USB Overview
Supported ICs[ RTL8721Dx ][ RTL8721F ]
USB (Universal Serial Bus) is a standard serial communication interface designed to interconnect host controllers and peripheral devices.
The complete USB specification suite, including electrical, mechanical, and protocol layer documentation, is accessible at http://www.usb.org/developers.
Note
Due to the incomplete official USB protocol stack of Zephyr, which only supports device mode. Therefore, we choose not to use the official source code, but instead provide the Realtek USB stack. This chapter briefly introduces the Realtek USB stack, and how to compile and run application examples.
Features
Supports USB2.0 high-speed (480 Mbps) and full-speed (12 Mbps) modes
Supports following USB IF standard device class drivers:
CDC ACM: Used for data transparent transmission or as a virtual serial port
HID: Used as USB keyboard, mouse and other devices to achieve human-computer interaction functions
MSC: Supports SD card or SRAM as storage medium for data storage
UAC: Used as USB audio device, it supports audio recording and playback
Supports following USB composite device class drivers:
CDC ACM + HID
CDC ACM + UAC
HID + UAC
Provides reference design for vendor-specific USB device drivers
Full descriptor customization:
VID and PID
String descriptors
Endpoint mapping
Configurable device core driver parameters
Speed mode
Data FIFO depth (only for dedicated FIFO mode)
Core ISR/thread priority
Hardware Features
Supports device mode
Supports USB 2.0 full-speed (12 Mbps) mode
Supports DMA mode
Endpoint configurations:
EP0: INOUT (for Control transfer only)
EP1: IN
EP2: OUT
EP3: IN
EP4: OUT
EP5: INOUT
Note
One and only one periodic IN endpoint is supported.
Shared FIFO with following configurations:
Total FIFO depth: 768 DWORD
Shared receive FIFO: 472 DWORD (max)
Shared non-periodic transmit FIFO: 32 DWORD (max)
Dedicated periodic transmit FIFO: 256 DWORD (max)
Note
8 DWORD FIFO shall be reserved for DMA registers in DMA mode.
Supports host and device modes
Supports USB 2.0 high-speed (480 Mbps) and full-speed (12 Mbps) modes
Supports DMA mode
Endpoint configurations in device mode:
EP0: INOUT (for Control transfer only)
EP1: IN
EP2: INOUT
EP3: INOUT
EP4: IN
EP5: OUT
EP6: INOUT
EP7: OUT
Supports 12 channels/pipes in host mode
Shared FIFO with following configurations:
Total FIFO depth: 1024 DWORD
Host mode:
Shared receive FIFO: 1024 DWORD (max)
Shared non-periodic transmit FIFO: 1024 DWORD (max)
Shared periodic transmit FIFO: 1024 DWORD (max)
Device mode:
Shared RX FIFO: 1024 DWORD (max)
6 dedicated transmit FIFOs for device IN transfers, max depth of each Tx FIFO:
Tx FIFO 0: 32
Tx FIFO 1: 16
Tx FIFO 2: 256
Tx FIFO 3: 32
Tx FIFO 4: 256
Tx FIFO 5: 128
Note
12 DWORD FIFO shall be reserved for DMA registers in DMA mode.
Integrated UTMI+ PHY
External hub connection support in host mode
Automatic ping in host mode
Software Stack Overview
Software stack architecture
Where:
USB HAL driver: Implements SoC-specific hardware driver for power management and phy calibration, exposing unified HAL API to upper-layer core drivers.
USB device core drivers: Integrates USB IP-specific hardware driver, device controller driver, enumeration state machine and transfer scheduling algorithms, exposing unified core API for designers to develop classes and applications.
USB device class drivers: Delivers USB-IF class-compliant drivers via class-specific API, enabling rapid deployment of standard USB solutions.
USB device applications: Provides reference designs for USB solutions.
Zephyr USB API is consistent with FreeRTOS USB API. For details, see FreeRTOS USB API
File Organization
SDK/modules/hal/realtek
├── ameba/common/usb/source
│ ├── common
│ │ ├── usb_ch9.h
│ │ └── usb_os.h
│ └── device
│ ├── cdc_acm
│ ├── composite
│ ├── hid
│ ├── inic_dplus
│ ├── msc
│ ├── uac
│ ├── vendor
│ └── core
│ └── usbd.h
└── zephyr/blobs/amebadplus/lib
└── lib_usbd.a
SDK/modules/hal/realtek
├── ameba/common/usb/source
│ ├── common
│ │ ├── usb_ch9.h
│ │ └── usb_os.h
│ └── device
│ ├── cdc_acm
│ ├── composite
│ ├── hid
│ ├── inic
│ ├── msc
│ ├── uac
│ ├── vendor
│ └── core
│ └── usbd.h
└── zephyr/blobs/amebaG2/lib
└── lib_usbd.a
USB Certification
USB certification generally refers to USB-IF certification, i.e. USB-IF compliance testing, which validates devices against USB-IF electrical, protocol and functional requirements to ensure interoperability, security and reliability. Certified devices are authorized to use USB logos.
While USB-IF certification is not mandatory, it is required when:
Products intend to use USB logos
Marketing materials claim USB specification compliance or certification
For certification procedures and requirements, refer to https://www.usb.org documentation or contact USB-IF authorized test labs.
Additional Compliance Requirements for USB Devices:
FCC/CE: Mandatory for USB devices sold in EU/US markets
ISO 26262: Required for automotive USB devices (functional safety)
Windows Logo Program: Necessary for “Certified for Windows” branding
MFi: Mandatory for Apple-specific USB accessories
