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參數(shù)資料
| 型號： | MC12439FN |
| 廠商： | FREESCALE SEMICONDUCTOR INC |
| 元件分類： | 時鐘產(chǎn)生/分配 |
| 英文描述： | 800 MHz, OTHER CLOCK GENERATOR, PQCC28 |
| 封裝： | PLASTIC, LCC-28 |
| 文件頁數(shù)： | 6/10頁 |
| 文件大?。?/td> | 155K |
| 代理商： | MC12439FN |
MC12439
374
FREESCALE SEMICONDUCTOR ADVANCED CLOCK DRIVERS DEVICE DATA
For input reference frequencies other than 16MHz, the set of
appropriate equations can be deduced from equation 1. For
computer applications, another useful frequency base would be
16.666MHz. From this reference, one can generate a family of
output frequencies at multiples of the 33.333 MHz PCI clock. As
an example, to generate a 533.333MHz clock from a
16.666MHz reference, the following M and N values would be
used:
FOUT = 16.666 x M
÷ N
Let N = 1, M = 533.333
÷ 16.666 = 32
The value for M falls within the constraints set for PLL
stability (400
÷16.666 ≤ M ≤ 800÷16.666; 24 ≤ M ≤ 48), therefore
N[1:0] = 11 and M[6:0} = 0100000. 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[6:0] 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[6:0] 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 12 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 M6).
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 3 illustrates the timing diagram
for both a parallel and a serial load of the MC12439 synthesizer.
M[6:0] 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 MC12439 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
MC12439 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 4 shows the functional setup of the PLL bypass mode.
Because the S_CLOCK is a CMOS level, the input frequency is
limited to 250 MHz or less. This means the fastest the FOUT pin
can be toggled via the S_CLOCK is 250 MHz as the minimum
divide ratio of the N counter is 1. 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.
Figure 3. Timing Diagram
T2
T1
T0
TEST (Pin 20)
0
1
0
1
0
1
0
1
0
1
0
1
0
1
SHIFT REGISTER OUT
HIGH
FREF
M COUNTER OUT/2
FOUT
LOW
M COUNTER/2 in PLL Bypass Mode
FOUT/4
T2
S_CLOCK
S_DATA
S_LOAD
M[6:0]
N[1:0]
P_LOAD
T1
T0
N1
N0
M6
M5
M4
M3
M2
M1
M0
M, N
First
Bit
Last
Bit
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