ESP32-S2: Changing UART0 speed also changes USB CDC monitor speed (USB_CDC_ON_BOOT = 1)

philtao · August 8, 2023, 6:17am

Hi,
I have been looking for two days on forums at potential interaction between USB CDC and UART0 on an ESP32-S2. All I could find indicate that once USB_CDC_ON_BOOT is enabled the Serial Monitor is using CDC instead of the UART0 and UART0 is available for other use.
I must have missed something - any clue about what I could be doing wrong?

I am using an ESP32-S2-DevKitM-1 to validate some code before moving to a custom board with ESP32-S2-SOLO module.
USB CDC: serial monitor (USB_CDC_ON_BOOT = 1) with monitor speed = 115200
UART0: RS485 / Modbus Slave with speed = 19200

My issue is: When I start the Modbus slave communication stack then the monitor starts displaying garbage (it seems to be a problem of communication speed as all is fine if Monitor and Modbus slave are both set at the same speed).

platformio.ini

[env]
platform = espressif32
framework = arduino
build_flags = 
   -DARDUINO_USB_CDC_ON_BOOT=1
   -DCONFIG_ARDUHAL_LOG_COLORS=1
   -DCORE_DEBUG_LEVEL=5

monitor_speed = 115200
monitor_filters = direct, esp32_exception_decoder
board_build.partitions = partitions.csv

[env:ESP32-S2-DevKitM-1]
board = esp32-s2-saola-1

Modbus slave setup:

#define GPIO_MB_SLAVE_TX    GPIO_NUM_43
#define GPIO_MB_SLAVE_RX    GPIO_NUM_44
#define GPIO_MB_485_SHDN    GPIO_NUM_15             // GPIO to shutdown RS485 (drive high for normal operation)
#define MB_SLAVE_UART       UART_NUM_0
#define MB_SLAVE_ADDR       1
#define MB_SLAVE_SPEED      19200
//#define MB_SLAVE_SPEED      115200

The code to setup the Modbus slave:

 esp_err_t mbSlaveSetup() 
{
  log_v("Setup RS485 port and Modbus slave driver");
  pinMode(GPIO_MB_485_SHDN, OUTPUT);
  digitalWrite(GPIO_MB_485_SHDN, HIGH);

  esp_err_t err = ESP_OK;
  mb_slave_setup.mode = MB_MODE_RTU;
  mb_slave_setup.slave_addr = MB_SLAVE_ADDR;
  mb_slave_setup.port = MB_SLAVE_UART;
  mb_slave_setup.baudrate = MB_SLAVE_SPEED;
  mb_slave_setup.parity = MB_PARITY_NONE;
  reg_area.type = MB_PARAM_HOLDING;
  reg_area.address = &reg;
  reg_area.start_offset = 0;
  reg_area.size = REG_NUM * 2;
  
  log_v("Initialize Modbus slave controller");
  err = mbc_slave_init(MB_PORT_SERIAL_SLAVE, &mb_slave_handler);
  if (err != ESP_OK) {
    log_e("Error initializing the serial port for Modbus: %i", err);
    return err;
  }

  log_v("setup Modbus slave");
  err = mbc_slave_setup((void*)&mb_slave_setup);
  if (err != ESP_OK) {
    log_e("Error Modbus slave setup: %i", err);
    return err;
  }
  
  log_v("Set Modbus slave registers structure");
  err = mbc_slave_set_descriptor(reg_area);
  if (err != ESP_OK) {
    log_e("Error setting Modbus slave registers structure: %i", err);
    return err;
  }

  log_v("Initialize Modbus communication stack");
  delay(100);
  err = mbc_slave_start();
  if (err != ESP_OK) {
    log_e("Error starting Modbus communication stack: %i", err);
    return err;
  }

  log_v("Set Modbus / RS485 UART pins - UART: %i  /  TX: %i  /  RX: %i", MB_SLAVE_UART, GPIO_MB_SLAVE_TX, GPIO_MB_SLAVE_RX);
  err = uart_set_pin(MB_SLAVE_UART, GPIO_MB_SLAVE_TX, GPIO_MB_SLAVE_RX, -1, -1);
  if (err != ESP_OK) {
    log_e("Error setting Modbus / RS485 UART pins: %i", err);
    return err;
  }
  
  log_v("Set RS485 UART mode");
  delay(100);
  err = uart_set_mode(MB_SLAVE_UART, UART_MODE_RS485_HALF_DUPLEX);
  if (err != ESP_OK) {
    log_e("Error setting Modbus UART mode: %i", err);
    return err;
  }
  return ESP_OK;
}

Output:

[  2075][V][mb_slave.cpp:11] mbSlaveSetup(): Setup RS485 port and Modbus slave driver
[  2076][V][mb_slave.cpp:26] mbSlaveSetup(): Initialize Modbus slave controller
[  2081][V][mb_slave.cpp:33] mbSlaveSetup(): setup Modbus slave
[  2087][V][mb_slave.cpp:40] mbSlaveSetup(): Set Modbus slave registers structure
[  2096][V][mb_slave.cpp:47] mbSlaveSetup(): Initialize Modbus communication stack
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if I set the MB_SLAVE_SPEED = 115200 the output is fine:

[  2074][V][mb_slave.cpp:11] mbSlaveSetup(): Setup RS485 port and Modbus slave driver
[  2075][V][mb_slave.cpp:26] mbSlaveSetup(): Initialize Modbus slave controller
[  2080][V][mb_slave.cpp:33] mbSlaveSetup(): setup Modbus slave
[  2086][V][mb_slave.cpp:40] mbSlaveSetup(): Set Modbus slave registers structure
[  2094][V][mb_slave.cpp:47] mbSlaveSetup(): Initialize Modbus communication stack
[  2203][V][mb_slave.cpp:55] mbSlaveSetup(): Set Modbus / RS485 UART pins - UART: 0  /  TX: 43  /  RX: 44
[  2203][V][mb_slave.cpp:62] mbSlaveSetup(): Set RS485 UART mode

maxgerhardt · August 8, 2023, 6:27am

…What? That board doesn’t have a USB conenctor that by default directly connects to the ESP32-S2. Per documentation and schematics,

So… it seems to me that you’ve programmed the ESP-32 to use the exact same UART peripheral for both serial monitor and modbus and you’re under the impression that by setting USB_CDC_ON_BOOT = 1 you free up the hardware UART for Modbus but you’re not actually connected to that USB-CDC (created by the ESP32-S2, not by the CP2102N) that would output the serial monitor messages at all.

Looking at the schematics.

USB_DM and USB_DP (D+, D-) go to the CP2102N USB-to-UART chip. GPIO19 and 20 go to the ESP32-S2.

R2 and R3 are NOT CONNECTED by default. Desolder R1 and R4 to sever the connection to the CP2102N and use those desolder resistors to fit R2, R3 instead. That will make the USB plug actually connected to the ESP32. You should still be able to flash the ESP32-S2 via its native USB bootloader then. You’ll then want to make sure that you receive the regular bootloader messages via that USB plug. Then you can think about using UART0 for modbus, because now they’re truely independent.

maxgerhardt · August 8, 2023, 6:30am

Actually, even that seems to be overcomplicating it massively. If the serial monitor and the Modbus connection are on different pins, and you have two UART peripherals per datasheet (https://www.espressif.com/sites/default/files/documentation/esp32-s2_datasheet_en.pdf)

Why not use the UART1 peripheral for Modbus and leave UART0 alone?

What happens whne you just change that to UART_NUM_1?

philtao · August 8, 2023, 7:38am

Hi @maxgerhardt ,
Thank you for your rapid and very comprehensive reply. I did not think about checking the devKitM diagram! The USB port is connected to UART0 on that development board… so what I observe is logical.

That is good news as my prototype PCB with ESP32-S2-SOLO will not have that issue (I will get them in a couple of days

Why not use the UART1 peripheral for Modbus and leave UART0 alone?

That was in fact my first reaction to go around the problem as I am not using UART1.
But the serial monitor stops printing anything when executing command
err = uart_set_pin(MB_SLAVE_UART, GPIO_MB_SLAVE_TX, GPIO_MB_SLAVE_RX, -1, -1);
output:

[  2090][V][mb_slave.cpp:47] mbSlaveSetup(): Initialize Modbus communication stack
[  3098][V][mb_slave.cpp:55] mbSlaveSetup(): Set Modbus / RS485 UART pins - UART: 1  /  TX: 43  /  RX: 44

So I thought it could be because the same GPIO’s are used for UART0 and UART1. So I changed GPIO pins for UART0 before setting the pins for UART1… Same result.
Now with your explanation that outcome is logical as the pins where the USB-UART Bridge is connected are now routed to UART1 where nothing happens!
Thank you again for this lesson.