AMB82-Mini USB Host CDC ECM fails to enumerate Quectel EC200U 4G modem — "ecm init fail"

FreeRTOS USB
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1

Hi Realtek team,

I am developing a 4G dashcam using AMB82-Mini ( RTL8735B) with a Quectel EC200U-CN 4G modem ( Quectel EC200U 4G Modem - CAPUF ). The modem is configured for USB CDC ECM mode via AT+QCFG=“usbnet”,1. UART AT commands work perfectly, the modem registers on the network, and gets an IP address. However, USB Host CDC ECM enumeration always fails with ecm init fail.

Hardware Setup

  • Board: AMB82-Mini EVB (Rev 2V0)

  • Modem: Quectel EC200U-CN

  • SDK: ameba-rtos-pro2 (FreeRTOS SDK, GCC build)

  • USB connection: EC200U USB header pins (5/P/M/G) → AMB82-Mini CON9 (USB Host port)

    • Pin P (USB D+) → CON9 D+ → R60 (33Ω) → USB_HSDP (module pin 85 as per schematic)

    • Pin M (USB D-) → CON9 D- → R61 (33Ω) → USB_HSDM (module pin 86 as per schematic)

    • Pin G (GND) → CON9 GND

    • Pin 5 (VBUS) → CON9 VBUS (5V confirmed with multimeter)

  • Debug serial: CON8 (CH340 debug port) — separate from USB Host

  • UART AT commands: PA_2 (TX) → EC200U RX, PA_3 (RX) → EC200U TX — working

What Works

  • UART AT commands: all working (AT+CPIN? → READY, AT+CREG? → registered, AT+CGPADDR → IP assigned)

  • Modem USB ECM mode confirmed: AT+QCFG=“usbnet” returns +QCFG: “usbnet”,1

  • Modem registered on Airtel 5G LTE (Band 40), signal CSQ 23

  • PDP context active, IP assigned

  • VBUS 5V verified at CON9 and at EC200U USB header pin

  • Tried both D+/D- orientations

What Fails

USB Host ECM enumeration fails every time. Serial log:

[ECM] USB host ECM demo started…

usb otg mode 1

USB_Init

[4G] Waiting for DHCP… (28)

[4G] Waiting for DHCP… (27)

…

ecm init fail

The bulk_task already deinit

The intr_task already deinit

USB_DeInit

ecm deinit ok

ecm_status 2

The USB host controller initializes (usb otg mode 1, USB_Init) but usbh_cdc_ecm_on() returns false — no USB device is detected on the bus.

Code Reference

I am calling ethernet_usb_init() from component/ethernet_mii/ethernet_usb.c (line 593), which creates the usbh_ecm_thread. Inside that thread, ecm_on() (line 290) calls usbh_cdc_ecm_on(), which fails.

My initialization code is as following:

/* In modem_task — called after modem AT init completes */

ethernet_usb_init();

/* Wait for DHCP */

int dhcp_timeout = 30;

while (ecm_lwip_dhcp_status() != 4 && dhcp_timeout-- > 0) {

***printf("\[4G\] Waiting for DHCP... (%d)\\r\\n", dhcp_timeout);***

***vTaskDelay(1000 / portTICK_PERIOD_MS);***

}

Build configuration in video_example_media_framework.cmake:

CONFIG_USBH_CDC_ECM=1

platform_opts.h:

#define CONFIG_ETHERNET 1

#define MII_INTERFACE 0

#define USB_INTERFACE 1

#define ETHERNET_INTERFACE USB_INTERFACE

What I’ve Verified

  1. Schematic confirmed: CON9 D+/D- route to USB_HSDP (pin 85) / USB_HSDM (pin 86) via 33Ω series resistors (R60/R61)

  2. CON8 is separate: Debug USB goes to CH340C (U9), not RTL8735B USB OTG

  3. VBUS detected by modem: USB PWR LED turned on in EC200U module when VBUS connected

  4. Tried with retries: Added 3 retry attempts with ethernet_usb_deinit() between each — same result

  5. Tried longer delays: Waited up to 15 seconds before calling ethernet_usb_init() — same result

Questions

  1. Has USB Host CDC ECM been tested with any device other than RTL8152B USB Ethernet dongle?

  2. Does usbh_cdc_ecm_on() in libusbd.a filter by USB VID/PID?

  3. Is there a way to enable verbose USB host debug logging to see if the host detects any electrical activity on D+/D-?

  4. Is there a known issue with USB Host enumeration on AMB82-Mini EVB ?

  5. Would it be possible to get source code for usbh_cdc_ecm_on() (currently in precompiled library form) so we can debug the enumeration failure?

Environment

  • SDK: ameba-rtos-pro2

  • Board: AMB82-Mini EVB 2V0

  • OS: Windows11

Warm Regards,

Pradeep