MPC973 View Datasheet(PDF) - Motorola => Freescale
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MPC973 Datasheet PDF : 16 Pages
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MPC973
Using the MPC973 as a Zero Delay Buffer
The external feedback of the MPC973 clock driver allows for
its use as a zero delay buffer. By using one of the outputs as a
feedback to the PLL the propagation delay through the device
is eliminated. The PLL works to align the output edge with the
input reference edge thus producing a near zero delay. The
reference frequency affects the static phase offset of the PLL
and thus the relative delay between the inputs and outputs.
Because the static phase offset is a function of the reference
clock the Tpd of the MPC973 is a function of the configuration
used.
When used as a zero delay buffer the MPC973 will likely be
in a nested clock tree application. For these applications the
MPC973 offers a LVPECL clock input as a PLL reference. This
allows the user to use LVPECL as the primary clock distribution
device to take advantage of its far superior skew performance.
The MPC973 then can lock onto the LVPECL reference and
translate with near zero delay to low skew LVCMOS outputs.
Clock trees implemented in this fashion will show significantly
tighter skews than trees developed from CMOS fanout buffers.
To calculate the overall uncertainty between the input
reference clock and the output clocks the following approach
should be used. Figure 4 through 7 contains performance
information to assist in calculating the overall uncertainty. Data
presented in Figures 4 through 7 is representative data but is
not guaranteed under all conditions. Since the overall skew
performance is a function of the input reference frequency all
of the graphs provide relavent data with respect to the input
reference frequency.
The overall uncertainty can be broken down into three parts;
the static phase offset variation (Tpd), the I/O phase jitter and
the output skew. If we assume that we have a 75 MHz reference
clock, from the graphs we can pull the following information for
static phase offset (SPO) and I/O jitter: the SPO variation will
be 300 ps (–100 ps to +200 ps assuming a TCLK is used) and
the I/O jitter will be ±105 ps (assuming a VCO/6 configuration
and a ±3 sigma for min and max). The nominal delay from
Figure 5 is 50 ps so that the propagation delay between the
reference clock and the feedback clock is 50 ps ±255 ps.
Figure 4 can now be used to establish the uncertainty
between the reference clock and all of the outputs for the
MPC973. Figure 4 provides the skew of the MC973 outputs with
respect to the feedback output. From Figure 4, if all of the
outputs are used the propagation delay of the device will range
from –555 ps (50 ps – 255 ps – 350 ps) to +705 ps (50 ps +
255 ps + 400 ps) for a total uncertainty of 1.26 ns. This 1.26ns
uncertainty would hold true if multiple 973’s are used in parallel
in the application given that the skew between the reference
clock for the devices were zero. Notice from the data in Figure
4 that if a subset of the outputs were used significant reductions
in uncertainty could be obtained.
SYNC Output Description
In situations where output frequency relationships are not
integer multiples of each other there is a need for a signal for
system synchronization purposes. The SYNC output of the
MPC973 is designed to specifically address this need. The
MPC973 monitors the relationship between the Qa and the Qc
banks of outputs. It provides a low going pulse, one period in
duration, one period prior to the coincident rising edges of the
Qa and Qc outputs. The duration and the placement of the
pulse is dependent on the higher of the Qa and Qc output
frequencies. The timing diagrams in the data sheet show the
various waveforms for the SYNC output. Note that the SYNC
output is defined for all possible combinations of the Qa and Qc
outputs even though under some relationships the lower
frequency clock could be used as a synchronizing signal.
1. Programmable Output Frequency Relationships (VCO_Sel=‘1’)
fsela1
fsela0
Qa
fselb1
fselb0
Qb
0
0
VCO/4
0
0
VCO/4
0
1
VCO/6
0
1
VCO/6
1
0
VCO/8
1
0
VCO/8
1
1
VCO/12
1
1
VCO/10
fselc1
0
0
1
1
fselc0
0
1
0
1
Qc
VCO/2
VCO/4
VCO/6
VCO/8
2. Programmable Output Frequency Relationships (VCO_Sel=‘1’)
fselFB2
fselFB1
fselFB0
0
0
0
0
0
1
0
1
0
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
QFB
VCO/4
VCO/6
VCO/8
VCO/10
VCO/8
VCO/12
VCO/16
VCO/20
MOTOROLA
7
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