How to select suitable endpoints for your STM32 USB application

FBL · ‎2024-10-23

Introduction

The article provides a comprehensive guide on selecting suitable endpoints for USB applications using STM32 microcontrollers. This involves understanding the types of data transfers required by the application and determining suitable endpoints. The article aims to assist in choosing the appropriate STM32 MCU and determining the necessary endpoints.

1. Understanding USB endpoints

USB endpoints are virtual communication channels between the host and the device. There are four primary types of endpoints.

1.1. Control endpoints

Used for configuration, command, and status operations. Every USB device must have at least one control endpoint (endpoint 0).

1.2. Bulk endpoints

Used for large, non-time-critical data transfers. Commonly used for data that can tolerate delays, such as file transfers.

1.3. Interrupt endpoints

Used for small, time-sensitive data transfers. Ideal for devices that need to send small amounts of data at regular intervals, like keyboards and mice.

1.4. Isochronous endpoints

Used for time-sensitive data transfers that require consistent data flow, such as audio and video streaming.

2. How to determine the number of endpoints

To determine the number of endpoints required for your application, consider the following general guidelines based on the types of data transfers your application needs:

Control endpoint: Typically, only one control endpoint (endpoint 0) is required.
Bulk endpoints: Determine if you need separate endpoints for IN (device to host) and OUT (host to device) data transfers.
Interrupt endpoints: Identify how many interrupt endpoints are needed based on the number of time-sensitive data streams.
Isochronous endpoints: Determine the number of isochronous endpoints based on the number of continuous data streams.

2.1. Example of a HID function

Here’s an example of endpoints that might be declared for an HID class:

1 x control endpoint (endpoint 0): Used for standard USB control transfers, such as configuration and enumeration.
1 x interrupt IN endpoint: For sending HID reports from the device to the host (for example, mouse movements, keyboard presses). This endpoint is used to send periodic data to the host with minimal latency. The HID descriptor defines the polling interval for this endpoint.
1 x interrupt OUT endpoint: For receiving HID reports from the host to the device (for example, LED status for keyboards, feature reports). This endpoint is used to receive data from the host that needs to be processed by the device.

2.2. Example of a CDC virtual communication port function

Here’s an example of endpoints that might be declared for a CDC ACM class implementation (PSTN subclass):

1 x bulk IN endpoint: For sending data from the STM32 device to the PC host. Data received over UART is saved in a buffer to be sent. Periodically, a timer callback checks the buffer state. If data is available, it’s transmitted in response to an IN token; otherwise, it’s NAKed.
1 x bulk OUT endpoint: For receiving data from the PC host to the STM32 device. Data received through this endpoint is saved in a receive buffer and then transmitted over UART using interrupt mode. Meanwhile, the OUT endpoint is NAKed. Once the transmission is complete, the OUT endpoint is prepared to receive the next packet in the UART transmit callback.
1 x interrupt IN endpoint: For setting and getting serial-port parameters. In this example, two requests can be 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.
- Other requests (send break, control line state) may be implemented.

2.3. Example of an audio function

Here’s an example of endpoints that might be declared for an audio class:

1 x control endpoint (endpoint 0): Handles standard USB control transfers.
1 x isochronous IN endpoint: Transfers audio data from the microphone to the host.
1 x isochronous OUT endpoint: Transfers audio data from the host to the speaker.

To learn about the number of endpoints available, refer to the associated STM32 datasheet or section 2.1 "USB implementation on STM32 products" in application note 4879.

3. Endpoint size and buffering

Consider the maximum packet size and buffering requirements for each endpoint:

	Control	Interrupt	Bulk	Isochronous
Max packet size in bytes in full speed	64	64	64	1023
Max packet size in bytes in high speed	64	1024	512	1024

Data payloads must be less than or equal to the endpoint’s max packet size. If a larger than expected data payload is received, the control transfer is aborted.

4. Endpoint addressing

Each endpoint has a unique address composed of a direction (IN or OUT) and an endpoint number to be defined in each endpoint descriptor. The endpoint address is an 8-bit value that contains both the endpoint number and the direction of data transfer. The format is as follows:

Bits 0-3: Endpoint number (0-15) if applicable.
Bit 7: Direction (0 for OUT, 1 for IN).
Bits 4-6: Reserved (usually set to 0).

Endpoint numbers typically range from 1 to 15 (0 is reserved for control endpoints). The definition of endpoint numbers must respect the order of classes instantiation. Here’s an example to initialize the endpoint address:

// Define endpoint addresses
#define EP1_OUT 0x01  // Endpoint 1, OUT direction
#define EP1_IN  0x81  // Endpoint 1, IN direction
#define EP2_OUT 0x02  // Endpoint 2, OUT direction
#define EP2_IN  0x82  // Endpoint 2, IN direction

// Initialize endpoint descriptors
USBD_EpDescTypedef ep1_out_descriptor = {
    .bLength = 7,               // Descriptor size
    .bDescriptorType = 0x05,    // Endpoint descriptor type
    .bEndpointAddress = EP1_OUT,// Endpoint address
    .bmAttributes = 0x02,       // Bulk transfer type
    .wMaxPacketSize = 64,       // Maximum packet size (64 bytes)
    .bInterval = 0              // Polling interval (ignored for bulk)
};

5. Explanation of FIFO operations in device mode in STM32 USB communication

Generally, FIFO sizes can be adjusted to optimize performance and reduce CPU overhead. For example, to handle more data and reduce the frequency of FIFO overflows, FIFO sizes should be larger in high speed mode. Also, bulk endpoints might require larger FIFOs to handle large data transfers efficiently compared to different types of endpoints.

Refer to the appropriate STM32 datasheet to check the size of dedicated FIFO SRAM. Furthermore, check if double buffer mode is supported, which would reduce the number of available endpoints. Here’s how USB FIFO is managed:

Rx FIFO: A single shared FIFO for all OUT endpoints, configured via OTG_GRXFSIZ. Efficiently handles incoming data, with status and data stored together. Generates interrupts when full or when data is available.
Tx FIFOs: Dedicated FIFOs for each IN endpoint, with sizes configured via specific registers (OTG_DIEPTXF0 and OTG_DIEPTXFx).

Proper management of FIFO sizes ensures efficient data transmission and reception without overflow. By configuring FIFO sizes appropriately, you can enhance the performance and reliability of your USB communication.

6. Additional considerations regarding MCU selection

When selecting an STM32 MCU for a USB application, it is crucial to ensure that the chosen MCU meets all the requirements of your project. Here are some key considerations:

Assess whether the device will be bus-powered or self-powered.
Estimate the amount of data that will be transferred and the required throughput.
Identify the number and types of endpoints needed (e.g., control, bulk, interrupt, isochronous).
Ensure the STM32 has the required USB version support and the necessary endpoints.
Check if the STM32’s clock speed can handle the data processing requirements.
Verify that the STM32 has sufficient RAM and Flash memory for your application.
Consider other interfaces (e.g., UART, SPI, I2C) that might be needed for your application.
Ensure the STM32 operates at a suitable voltage for your application.
Look for power-saving modes if your application requires low power consumption.

Conclusion

By understanding the types of data transfers your USB application requires, you can determine the suitable endpoints and associated configuration. This ensures efficient and reliable communication between the USB device and the host. Always refer to the USB specification and the STM32 MCU’s datasheet for detailed information on endpoint capabilities and limitations.