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• Reduce the clock pins used in the FPGA by synthesizing multiple clock frequencies from a single reference
clock source
• Compensate clock network delay
• Zero delay buffering
• Transmit clocking for transceivers
PLL Cascading
Cyclone V devices support two types of PLL cascading.
PLL-to-PLL Cascading
This cascading mode synthesizes a more precise output frequency than a single PLL in integer mode.
Cascading two PLLs in integer mode expands the effective range of the pre-scale counter,
N
and the multiply
counter,
M
.
Cyclone V devices only use
adjpllin
input clock source for inter-cascading between fracturable fractional
PLLs.
Altera recommends using a low bandwidth setting for the source (upstream) PLL and a high bandwidth
setting for destination (downstream) PLL.
Counter-Output-to-Counter-Output Cascading
This cascading mode synthesizes a lower frequency output than a single post-scale counter,
C
. Cascading
two
C
counters expands the effective range of
C
counters.
PLL External Clock I/O Pins
All Cyclone V external clock outputs for corner fractional PLLs (that are not from the PLL strips) are dual-
purpose clock I/O pins. Two external clock output pins associated with each corner fractional PLL are
organized as one of the following combinations:
• Two single-ended clock outputs
• One differential clock output
• Two single-ended clock outputs and one single-ended clock input in the I/O driver feedback for zero
delay buffer (ZDB) mode support
• One single-ended clock output and one single-ended feedback input for single-ended external feedback
(EFB) mode support
• One differential clock output and one differential feedback input for differential EFB support
The external clock outputs support is dependent on the device density and package.
Note:
The following figure shows that any of the output counters (
C[0..8]
) or the
M
counter on the PLLs can feed
the dedicated external clock outputs. Therefore, one counter or frequency can drive all output pins available
from a given PLL.
Altera Corporation
Clock Networks and PLLs in Cyclone V Devices
4-23
PLL Cascading
CV-52004
2014.01.10