FAQ

Make boot silent

On every reset, the ESP ROM bootloader (stage 1) prints a diagnostic report on the default console (UART0 and/or USB Serial/JTAG). This output can disturb the communication between the ESP and the connected controller (3D printer, CNC board, etc.).

The output varies from chip to chip but generally looks like this:

ets Jun  8 2016 00:22:57

rst:0x1 (POWERON_RESET),boot:0x17 (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:2
load:0x3fff0030,len:1184
load:0x40078000,len:13104
load:0x40080400,len:3036
entry 0x400805e4

Silencing it is often a must for production use.

Note :

The ROM bootloader output is separate from the second-stage bootloader and application log output. Those are controlled via menuconfig:

Bootloader config → Bootloader log verbosity → No output
Component config → Log output → Default log verbosity → No output

On ESP32 (classic)

The ROM messages can be silenced simply by pulling GPIO15 to GND during reset. GPIO15 has an internal pull-up, so when left floating it goes HIGH and the ROM prints normally. Driving it LOW at reset silences the ROM bootloader.

This is a runtime choice: remove the pull-down and the ROM output comes back on the next reset. It is fully reversible.

On ESP32-S2 / S3 / C3 / C6 / H2 / P4

On these newer targets, silencing the ROM log is done exclusively via eFuses. Two eFuse fields are relevant, depending on where the ROM prints:

eFuse field	Output channel	Availability
`UART_PRINT_CONTROL`	UART0 (`U0TXD`)	S2, S3, C3, C6, H2, P4
`DIS_USB_SERIAL_JTAG_ROM_PRINT`	USB Serial/JTAG controller	S3, C3, C6, H2, P4 (not S2)

UART_PRINT_CONTROL is a 2-bit field (not a simple on/off). Its values are:

Value	Behavior
`0b00`	Force enabled (default)
`0b01`	Controlled by GPIO (pin level decides at reset)
`0b10`	Controlled by GPIO (inverted logic)
`0b11`	Force disabled (ROM log always silent)

Note :

eFuses are one-time programmable.** Bits can only go from 0 to 1, never back. Once you burn UART_PRINT_CONTROL = 0b11 (force silent), the ROM log is permanently disabled and cannot be re-enabled for debugging. Similarly, DIS_USB_SERIAL_JTAG_ROM_PRINT = 1 is irreversible.

Test your firmware thoroughly before burning these fuses.

The “GPIO-controlled” modes (0b01 / 0b10) give you a compromise: a chosen GPIO level at reset decides whether the ROM prints, similar in spirit to GPIO15 on the classic ESP32 — but the pin and polarity are fixed by the eFuse value and cannot be changed afterwards.

Programmatic access — ESP-IDF exposes a helper to set the scheme from firmware:

#include "esp_efuse.h"

// Permanently silence UART ROM log (writes UART_PRINT_CONTROL eFuse)
esp_efuse_set_rom_log_scheme(ESP_EFUSE_ROM_LOG_ALWAYS_OFF);

Other values are ESP_EFUSE_ROM_LOG_ALWAYS_ON, ESP_EFUSE_ROM_LOG_ON_GPIO_LOW, and ESP_EFUSE_ROM_LOG_ON_GPIO_HIGH.

From the command line — espefuse.py can burn the fuses directly:

# Permanently disable UART ROM boot log
espefuse.py --port /dev/ttyUSB0 burn_efuse UART_PRINT_CONTROL 3

# Permanently disable USB Serial/JTAG ROM boot log (S3/C3/C6/H2/P4)
espefuse.py --port /dev/ttyUSB0 burn_efuse DIS_USB_SERIAL_JTAG_ROM_PRINT 1

Using ESP3D

ESP3D exposes the [ESP999] command to burn the appropriate eFuse without needing espefuse.py or a separate firmware:

[ESP999]QUIETBOOT pwd=<admin password>

Supported on ESP32-S2, S3, C3, C6 (and newer variants as they are added). This action is permanent — same irreversibility rules as above apply.

On ESP8266

There is no way to silence the ROM bootloader on ESP8266. The initial messages (on GPIO1 / U0TXD at 74880 baud, then 115200 once the app starts) are always printed and cannot be disabled.

Workarounds, depending on the hardware design:

Route the printer/host connection on a different UART than UART0, so the ROM chatter stays on the debug port.
Remap UART0 TX to a pin not connected to the host (ESP8266 allows swapping U0TXD to GPIO15).
Accept the noise and make the host firmware tolerant to the initial garbage.