MPC970

TIMING SOLUTIONS

BR1333 — Rev 6

9

MOTOROLA

output divide ratio. If the VCO_Sel pin is “LOW” the

multiplication factor will be reduced further by 2. (See “Using

the On–Board Crystal Oscillator” section of this datasheet.)

Using the External Feedback Feature of the MPC970/71

In applications where the relationship between the output

waveforms and the input waveforms are critical the external

feedback option will likely be used. Table 1 and Table 2 are

still appropriate for establishing the potential output

frequency relationships. The input reference frequency for

external feedback applications will be equal to the frequency

of the feedback signal. As a result the use of the external

feedback yields a number of potential input to output

frequency multiplication factors which are not available using

the internal feedback. Using the external feedback the device

can function as a zero delay buffer and could multiply the

input from 4 to as much as 48. In practice however the

multiplication factor is limited by the loop dynamics of the

PLL. The MPC970 PLL was optimized for an input reference

frequency or greater than 10MHz. Frequencies lower than

10MHz will tend to pass through the filter and add jitter to the

PLL. In addition the PLL was optimized for feedback divide

ratios of between 8 and 64. The user should avoid using the

device with feedback divide ratios outside of this range. For

the external feedback case the feedback divide ratio will

include the

÷2 (if VCO_Sel is LOW) plus the output divider for

the feedback output. If, for instance the MPC970 is to be

used as a zero delay buffer the VCO_Sel pin should be pulled

LOW and all of the outputs should be set in a VCO/4 mode.

This would produce a feedback ratio of

÷8. Several potential

configurations using the external feedback are pictured in

Figure 4 through Figure 7.

The external feedback option of the MPC970 is critical for

applications in which more than one clock driver need to be

synchronized. The external feedback option ensures that the

feed through delay is the same as the feedback delay. This

functionality removes propagation delay variation as a factor

in the determination of part to part skew. The low jitter PLL

used in the MPC970 has a near zero deadband phase

detector and very little part to part variability. The result is a

very low phase error variability in the product. When coupled

with the output to output skew the phase error variability

accounts for the part to part skew of the device. From the

specification table one sees that the worst case part to part

skew of the device is 800ps, assuming that there is zero

skew in the multiple reference inputs. For multiple MPC970

applications if the lowest generated output frequency is used

as the feedback signal the devices will be guaranteed to be

synchronized. For applications where the lowest frequency is

not used as the reference or where the internal feedback is

used there is no way to guarantee that the multiple devices

will be phase synchronized.

Table 1. Programmable Output Frequency Relationships (MPC601_Clks = ‘HIGH’; VCO_Sel = ‘HIGH’)

INPUTS

OUTPUTS

PCI_Div1

PCI_Div0

BCLK_Div1

BCLK_Div0

2x_PCLK

PCLKEN

BCLKEN*

BCLK

PCI_CLK

0

0

0

0

VCO/2

VCO/4

VCO/4

VCO/4

VCO/4

0

0

0

1

VCO/2

VCO/4

VCO/8

VCO/8

VCO/8

0

0

1

0

VCO/2

VCO/4

VCO/12

VCO/12

VCO/12

0

0

1

1

VCO/2

VCO/4

VCO/16

VCO/16

VCO/16

0

1

0

0

VCO/2

VCO/4

VCO/4

VCO/4

VCO/8

0

1

0

1

VCO/2

VCO/4

VCO/8

VCO/8

VCO/16

0

1

1

0

VCO/2

VCO/4

VCO/12

VCO/12

VCO/24

0

1

1

1

VCO/2

VCO/4

VCO/16

VCO/16

VCO/32

1

0

0

0

VCO/2

VCO/4

VCO/4

VCO/4

VCO/12

1

0

0

1

VCO/2

VCO/4

VCO/8

VCO/8

VCO/24

1

0

1

0

VCO/2

VCO/4

VCO/12

VCO/12

VCO/36

1

0

1

1

VCO/2

VCO/4

VCO/16

VCO/16

VCO/48

1

1

0

0

VCO/2

VCO/4

VCO/4

VCO/4

VCO/4

1

1

0

1

VCO/2

VCO/4

VCO/8

VCO/8

VCO/4

1

1

1

0

VCO/2

VCO/4

VCO/12

VCO/12

VCO/4

1

1

1

1

VCO/2

VCO/4

VCO/16

VCO/16

VCO/4

* BCLK_En output is delayed relative to other outputs