The Xilinx CIC Compiler provides the ability to design and implement AXI4-Stream-compliant Cascaded Integrator-Comb (CIC) filters for a variety of Xilinx FPGA devices.
CIC filters, also known as Hogenauer filters, are multi-rate filters often used for implementing large sample rate changes in digital systems. They are typically employed in applications that have a large excess sample rate. That is, the system sample rate is much larger than the bandwidth occupied by the processed signal as in digital down converters (DDCs) and digital up converters (DUCs). Implementations of CIC filters have structures that use only adders, subtractors, and delay elements. These structures make CIC filters appealing for their hardware-efficient implementations of multi-rate filtering.
Sample Rates and the CIC Compiler Block
The CIC Compiler block must always run at the system rate because the CIC Compiler block has a programmable rate change option and Simulink® cannot inherently support it. You should use the "ready" output signal to indicate to downstream blocks when a new sample is available at the output of the CIC Compiler block.
The CIC will downsample the data, but the sample rate will remain at the clock
rate. If you look at the output of the CIC Compiler block, you will see each output
data repeated R times for a rate change of R while the
signal pulses once every R cycles. The downstream blocks can be clocked at
lower-than-system rates without any problems as long as the clock is never slower
than the rate change R.
There are several different ways this can be handled. You can leave the entire design running at the system rate then use registers with enables, or enables on other blocks to capture data at the correct time. Or alternatively, you can use a downsample block corresponding to the lowest rate change R, then again use enable signals to handle the cases when there are larger rate changes.
If there are not many required rate changes, you can use MUX blocks and use a different downsample block for each different rate change. This might be the case if the downstream blocks are different depending on the rate change, basically creating different paths for each rate. Using enables as described above will probably be the most efficient method.
If you are not using the CIC Compiler block in a programmable mode, you can place an up/down sample block after the CIC Compiler to correctly pass on the sample rate to downstream blocks that will inherit the rate and build the proper CE circuitry to automatically enable those downstream blocks at the new rate.
- Filter Specification tab
- Parameters specific to the Filter
Specification tab are as follows.
- Filter Specification
- Filter Type
- The CIC core supports both interpolation and decimation architectures. When the filter type is selected as decimator the input sample stream is down-sampled by the factor R. When an interpolator is selected the input sample is up-sampled by R.
- Number of Stages
- Number of integrator and comb stages. If N stages are specified, there are N integrators and N comb stages in the filter. The valid range for this parameter is 3 to 6.
- Differential Delay
- Number of unit delays employed in each comb filter in the comb section of either a decimator or interpolator. The valid range of this parameter is 1 or 2.
- Number of Channels
- Number of channels to support in implementation. The valid range of this parameter is 1 to 16.
- Sample Rate Change Specification
- Sample Rate Changes
- Option to select between Fixed or Programmable.
- Fixed or Initial Rate(ir)
- Specifies initial or fixed sample rate change value for the CIC. The valid range for this parameter is 4 to 8192.
- Minimum Rate
- The minimum rate change value for programmable rate change. The valid range for this parameter is 4 to fixed rate (ir).
- Maximum Rate
- The maximum rate change value for programmable rate change. The valid range for this parameter is fixed rate (ir) to 8192.
- Hardware Oversampling Specification
- Select format
- Choose Maximum_Possible, Sample_Period, or Hardware Oversampling Rate. Selects which method is used to specify the hardware oversampling rate. This value directly affects the level of parallelism of the block implementation and resources used. When “Maximum Possible” is selected, the block uses the maximum oversampling given the sample period of the signal connected to the Data field of the s_axis_data_tdata port. When you select “Hardware Oversampling Rate”, you can specify the oversampling rate. When “Sample Period” is selected, the block clock is connected to the system clock and the value specified for the Sample Period parameter sets the input sample rate the block supports. The Sample Period parameter also determines the hardware oversampling rate of the block. When “Sample Period” is selected, the block is forced to use the s_axis_data_tvalid control port.
- Sample period
- Integer number of clock cycles between input samples. When the multiple channels have been specified, this value should be the integer number of clock cycles between the time division multiplexed input sample data stream.
- Hardware Oversampling Rate
- Enter the hardware oversampling rate if you select Hardware_Oversampling_Rate as the format.
- Implementation tab
- Numerical Precision
- Can be specified as Full_Precision or
Truncation.Note: Truncation occurs at the output stage only.
- Output Data Width
- Can be specified up to 48 bits for the Truncation option above.
- Use Xtreme DSP slice
- This field specifies that if possible, use the XtremeDSP slice (DSP48 type element) in the target device.
- Use Streaming Interface
- Specifies whether or not to use a streaming interface for multiple channel implementations.
- Control Options
- Specifies if the block has a clock enable port (the equivalent of selecting the Has ACLKEN option in the CORE Generator GUI).
- Specifies that the block has a reset port. Active-Low synchronous clear. A minimum ARESETn pulse of two cycles is required.
- Has TREADY
- Specifies if the block has a TREADY port for the Data Output Channel (the equivalent of selecting the Has_DOUT_TREADY option in the CORE Generator GUI).
Other parameters used by this block are explained in the topic Common Options in Block Parameter Dialog Boxes.
CIC Compiler LogiCORE IP Product Guide (PG140)