CY7B9911
RoboClock+™
Document Number: 38-07209 Rev. *C
Page 9 of 13
Operational Mode Descriptions
Figure 2 shows the PSCB configured as a zero skew clock buffer. In this mode the 7B9911 is used as the basis for a low skew clock
distribution tree. When all of the function select inputs (xF0, xF1) are left open, the outputs are aligned and each drive a terminated
transmission line to an independent load. The FB input is tied to any output in this configuration and the operating frequency range
is selected with the FS pin. The low skew specification, coupled with the ability to drive terminated transmission lines (with impedances
as low as 50 ohms), enables efficient printed circuit board design.
Figure 3 shows a configuration to equalize skew between metal
traces of different lengths. In addition to low skew between
outputs, the PSCB is programmed to stagger the timing of its
outputs. The four groups of output pairs are each programmed
to different output timing. Skew timing is adjusted over a wide
range in small increments with the appropriate strapping of the
function select pins. In this configuration the 4Q0 output is sent
to FB and configured for zero skew. The other three pairs of
outputs are programmed to yield different skews relative to the
feedback. By advancing the clock signal on the longer traces or
retarding the clock signal on shorter traces, all loads receive the
clock pulse at the same time.
In this illustration the FB input is connected to an output with 0
ns skew (xF1, xF0 = MID) selected. The internal PLL synchro-
nizes the FB and REF inputs and aligns their rising edges to
make certain that all outputs have precise phase alignment.
Clock skews is advanced by ±6 time units (tU) when using an
output selected for zero skew as the feedback. There is a wider
Figure 3. Zero Skew and Zero Delay Clock Driver
SYSTEM
CLOCK
L1
L2
L3
L4
LENGTH L1 = L2 = L3 = L4
FB
REF
FS
4F0
4F1
3F0
3F1
2F0
2F1
1F0
1F1
4Q0
4Q1
3Q0
3Q1
2Q0
2Q1
1Q0
1Q1
TEST
Z0
LOAD
LOAD
LOAD
LOAD
REF
Z0
Z0
Z0
Figure 4. Programmable Skew Clock Driver
LENGTH L1 = L2
L3 < L2 by 6 inches
L4 > L2 by 6 inches
SYSTEM
CLOCK
L1
L2
L3
L4
FB
REF
FS
4F0
4F1
3F0
3F1
2F0
2F1
1F0
1F1
4Q0
4Q1
3Q0
3Q1
2Q0
2Q1
1Q0
1Q1
TEST
Z0
LOAD
LOAD
LOAD
LOAD
REF
Z0
Z0
Z0
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