Chapter 5: Clock Networks and PLLs in Cyclone IV Devices
5–33
PLL Cascading
October 2012
Altera Corporation
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,
CLK0
is based on 0°
phase from the VCO and has the C
value for the counter set to one. The
CLK1
signal is divided by four, two VCO clocks
for high time and two VCO clocks for low time.
CLK1
is based on the 135° phase tap
from the VCO and has the C value for the counter set to one. The
CLK1
signal is also
divided by four. In this case, the two clocks are offset by 3
fine
.
CLK2
is 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
coarse
(two complete VCO periods).
You can use the coarse and fine phase shifts to implement clock delays in
Cyclone IV devices.
Cyclone IV devices support dynamic phase shifting of VCO phase taps only. The
phase shift is configurable for any number of times. Each phase shift takes about one
scanclk
cycle, allowing you to implement large phase shifts quickly.
PLL Cascading
Cyclone IV devices allow cascading between general purpose PLLs and multipurpose
PLLs in normal or direct mode through the GCLK 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.
1
PLL_6
and
PLL7
have upstream cascading capability only.
1
PLL cascading is not supported when used in transceiver applications.
Figure 5–21. Delay Insertion Using VCO Phase Output and Counter Delay Time
t
d0-1
t
d0-2
1/8 t
VCO
t
VCO
0
90
135
1
8
0
225
270
315
CLK0
CLK1
CLK2
45
Summary of Contents for Cyclone IV
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