5

SY89429A

Micrel, Inc.

M,N

M[ 8 :0 ]

N[1 :0 ]

T 2 T 1 T 0 N1 N0 M8 M7 M6 M5 M4 M3 M2 M1 M0

PROGRAMMING INTERFACE

Programming the device is accomplished by properly

configuring the internal dividers to produce the desired

frequency at the outputs. The output frequency can be

represented by this formula:

modulus, and N is the output divider modulus. Note that it is

possible to select values of M such that the PLL is unable to

achieve loop lock. To avoid this, always make sure that M is

selected to be 200 M 400 for a 16MHz input reference.

M[8:0] and N[1:0] are normally specified once at power-on,

through the parallel interface, and then possibly again through

the serial interface. This approach allows the designer to

bring up the application at one frequency and then change or

fine-tune the clock, as the ability to control the serial interface

becomes available. To minimize transients in the frequency

domain, the output should be varied in the smallest step size

possible.

The TEST output provides visibility for one of several

internal nodes (as determined by the T[1:0] 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 TTL output may not be able to

toggle fast enough for some of the higher output frequencies.

The T2, T1, T0 configuration latches are preset to 000 when

P_LOAD is low, so that the FOUT outputs are as jitter-free as

possible. The serial configuration port can be used to select

one of the alternate functions for this pin.

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

the most significant bit is loaded first (T2, N1 and M8).

When T[2:0] is set to 100 the SY89429A is placed in PLL

into the M and N dividers. The N divider drives the FOUT

differential pair and the M counter drives the TEST output

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 (See detailed

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

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.

First

Bit

Last

Bit

T2

T1

T0

TEST

FOUT/FOUT

0

0

0

Data Out – Last Bit SR

FVCO ÷ N

0

0

1

HIGH

FVCO ÷ N

0

1

0

FREF

FVCO ÷ N

0

1

1

M Counter Output

FVCO ÷ N

1

0

0

FOUT

FVCO ÷ N

1

0

1

LOW

FVCO ÷ N

1

1

0

1

1

1

FOUT ÷ 4

FVCO ÷ N