This application provides an example of Azure RTOS USBX stack usage on NUCLEO-C071RB board, it shows how to develop USB Device communication Class “CDC_ACM” based application. The application is designed to emulate an USB-to-UART bridge following the Virtual COM Port (VCP) implementation, the code provides all required device descriptors framework and associated Class descriptor report to build a compliant USB CDC_ACM device. At the beginning ThreadX call the entry function tx_application_define(), at this stage, all USBx resources are initialized, the CDC_ACM Class driver is registered and the application creates 3 threads with the same priorities :
During enumeration phase, three communication pipes “endpoints” are declared in the CDC class implementation :
1 x Bulk IN endpoint for receiving data from STM32 device to PC host: When data are received over UART they are saved in the buffer “UserTxBufferFS”. Periodically, in a usbx_cdc_acm_write_thread_entry the state of the buffer “UserTxBufferFS” is checked. If there are available data, they are transmitted in response to IN token otherwise it is NAKed.
1 x Bulk OUT endpoint for transmitting data from PC host to STM32 device: When data are received through this endpoint they are saved in the buffer “UserRxBufferFS” then they are transmitted over UART using DMA mode and in meanwhile the OUT endpoint is NAKed. Once the transmission is over, the OUT endpoint is prepared to receive next packet in HAL_UART_RxCpltCallback().
1 x Interrupt IN endpoint for setting and getting serial-port parameters: When control setup is received, the corresponding request is executed in USBD_CDC_ACM_ParameterChange.
In this application, two requests are implemented:
- Set line: Set the bit rate, number of stop bits, parity, and number of data bits
- Get line: Get the bit rate, number of stop bits, parity, and number of data bits
The other requests (send break, control line state) are not implemented.
When plugged to PC host, the NUCLEO-C071RB must be properly enumerated as an USB Serial device and an STlink Com port. During the enumeration phase, the device must provide host with the requested descriptors (Device descriptor, configuration descriptor, string descriptors). Those descriptors are used by host driver to identify the device capabilities. Once NUCLEO-C071RB USB device successfully completed the enumeration phase, Open two hyperterminals (USB com port and UART com port(USB STLink VCP)) to send/receive data to/from host from/to device.
Host PC shows that USB device does not operate as designed (CDC Device enumeration failed, PC and Device can not communicate over VCP ports).
User is familiar with USB 2.0 “Universal Serial BUS” specification and CDC_ACM class specification.
None.
Receiving data over UART is handled by interrupt while transmitting is handled by DMA allowing hence the application to receive data at the same time it is transmitting another data (full- duplex feature).
CDC ACM non-blocking transmission by default disabled, to enable non-blocking transmission UX_DEVICE_CLASS_CDC_ACM_TRANSMISSION_DISABLE must be disabled and 2048 additional in USBX byte pool and USBX_MEMORY_SIZE.
place in RAM_region { last section FREE_MEM };
either define the RW_IRAM1 region in the ".sct" file
or modify the line below in "tx_initialize_low_level.S to match the memory region being used
LDR r1, =|Image$$RW_IRAM1$$ZI$$Limit|
._threadx_heap :
{
. = ALIGN(8);
__RAM_segment_used_end__ = .;
. = . + 64K;
. = ALIGN(8);
} >RAM_D1 AT> RAM_D1
The simplest way to provide memory for ThreadX is to define a new section, see ._threadx_heap above.
In the example above the ThreadX heap size is set to 64KBytes.
The ._threadx_heap must be located between the .bss and the ._user_heap_stack sections in the linker script.
Caution: Make sure that ThreadX does not need more than the provided heap memory (64KBytes in this example).
Read more in STM32CubeIDE User Guide, chapter: "Linker script".
RTOS, ThreadX, USBX, USBXDevice, USB_DRD, Full Speed, CDC, VCP, USART, DMA.
This application has been tested with STMicroelectronics NUCLEO-C071RB boards revision MB2046-C071RB-B01. and can be easily tailored to any other supported device and development board.
In order to make the program work, you must do the following :