Espressif ESP32 Technical Reference Manual page 351

13 UART Controller (UART)
13.3.6 AT_CMD Character Structure
Figure 13-4 shows a special AT_CMD character format. If the receiver constantly receives UART_AT_CMD_CHAR
characters and these characters satisfy the following conditions, interrupt UART_AT_CMD_CHAR_DET_INT will
be generated.
• Between the first UART_AT_CMD_CHAR and the last non-UART_AT_CMD_CHAR, there are at least UART_
PER_IDLE_NUM APB clock cycles.
• Between every UART_AT_CMD_CHAR character there must be less than
cycles.
• The number of received UART_AT_CMD_CHAR characters must be equal to, or greater than, UART_CHAR_NUM.
• Between the last UART_AT_CMD_CHAR character received and the next non-UART_AT_CMD_CHAR, there
are at least UART_POST_IDLE_NUM APB clock cycles.
13.3.7 Flow Control
UART controller supports both hardware and software flow control. Hardware flow control regulates data flow
through input signal dsrn_in and output signal rtsn_out. Software flow control regulates data flow by inserting
special characters in the flow of sent data and by detecting special characters in the flow of received data.
Espressif Systems
Figure 13-4. AT_CMD Character Format
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UART_RX_GAP_TOUT APB clock
ESP32 TRM (Version 5.2)