Cypress Semiconductor CY7B991 Lembar Spesifikasi - Halaman 13

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the FB and REF inputs and aligns their rising edges to ensure

that all outputs have precise phase alignment.

Clock skews are advanced by ±6 time units (tU) when using an

output selected for zero skew as the feedback. A wider range of

delays is possible if the output connected to FB is also skewed.

Since "Zero Skew", +tU, and –tU are defined relative to output

groups, and since the PLL aligns the rising edges of REF and

FB, you can create wider output skews by proper selection of the

xFn inputs. For example, a +10 tU between REF and 3Qx is

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 are realized by skewing both the outputs used as

the FB input and skewing the other outputs.

Figure 4. Inverted Output Connections

REF

4Q0

4F0

4Q1

4F1

3Q0

3F0

3Q1

3F1

2Q0

2F0

2Q1

2F1

1Q0

1F0

1Q1

1F1

TEST

Figure 4

shows an example of the invert function of the PSCB.

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.

It is possible to have 2 inverted and 6 non-inverted outputs or 6

inverted and 2 non-inverted outputs by selecting the output

connected to FB. The correct configuration is 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.

Document Number: 38-07138 Rev. *B

REF

Figure 5. Frequency Multiplier with Skew Connectrions

REF

20 MHz

REF

4Q0

4F0

4Q1

4F1

3Q0

3F0

3Q1

3F1

2Q0

2F0

2Q1

2F1

1Q0

1F0

1Q1

1F1

TEST

Figure 5

shows the PSCB configured as a clock multiplier. The

3Q0 output is programmed to divide by four and is sent 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, that results in a 40 MHz waveform

at these outputs. Note that the 20 and 40 MHz clocks fall simul-

taneously and are out of phase on their rising edge. This enables

the designer to use the rising edges of the

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. Frequency Divider Connections

20 MHz

REF

4Q0

4F0

4Q1

4F1

3Q0

3F0

3Q1

3F1

2F0

2Q0

2Q1

2F1

1F0

1Q0

1F1

1Q1

TEST

Figure 6

demonstrates the PSCB in a clock divider application.

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 enables the use of rising edges of the

⁄

frequency and

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 MHz to 30 MHz

CY7B991

CY7B992

40 MHz

20 MHz

80 MHz

⁄

frequency and

REF

10 MHz

5 MHz

20 MHz

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