# USB Host CDC-ACM Class Driver
This component contains an implementation of a USB CDC-ACM Host Class Driver that is implemented on top of the [USB Host Library](https://docs.espressif.com/projects/esp-idf/en/latest/esp32s2/api-reference/peripherals/usb_host.html).
## Supported Devices
The CDC-ACM Host driver supports the following types of CDC devices:
1. CDC-ACM devices
2. CDC-like vendor specific devices (usually found on USB to UART bridge devices or cellular modems)
### CDC-ACM Devices
The CDC-ACM Class driver supports CDC-ACM devices that meet the following requirements:
- The device class code must be set to the CDC class `0x02` or implement Interface Association Descriptor (IAD)
- The CDC-ACM must contain the following interfaces:
- A Communication Class Interface containing a management element (EP0) and may also contain a notification element (an interrupt endpoint). The driver will check this interface for CDC Functional Descriptors.
- A Data Class Interface with two BULK endpoints (IN and OUT). Other transfer types are not supported by the driver
### CDC-Like Vendor Specific Devices
The CDC-ACM Class driver supports CDC-like devices that meet the following requirements:
- The device class code must be set to the vendor specific class code `0xFF`
- The device needs to provide and interface containing the following endpoints:
- (Mandatory) Two Bulk endpoints (IN and OUT) for data
- (Optional) An interrupt endpoint (IN) for the notification element
For CDC-like devices, users are responsible for ensuring that they only call APIs (e.g., `cdc_acm_host_send_break()`) that are supported by the target device.
The following steps outline the typical API call pattern of the CDC-ACM Class Driver
1. Install the USB Host Library via `usb_host_install()`
2. Install the CDC-ACM driver via `cdc_acm_host_install()`
3. Call `cdc_acm_host_open()`/`cdc_acm_host_open_vendor_specific()` to open a target CDC-ACM/CDC-like device. These functions will block until the target device is connected or time-out
4. To transmit data, call `cdc_acm_host_data_tx_blocking()`
5. When data is received, the driver will automatically run the receive data callback
6. An opened device can be closed via `cdc_acm_host_close()`
7. The CDC-ACM driver can be uninstalled via `cdc_acm_host_uninstall()`
- For an example with a CDC-ACM device, refer to [cdc_acm_host](https://github.com/espressif/esp-idf/tree/master/examples/peripherals/usb/host/cdc/cdc_acm_host)
- For an example with a CDC-like device, refer to [cdc_acm_host_bg96](https://github.com/espressif/esp-idf/tree/master/examples/peripherals/usb/host/cdc/cdc_acm_bg96)
- For an example with Virtual COM devices, refer to [cdc_acm_vcp](https://github.com/espressif/esp-idf/tree/master/examples/peripherals/usb/host/cdc/cdc_acm_vcp)
- For examples with [esp_modem](https://components.espressif.com/components/espressif/esp_modem), refer to [esp_modem examples](https://github.com/espressif/esp-protocols/tree/master/components/esp_modem/examples)
- Return an error if the selected interface does not have IN and OUT bulk endpoints
- Add support for USB "triple null" devices, which use USB interface association descriptors, but have Device Class, Device Subclass, and Device Protocol all set to 0x00, instead of 0xEF, 0x02, and 0x01 respectively. USB Standard reference: https://www.usb.org/defined-class-codes, Base Class 00h (Device) section.
- New function `cdc_acm_host_register_new_dev_callback`. This allows you to get New Device notifications even if you use the default driver.
- Receive buffer has configurable size. This is useful if you expect data transfers larger then Maximum Packet Size.
- Receive buffer has 'append' function. In the Data Received callback you can signal that you wait for more data and the current data were not yet processed. In this case, the CDC driver appends new data to the already received data. This is especially useful if the upper layer messages consist of multiple USB transfers and you don't want to waste more RAM and CPU copying the data around.
- C++ methods are now virtual to allow derived classes to override them.
- Initial version