{"id":8716,"date":"2024-04-17T08:30:34","date_gmt":"2024-04-17T15:30:34","guid":{"rendered":"https:\/\/visualgdb.com\/w\/?p=8716"},"modified":"2024-04-17T08:30:34","modified_gmt":"2024-04-17T15:30:34","slug":"optimizing-stm32-usb-high-speed-performance-with-tracing","status":"publish","type":"post","link":"https:\/\/visualgdb.com\/tutorials\/tracing\/embedded\/usb\/optimizing\/","title":{"rendered":"Optimizing STM32 USB High-Speed Performance with Tracing"},"content":{"rendered":"

This tutorial shows how to use several optimization techniques to get the maximum performance out of the high-speed USB interface on the STM32F7-Discovery board. We will show how raise the throughput in a single-direction mode more than twice the baseline, and how to use the linker-time optimization (LTO) to considerably drop the CPU utilization by the USB stack.<\/p>\n

We will use VisualGDB’s software tracing<\/a> to get the cycle-accurate timings between different parts of the USB communication and the Memory Explorer<\/a> to understand the effects of the linker-time optimization on the generated code.<\/p>\n

If you are just looking for the final numbers, scroll back to the end of the tutorial or click here<\/a>.<\/p>\n

Before you begin, install VisualGDB 6.0 or later.<\/p>\n

    \n
  1. Start Visual Studio and Open the VisualGDB Embedded Project Wizard:\"\"<\/a><\/li>\n
  2. Enter the name and location for your project:
    \n    \"\"<\/a><\/li>\n
  3. Proceed with creating a new embedded application using Advanced CMake: \"\"<\/a><\/li>\n
  4. Choose the latest ARM toolchain and select your device from the list. In this tutorial we will use the STM32F7-Discovery board that uses the STM32F746NG device: \"\"<\/a><\/li>\n
  5. Select the USB Communications Device<\/strong> sample on the next page and click “Next”:\"\"<\/a><\/li>\n
  6. Connect your on-board ST-Link debugger to the USB port and make sure VisualGDB recognizes it. Then ensure you have the software tracing and FLASH memory patching enabled and click “Finish” to generate the project: \"\"<\/a><\/li>\n
  7. VisualGDB will create a sample project, however it will use the default clock settings that may not match your board, so the project may not run correctly:
    \n    \"\"<\/a><\/li>\n
  8. To fix this, create another project based on the STM32CubeMX Samples -> USB_Device -> MSC_Standalone -> [board name]-HS<\/strong>:\"\"<\/a><\/li>\n
  9. Then, copy the following from the cloned MSC sample into the main project:
    \n\n\n\n\n\n\n\n
    Entity<\/td>\nFunctionality<\/td>\n<\/tr>\n
    SystemClock_Config()<\/strong> function<\/td>\nConfigures PLL to match the on-board crystal<\/td>\n<\/tr>\n
    Entire usbd_conf.c <\/strong>file
    \n<\/strong><\/td>\n    		Handles low-level aspects of the USB configuration<\/td>\n<\/tr>\n
    Entire system_stm32f7xx.c<\/strong> file<\/td>\nEnables optimal FLASH and power settings<\/td>\n<\/tr>\n
    Entire stm32f7xx_hal_conf.h<\/strong> file<\/td>\nSpecifies correct on-board crystal frequency and other settings<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    If usbd_conf.c doesn’t build due to missing main.h, replace #include <main.h> with the following code:<\/p>\n

    #include <usbd_core.h>\r\n#include <usbd_cdc.h><\/pre>\n
    \"\"<\/a>\r\n<\/pre>\n
    You can see the exact changes needed for the STM32F7Discovery board in this commit<\/a>.<\/li>\n
  10. Finally, ensure that the USB Device Speed is in VisualGDB Project Properties -> Embedded Frameworks<\/strong> is set to USB 2.0 High Speed:
    \n    \"\"<\/a><\/li>\n
  11. Press F5 to start debugging and ensure the USB-HS port on the board is connected to your computer. A new USB-based COM port will appear, echoing any characters typed into it:\"\"<\/a><\/li>\n
  12. Now we will modify the main()<\/strong> function to unconditionally send a stream of data via the USB port. Replace the code after the call to USBD_Start()<\/strong> with this:\n
    static char data[512];\r\nchar byte;\r\nwhile (!USBD_Device.pClassData)\r\n    asm(\"nop\");\r\nfor (;;)\r\n    VCP_write(&data, sizeof(data));<\/pre>\n
    Then, create a basic C# program that will continuously read the data and show the throughput:<\/p>\n
    static void Main(string[] args)\r\n{\r\n\tSerialPort port = new SerialPort(\"COM20\");\r\n\tport.Open();\r\n\r\n\tvar buf = new byte[65536];\r\n\tfor (int i = 0; i < buf.Length; i++)\r\n\t\tbuf[i] = (byte)i;\r\n\r\n\tlong bytesRead = 0;\r\n\tDateTime start = DateTime.Now;\r\n\r\n\tfor (; ; )\r\n\t{\r\n\t\tint done = port.Read(buf, 0, buf.Length);\r\n\t\tbytesRead += done;\r\n\t\tdouble msec = (DateTime.Now - start).TotalMilliseconds;\r\n\t\tdouble bytesPerSecond = (bytesRead * 1000) \/ msec;\r\n\t\tdouble kbPerSecond = bytesPerSecond \/ 1024;\r\n\t\tConsole.Write($\"\\rRead: {bytesRead \/ 1024}KB; \" +\r\n\t\t\t$\"Average speed: {kbPerSecond:f0} KB\/sec\");\r\n\t}\r\n}<\/pre>\n<\/li>\n
  13. Replace COM20<\/strong> with the COM port number on your machine and run the benchmark program to measure the speed:\"\"<\/a>In our experiments, the unoptimized (Debug) build yielded about 4 MB\/sec.<\/li>\n
  14. Switch the solution configuration to RelWithDebInfo <\/strong>and try measuring the speed again:\"\"<\/a>In our experiments, this raised the USB throughput to about 7.7 MB\/sec.<\/li>\n
  15. Note that the default STM32 USB configuration does not use DMA for the USB high-speed endpoint. You can override by setting dma_enable<\/strong> to 1 in the USBD_LL_Init()<\/strong> function in usbd_conf.c<\/strong>:\"\"<\/a>This brings the throughput up to 9.1 MB\/s.<\/li>\n
  16. Now we will use the VisualGDB’s Software Tracing<\/a> to understand the exact timing of the USB communication and find bottlenecks. Sending large amounts of data via the bulk endpoint would normally involve a loop like this:\n