SY89429 查看數據表（PDF） - Micrel
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SY89429 Datasheet PDF : 9 Pages
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Micrel
ClockWorks™
SY89429A
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:
FOUT
=
(
FXTAL
8
)
x
M
N
Where FXTAL is the crystal frequency, M is the loop divider
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.
T2 T1 T0
000
001
010
011
100
101
110
111
TEST
Data Out – Last Bit SR
HIGH
FREF
M Counter Output
FOUT
LOW
S_CLOCK ÷ M
FOUT ÷ 4
FOUT / FOUT
FVCO ÷ N
FVCO ÷ N
FVCO ÷ N
FVCO ÷ N
FVCO ÷ N
FVCO ÷ N
S_CLOCK ÷ N
FVCO ÷ N
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
bits of the data stream on the S_DATA input. For each register
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
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 (See detailed Block Diagram).
Because the S_CLOCK is a TTL 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.
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
First
Last
Bit
Bit
M,N
Input S_DATA to M0 then M1, then M2, etc., as indicated above.
5
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