Make boot silent

At start the ESP board bootloader generate a report displayed on serial, this report may disturb communication with connected board. It may vary depending on boards but generally look 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
So make it silent is a must.

On ESP32

Bootloader report is not displayed if GPIO15 is connected to GND at startup.

On ESP32-S3

On ESP32-S3 bootloader report is always displayed by default, changing the behavior can be done only once for the pin GPIO46 in sdkconfig file change : 

#
# Boot ROM Behavior
#
CONFIG_BOOT_ROM_LOG_ALWAYS_ON=y
# CONFIG_BOOT_ROM_LOG_ALWAYS_OFF is not set
# CONFIG_BOOT_ROM_LOG_ON_GPIO_HIGH is not set
# CONFIG_BOOT_ROM_LOG_ON_GPIO_LOW is not set
# end of Boot ROM Behavior
by 
#
# Boot ROM Behavior
#
# CONFIG_BOOT_ROM_LOG_ALWAYS_ON=y
# CONFIG_BOOT_ROM_LOG_ALWAYS_OFF is not set
CONFIG_BOOT_ROM_LOG_ON_GPIO_HIGH=y
# CONFIG_BOOT_ROM_LOG_ON_GPIO_LOW is not set
# end of Boot ROM Behavior

Then flash the board, you can reverse the change in SDK config file after as it won't take in account anymore because the change is only allowed once. On Panlee and HMI S3 the pin that trigger the bootloader report is the GPIO 46 and it is low at boot.