLOCK input is fed directly into the M and N dividers. The

N divider drives the FOUT differential pair and the M counter

drives the TEST output pin. In this mode the S_CLOCK input

could be used for low speed board level functional test or

debug. Bypassing the PLL and driving FOUT directly gives

the user more control on the test clocks sent through the

clock tree. Figure 5 shows the functional setup of the PLL

bypass mode. Because the S_CLOCK is a CMOS level the

input frequency is limited to 250MHz or less. This means the

fastest the FOUT pin can be toggled via the S_CLOCK is

125MHz as the minimum divide ratio of the N counter is 2.

Note that the M counter output on the TEST output will not be

a 50% duty cycle due to the way the divider is implemented.

T2

T1

T0

TEST (Pin 20)

0

0

0

0

1

1

1

1

0

0

1

1

0

0

1

1

0

1

0

1

0

1

0

1

SHIFT REGISTER OUT

HIGH

FREF

M COUNTER OUT

FOUT

LOW

MCNT

FOUT/4

Figure 4. Timing Diagram

S_CLOCK

S_DATA

S_LOAD

M[8:0]

N[1:0]

P_LOAD

T2

T1

T0

N1

N0

M8

M7

M6

M5

M4

M3

M2

M1

M0

M, N

First

Bit

Last

Bit