5–12
Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family
Clock Feedback Modes
1
Input and output delays are fully compensated by the PLL only when you are using
the dedicated clock input pins associated with a given PLL as the clock sources. For
example, when using PLL1 in normal mode, the clock delays from the input pin to the
PLL and the PLL clock output-to-destination register are fully compensated, provided
that the clock input pin is one of the following four pins:
■
■
■
■
CLK0
CLK1
CLK2
CLK3
When driving the PLL using the GCLK network, the input and output delays may not
be fully compensated in the Quartus II software.
Source-Synchronous Mode
If the data and clock arrive at the same time at the input pins, the phase relationship
between the data and clock remains the same at the data and clock ports of any I/O
element input register.
Figure 5–8 shows an example waveform of the data and clock in this mode. Use this
mode for source-synchronous data transfers. Data and clock signals at the I/O
element experience similar buffer delays as long as the same I/O standard is used.
Figure 5–8. Phase Relationship Between Data and Clock in Source-Synchronous Mode
Data pin
PLL reference
clock at input pin
Data at register
Clock at register
Source-synchronous mode compensates for delay of the clock network used,
including any difference in the delay between the following two paths:
■
Data pin to I/O element register input
■
Clock input pin to the PLL phase-frequency detector (PFD) input
1
Set the input pin to the register delay chain in the I/O element to zero in the
Quartus II software for all data pins clocked by a source-synchronous mode PLL.
Also, all data pins must use the PLL COMPENSATED logic option in the Quartus II
software.
Cyclone III Device Handbook
Volume 1
July 2012 Altera Corporation
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