參數(shù)資料
型號: MPC9774FA
廠商: FREESCALE SEMICONDUCTOR INC
元件分類: 時(shí)鐘及定時(shí)
英文描述: 9774 SERIES, PLL BASED CLOCK DRIVER, 14 TRUE OUTPUT(S), 0 INVERTED OUTPUT(S), PQFP52
封裝: PLASTIC, LQFP-52
文件頁數(shù): 8/10頁
文件大小: 186K
代理商: MPC9774FA
MPC9774
230
FREESCALE SEMICONDUCTOR ADVANCED CLOCK DRIVERS DEVICE DATA
Using the MPC9774 in Zero-Delay Applications
Nested clock trees are typical applications for the MPC9774.
Designs using the MPC9774 as LVCMOS PLL fanout buffer
with zero insertion delay will show significantly lower clock skew
than clock distributions developed from CMOS fanout buffers.
The external feedback of the MPC9774 clock driver allows for
its use as a zero delay buffer. The PLL aligns the feedback clock
output edge with the clock input reference edge resulting a near
zero delay through the device (the propagation delay through
the device is virtually eliminated). The maximum insertion delay
of the device in zero-delay applications is measured between
the reference clock input and any output. This effective delay
consists of the static phase offset, I/O jitter (phase or long-term
jitter), feedback path delay and the output-to-output skew error
relative to the feedback output.
Calculation of Part-to-Part Skew
The MPC9774 zero delay buffer supports applications where
critical clock signal timing can be maintained across several
devices. If the reference clock inputs of two or more MPC9774
are connected together, the maximum overall timing uncertainty
from the common CCLK input to any output is:
tSK(PP) = t() + tSK(O) + tPD, LINE(FB) + tJIT() CF
This maximum timing uncertainty consist of 4 components:
static phase offset, output skew, feedback board trace delay
and I/O (phase) jitter:
Figure 5. MPC9774 Maximum Device-to-Device Skew
Due to the statistical nature of I/O jitter a RMS value (1
σ) is
specified. I/O jitter numbers for other confidence factors (CF)
can be derived from Table 10.
The feedback trace delay is determined by the board layout
and can be used to fine-tune the effective delay through each
device.
Due to the frequency dependence of the static phase offset
and I/O jitter, using Figure 6 and Figure 7 to predict a maximum
I/O jitter and the specified t() parameter relative to the input
reference frequency results in a precise timing performance
analysis.
In the following example calculation a I/O jitter confidence
factor of 99.7% (
± 3σ) is assumed, resulting in a worst case
timing uncertainty from the common input reference clock to
any output of –470 ps to +320 ps relative to CCLK (PLL
feedback =
÷ 8, reference frequency = 50 MHz, VCO
frequency = 400 MHz, I/O jitter = 15 ps RMS max., static phase
offset t() = –250 ps to +100 ps):
tSK(PP) = [–250 ps...+100 ps] + [-175 ps...175 ps] +
[(15 ps –3)...(15 ps 3)] + tPD, LINE(FB)
tSK(PP) = [–470 ps...+320 ps] + tPD, LINE(FB)
Figure 6. MPC9774 I/O Jitter
Figure 7. MPC9774 I/O Jitter
tPD,LINE(FB)
tJIT()
±tSK(O)
–t()
+t()
tJIT()
±tSK(O)
tSK(PP)
Max. skew
CCLKCommon
QFBDevice 1
Any QDevice 1
QFBDevice2
Any QDevice 2
Table 10. Confidence Factor CF
CF
Probability of Clock Edge within the Distribution
± 1σ
0.68268948
± 2σ
0.95449988
± 3σ
0.99730007
± 4σ
0.99993663
± 5σ
0.99999943
± 6σ
0.99999999
VCO Frequency [MHz]
200
250
300
350
400
450
500
100
80
60
40
20
0
FB =
÷ 8
FB =
÷ 32
FB =
÷ 16
Maximum I/O Phase Jitter (RMS) versus Frequency Parameter:
PLL Feedback Divider FB
t jit[
φ]
[p
s]
RM
S
t jit[
φ]
[p
s]
RMS
VCO Frequency [MHz]
200
250
300
350
400
450
500
120
100
80
60
40
20
0
FB =
÷ 12
FB =
÷ 24
Maximum I/O Phase Jitter (RMS) versus Frequency Parameter:
PLL Feedback Divider FB
FB =
÷ 48
160
140
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