esp_modem_usb_dte on ESP32-P4: how to properly initialize FS PHY2 (IEC-485)

[dmartauz]

Answers checklist.

• I have read the documentation of the component in question and the issue is not addressed there.
• I have searched the issue tracker for a similar issue and not found a similar issue.

General issue report

It seems that esp_modem/esp_modem_usb_dte examples initialize HS PHY by default.

I tried to disable auto phy setup by usb_host_install() with host_config.skip_phy_setup = true and initialize PHY manually before calling usb_host_install():

        usb_phy_handle_t phy_hdl = NULL;
        usb_phy_config_t phy_conf = {
            .controller = USB_PHY_CTRL_OTG,
            .target = USB_PHY_TARGET_INT,
            .otg_mode = USB_OTG_MODE_HOST,
            .otg_speed = USB_PHY_SPEED_FULL
        };
        esp_err_t ret = usb_new_phy(&phy_conf, &phy_hdl);

but this did not work as expected.

[github-actions]

Hi @dmartauz! Please be aware that the following suggestions are generated by a bot and haven't been fact-checked by Espressif Systems. We hope that this message will help you until an Espressif Engineer looks at your issue.

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You are right that the default esp_modem USB DTE examples will end up using the HS PHY on ESP32‑P4. On P4 there are three USB controllers and three PHYs, and you need to tell the USB Host driver which one to use.

From the available docs we know:

• ESP32‑P4 has HS PHY (pin 49/50) plus two FS PHYs:
  • FS_PHY1: D+ on GPIO25, D‑ on GPIO24
  • FS_PHY2: D+ on GPIO27, D‑ on GPIO26 [USB PHY intro].
• By default, FS_PHY1 is wired to USB‑Serial‑JTAG and FS_PHY2 to the Full‑Speed OTG controller; this mapping can be swapped via EFUSE_USB_PHY_SEL [ESP32‑P4 USB overview; FAQ pins].
• To use the FS PHY as a host with ESP32‑P4, you must select it via the peripheral_map field when calling usb_host_install() [FAQ FS host].

So, just calling usb_new_phy() with USB_PHY_TARGET_INT is not enough; the host driver still needs to be told to bind to the FS OTG controller instead of the HS one.

According to the FAQ, the key is:

When calling the USB host initialization function usb_host_install, you can use the USB FS PHY as the host by setting the value of peripheral_map in the structure parameter to BIT1. [FAQ FS host]

Putting this together, the recommended approach for ESP32‑P4 FS PHY2 host is:

1. Configure the host driver to use the FS OTG controller via peripheral_map = BIT1 in usb_host_config_t.
2. If you still want to manage PHY init yourself, keep skip_phy_setup = true and call usb_new_phy() before usb_host_install(); but the crucial part is that peripheral_map must select the FS controller.

A minimal sketch (structure only; adapt to your project) would look like:

// 1) (Optional) Manual PHY init – FS, internal, OTG host
usb_phy_handle_t phy_hdl = NULL;
usb_phy_config_t phy_conf = {
    .controller = USB_PHY_CTRL_OTG,
    .target     = USB_PHY_TARGET_INT,
    .otg_mode   = USB_OTG_MODE_HOST,
    .otg_speed  = USB_PHY_SPEED_FULL,
};
ESP_ERROR_CHECK(usb_new_phy(&phy_conf, &phy_hdl));

// 2) USB Host config – select FS controller via peripheral_map
usb_host_config_t host_config = {
    .skip_phy_setup = true,   // because we called usb_new_phy() above
    .peripheral_map = BIT1,   // this selects the FS PHY host on ESP32‑P4
    // ... other fields as needed ...
};

ESP_ERROR_CHECK(usb_host_install(&host_config));

Notes / things to double‑check on your board:

• Make sure your 4G modem (or other device) is wired to FS_PHY2 pins: D+ = GPIO27, D‑ = GPIO26, unless you have changed EFUSE_USB_PHY_SEL and intentionally swapped FS_PHY1/FS_PHY2 [USB PHY intro; ESP32‑P4 FS OTG pins].
• If you have burned EFUSE_USB_PHY_SEL, remember that FS_PHY1/FS_PHY2 roles for OTG_FS vs USB‑Serial‑JTAG are swapped; the host controller is still the same OTG_FS block, but now connected to the other FS PHY pair [ESP32‑P4 USB overview].

If after setting peripheral_map = BIT1 it still falls back to HS, then the issue is likely in esp_modem’s use of usb_host_install() (e.g., overriding your config). In that case, sharing the exact usb_host_config_t used by esp_modem in the issue would be the next step.

[dmartauz]

Setting peripheral_map = BIT1 worked, problem solved.