5–24
Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family
PLL Cascading
Figure 5–18 shows an example of phase shift insertion using fine resolution through
VCO phase taps method. The eight phases from the VCO are shown and labeled for
reference. In this example, CLK0is based on 0° phase from the VCO and has the C
value for the counter set to one. The CLK1signal is divided by four, two VCO clocks
for high time and two VCO clocks for low time. CLK1is based on the 135° phase tap
from the VCO and has the C value for the counter set to one. The CLK1signal is also
divided by four. In this case, the two clocks are offset by 3 Φfine. CLK2is based on the
0° phase from the VCO but has the C value for the counter set to three. This creates a
delay of two Φcoarse (two complete VCO periods).
Figure 5–18. Delay Insertion Using VCO Phase Output and Counter Delay Time
1/8 t
t
VCO
VCO
0
45
90
135
180
225
270
315
CLK0
t
d0-1
CLK1
CLK2
t
d0-2
You can use the coarse and fine phase shifts to implement clock delays in the
Cyclone III device family.
The Cyclone III device family supports dynamic phase shifting of VCO phase taps
only. The phase shift is configurable for any number of times. Each phase shift takes
about one scanclkcycle, allowing you to implement large phase shifts quickly.
PLL Cascading
Two PLLs are cascaded to each other through the clock network. If your design
cascades PLLs, the source (upstream) PLL must have a low-bandwidth setting, while
the destination (downstream) PLL must have a high-bandwidth setting.
Cyclone III Device Handbook
Volume 1
December 2011 Altera Corporation
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