Difference between revisions of "RFNoC Frequently Asked Questions"
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=== How do I add a clock with a different frequency? === | === How do I add a clock with a different frequency? === | ||
| − | Adding custom clocks is not directly supported yet. | + | Adding custom clocks is not directly supported yet. Describing them in the YAML file will not cause them to be generated for you. If you can't use any of the available clocks, you can modify the HDL code to generate a clock. |
| − | + | If you only need the clock within your own RFNoC block, you can modify the HDL for your block to generate the clock that you need from one of the available clocks. To do this, add a new clock to your block's YAML description, connect the available clock to your block in the YAML description of your RFNoC image, then add a Xilinx MMCM IP instance to your block's HDL and connect the available clock to its input. | |
| − | + | ||
| + | If the clock is needed by multiple RFNoC blocks, or if you want to change an existing clock, you can modify the HDL for the USRP you are using to add or change a clock. If you add a new clock to the RFNoC image core, you must also update the BSP YAML file (located in [https://github.com/EttusResearch/uhd/tree/master/host/include/uhd/rfnoc/core <repo>/host/include/uhd/rfnoc/core]) so that the <code>rfnoc_image_builder</code> knows that the clock exists. How and where the clocks are generated varies between USRPs. Please refer to the source code for that USRP ([https://github.com/EttusResearch/uhd/tree/master/fpga/usrp3/top <repo>/fpga/usrp3/top]). | ||
Revision as of 10:26, 18 April 2022
Configuring the Stream Endpoint Buffer Size in RFNoC
What is the SEP buffer size?
Each stream endpoint (SEP) has an ingress buffer to store data received from others stream endpoints. This size of this buffer affects the data transfer rate that can be achieved when streaming to that endpoint. A larger ingress buffer in the stream endpoint means that there is more space to put data, minimizing idle time on the network. Additionally, streamers can queue up data before it is needed, reducing the chance of a buffer underflow.
How do I set the SEP buffer size?
The stream endpoint buffer size is set by adding a parameter under the endpoint you want to configure in the RFNoC image core YAML file. There are two parameters you can use to set the stream endpoint ingress buffer size in your RFNoC image core YAML file.
-
buff_size: Buffer size in CHDR words. The size in bytes depends on the CHDR width. For example, if thechdr_widthparameter for the device is 64, then each CHDR word is 8 bytes. So a buff size of 32768 would be 262,144 bytes or 256 KiB. See here for an example. -
buff_size_bytes: Buffer size in bytes. See here for an example.
To what value should I set the SEP buffer size?
The buffer size should be a power of two in size to make optimal use of FPGA RAM resources. The default FPGA bitstreams typically set them to the largest size the FPGA can fit in order to maximize performance. Here are some general recommendations:
- Set to
0if you don't need to send data to that SEP. - Set to
8192bytes (8 KiB = 1 MTU) minimum in order to stream data packets. - Set to
32768bytes (32 KiB = 4 MTU) in order to stream at maximum rates between SEPs on the same FPGA. - Set to
262144bytes (256 KiB = 32 MTU) or lager for high performance streaming between a host computer and the FPGA.
Note that the requirements are application-dependent, so optimal sizes for your application may be different. MTU refers to the maximum transmission unit, which is the largest CHDR packet supported by the FPGA.
If you need to free up FPGA resources (particularly block RAM) for your application, you can reduce the SEP buffer sizes. Just keep in mind that the maximum streaming rate may be affected.
RFNoC Clocks
What clocks are available for me to use?
Each device has different clocks available. See below for a list of clocks exposed to RFNoC. Although they have intended purposes, you can use any of these clocks for any purpose. The rfnoc_chdr_clock is a good default choice. This clock is always available in your block, even if it is not explicitly connected in the RFNoC image YAML description.
What are the clock frequencies?
See the table below for the clock rates. The radio clock rate depends on the master clock rate.
E31x
| Clock Name | Description | Frequency |
|---|---|---|
rfnoc_chdr |
RFNoC CHDR clock | 100 MHz |
dram |
DRAM interface clock | 100 MHz |
radio |
Radio interface clock | Same as master clock rate |
E320
| Clock Name | Description | Frequency |
|---|---|---|
rfnoc_chdr |
RFNoC CHDR clock | 200 MHz |
dram |
DRAM interface clock | 166.667 MHz |
radio |
Radio interface clock | Same as master clock rate (200 kHz to 61.44 MHz) |
N300/N310
| Clock Name | Description | Frequency |
|---|---|---|
rfnoc_chdr |
RFNoC CHDR clock | 200 MHz |
dram |
DRAM interface clock | 303.189 MHz |
radio |
Radio interface clock | Same as master clock rate (122.88 MHz, 125.0 MHz, or 153.6 MHz) |
N32x
| Clock Name | Description | Frequency |
|---|---|---|
rfnoc_chdr |
RFNoC CHDR clock | 187.5 MHz |
dram |
DRAM interface clock | 303.819 MHz |
radio |
Radio interface clock | Same as master clock rate (200 MHz, 245.76 MHz, or 250 MHz) |
X310
| Clock Name | Description | Frequency |
|---|---|---|
rfnoc_chdr |
RFNoC CHDR clock | 187.5 MHz |
ce |
Compute Engine clock | 214.286 MHz |
dram |
DRAM interface clock | 300 MHz |
radio |
Radio interface clock | Same as master clock rate (184.32 MHz or 200 MHz) |
X410
| Clock Name | Description | Frequency |
|---|---|---|
rfnoc_chdr |
RFNoC CHDR clock | 200 MHz |
dram |
DRAM interface clock | 250 MHz |
radio |
Radio interface clock | 122.88 MHz when master clock rate is 122.88, 245.76, or 491.52 MHz 125 MHz when master clock rate is 125, 250, or 500 MHz |
radio_2x |
Radio interface clock 2x | Twice the frequency of radio_clk
|
How do I add a clock with a different frequency?
Adding custom clocks is not directly supported yet. Describing them in the YAML file will not cause them to be generated for you. If you can't use any of the available clocks, you can modify the HDL code to generate a clock.
If you only need the clock within your own RFNoC block, you can modify the HDL for your block to generate the clock that you need from one of the available clocks. To do this, add a new clock to your block's YAML description, connect the available clock to your block in the YAML description of your RFNoC image, then add a Xilinx MMCM IP instance to your block's HDL and connect the available clock to its input.
If the clock is needed by multiple RFNoC blocks, or if you want to change an existing clock, you can modify the HDL for the USRP you are using to add or change a clock. If you add a new clock to the RFNoC image core, you must also update the BSP YAML file (located in <repo>/host/include/uhd/rfnoc/core) so that the rfnoc_image_builder knows that the clock exists. How and where the clocks are generated varies between USRPs. Please refer to the source code for that USRP (<repo>/fpga/usrp3/top).
