Cypress Semiconductor CY7B991 Технический паспорт - Страница 12

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Operational Mode Descriptions

SYSTEM

CLOCK

Figure 2

shows the PSCB configured as a zero skew clock buffer. In this mode the 7B991/992 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 drives 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.

SYSTEM

CLOCK

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. Each of the four groups of output pairs are 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 fed

Document Number: 38-07138 Rev. *B

Figure 2. Zero Skew and Zero Delay Clock Driver

REF

4Q0

4F0

4Q1

4F1

3Q0

3F0

3Q1

3F1

2F0

2Q0

2F1

2Q1

1F0

1Q0

1F1

1Q1

TEST

LENGTH L1 = L2 = L3 = L4

Figure 3. Programmable Skew Clock Driver

REF

4Q0

4F0

4Q1

4F1

3Q0

3F0

3Q1

3F1

2F0

2Q0

2F1

2Q1

1F0

1Q0

1F1

1Q1

TEST

LENGTH L1 = L2

L3 < L2 by 6 inches

L4 > L2 by 6 inches

back 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 can 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 synchronizes

CY7B991

CY7B992

LOAD

Page 12 of 19

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