Altera CIC MegaCore Function - Manual (Page 18)

Variable Rate Change Factors
You can optionally set minimum and maximum values for the decimator or interpolator rate change
factors and enable the rate change factors to be set at run time. With these options, the CIC provides an
additional
rate
port that you can use to specify the rate change factor.
Note: With variable rate change factors, reset the IP core when you change the rate change factor,
otherwise the CIC uses previous memory and register values. You cannot change the filter mode
(interpolation or decimation) at run time.
Figure 3-2: Variable Rate Change Decimation CIC Filter Timing Diagram
The out_valid signal changes its period according to the variable rate change.
clk
reset_n
clken
in_error
rate
in_valid
out_error
out_ready
in_ready
out_valid
00
8 2 8
00
Multichannel Support
Often many channels of data in a digital signal processing (DSP) system require filtering by CIC filters
with the same configuration. You can combine them into one filter, which shares the adders that exist in
each stage and reduces the overall resource utilization.
Using a combined filter uses fewer resources than using many individual CIC filters. For example, a two-
channel parallel filter requires two clock cycles to calculate two outputs. The resulting hardware needs to
run at twice the data rate of an individual filter, which is especially useful for higher rate changes where
adders grow particularly large.
Note: To minimize the number of logic elements, use a multiple input single output (MISO) architecture
for decimation filters, and a single input multiple output (SIMO) architecture for interpolation
filters.
Multiple Input Single Output (MISO)
In many practical designs, channel signals come from different input interfaces. On each input interface,
the same parameters including rate change factors apply to the channel data that the CIC filter is going to
process. The CIC IP core allows multiple input single output (MISO) decimation filters, which allows the
flexibility to exploit time sharing of the low-rate differentiator sections.
The CIC achieves time sharing by providing multiple input interfaces and processing chains for the high
rate portions. It then combines all of the processing associated with the lower rate portions into a single
3-2 Variable Rate Change Factors
UG-CIC
2014.12.15
Altera Corporation CIC IP Core Functional Description
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