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PCI CD/CDa Configurable DMA Interface User’s Guide Generating an Output Clock

EDT, Inc. May 2007 11

The formula for calculating the output frequency is:

out

= (N * V * f

xtal

) / (m * R * H * L * X * 2)

Figure 1. Setting the Output Clock Frequency

For example, for an output clock of15 MHz using a PCI CD-20 (f

xtal

= 10 MHz), the following numbers

work, although they are not unique: N=60, V=1, M=10, R=2, H=1, L=1, X=1

For an output clock of 125 Hz in a PCI CD-20 (f

xtal

= 10 MHz): N=50, V=1, M=10, R=8, H=5, L=50,

X=100

The output clock has a wide range of values, but the frequency limitations at different stages limits the

ultimate ability to exactly reach any specific frequency.

For example, the PLL reference frequency can be as low as 200 KHz, which would seem to allow steps

of 200,000 in the VCO output. Unfortunately, since the maximum VCO output is 50 MHz and the n-

programmable divider only goes to 127, the loop cannot lock unless V is set to 8, giving 1.6 MHz

minimum steps. If, however, R is set to 8, we can get 200 KHz steps at f

xilinx

. The lowest frequency in

this case is at N=32 (6.4 MHz) to N=127 (25.4 MHz), in 200 KHz steps.

The following three library routines, documented in the EDT DMA Software Library, help compute the

output clock frequency:

edt_find_vco_frequency

Computes the phase-locked loop parameters necessary to match or approximate

the supplied target frequency.

edt_set_pll_clock

Sets the phase-locked loop circuit to the value computed by

edt_find_vco_frequency. Includes debugging information.

edt_set_out_clock

Sets the phase-locked loop circuit to the value computed by

edt_find_vco_frequency. Does not include debugging information.

crystal

oscillator

ref divider

M=3 to 127

VCO output div

R=1,2,4, or 8

feedback prescale

V=1 or 8

feedback div

N=3 to 127

ref

fback

VCO

vco

xilinx

high-speed odd div

H=1,3,5, or 7

low

1st divide by n

L=1–64

2nd divide by n

X=1–256

divide by 2

for clock symmetry

out

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 39 40

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