PRELIMINARY
CY7C9915
Document #: 38-07687 Rev. *A
Page 5 of 14
proper selection of the xFn inputs. For example a +10 tU
between REF and 3Qx can be achieved by connecting 1Q0 to
FB and setting 1F0 = 1F1 = GND, 3F0 = MID, and 3F1 = High.
(Since FB aligns at –4 tU and 3Qx skews to +6 tU, a total of
+10 tU skew is realized.) Many other configurations can be
realized by skewing both the output used as the FB input and
skewing the other outputs.
Figure 4 shows an example of the invert function of the
LVPSCB. In this example the 4Q0 output used as the FB input
is programmed for invert (4F0 = 4F1 = HIGH) while the other
three pairs of outputs are programmed for zero skew. When
4F0 and 4F1 are tied HIGH, 4Q0 and 4Q1 become inverted
zero phase outputs. The PLL aligns the rising edge of the FB
input with the rising edge of the REF. This causes the 1Q, 2Q,
and 3Q outputs to become the “inverted” outputs with respect
to the REF input. By selecting which output is connect to FB,
it is possible to have 2 inverted and 6 non-inverted outputs or
6 inverted and 2 non-inverted outputs. The correct configu-
ration would be determined by the need for more (or fewer)
inverted outputs. 1Q, 2Q, and 3Q outputs can also be skewed
to compensate for varying trace delays independent of
inversion on 4Q.
Figure 5 illustrates the LVPSCB configured as a clock multi-
plier. The 3Q0 output is programmed to divide by four and is
fed back to FB. This causes the PLL to increase its frequency
until the 3Q0 and 3Q1 outputs are locked at 20 MHz while the
1Qx and 2Qx outputs run at 80 MHz. The 4Q0 and 4Q1
outputs are programmed to divide by two, which results in a
40-MHz waveform at these outputs. Note that the 20- and
40-MHz clocks fall simultaneously and are out of phase on
their rising edge. This will allow the designer to use the rising
edges of the 1
⁄2 frequency and 1⁄4 frequency outputs without
concern for rising-edge skew. The 2Q0, 2Q1, 1Q0, and 1Q1
outputs run at 80 MHz and are skewed by programming their
select inputs accordingly. Note that the FS pin is wired for
80-MHz operation because that is the frequency of the fastest
output.
Figure 6 demonstrates the LVPSCB in a clock divider appli-
cation. 2Q0 is fed back to the FB input and programmed for
zero skew. 3Qx is programmed to divide by four. 4Qx is
programmed to divide by two. Note that the falling edges of the
4Qx and 3Qx outputs are aligned. This allows use of the rising
edges of the 1
⁄2 frequency and 1⁄4 frequency without concern
for skew mismatch. The 1Qx outputs are programmed to zero
skew and are aligned with the 2Qx outputs. In this example,
the FS input is grounded to configure the device in the 15- to
30-MHz range since the highest frequency output is running at
20 MHz.
Figure 7 shows some of the functions that are selectable on
the 3Qx and 4Qx outputs. These include inverted outputs and
outputs that offer divide-by-2 and divide-by-4 timing. An
inverted output allows the system designer to clock different
subsystems on opposite edges, without suffering from the
pulse asymmetry typical of non-ideal loading. This function
allows the two subsystems to each be clocked 180 degrees
out of phase, but still to be aligned within the skew spec.
The divided outputs offer a zero-delay divider for portions of
the system that need the clock to be divided by either two or
four, and still remain within a narrow skew of the “1X” clock.
Without this feature, an external divider would need to be
added, and the propagation delay of the divider would add to
the skew between the different clock signals.
These divided outputs, coupled with the Phase Locked Loop,
allow the LVPSCB to multiply the clock rate at the REF input
by either two or four. This mode will enable the designer to
distribute a low-frequency clock between various portions of
the system, and then locally multiply the clock rate to a more
suitable frequency, while still maintaining the low-skew charac-
teristics of the clock driver. The LVPSCB can perform all of the
functions described above at the same time. It can multiply by
Figure 4. Inverted Output Connections
Figure 5. Frequency Multiplier with Skew Connections
FB
REF
FS
4F0
4F1
3F0
3F1
2F0
2F1
1F0
1F1
4Q0
4Q1
3Q0
3Q1
2Q0
2Q1
1Q0
1Q1
TEST
REF
FB
REF
FS
4F0
4F1
3F0
3F1
2F0
2F1
1F0
1F1
4Q0
4Q1
3Q0
3Q1
2Q0
2Q1
1Q0
1Q1
TEST
REF
20 MHz
20 MHz
40 MHz
80 MHz
Figure 6. Frequency Divider Connections
FB
REF
FS
4F0
4F1
3F0
3F1
2F0
2F1
1F0
1F1
4Q0
4Q1
3Q0
3Q1
2Q0
2Q1
1Q0
1Q1
TEST
REF
20 MHz
5 MHz
10 MHz
20 MHz
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