ESP32 Forum

In https://www.espressif.com/sites/default ... ual_en.pdf

UART_STOP_BIT_NUM This register is used to set the length of the stop bit; 1: 1 bit, 2: 1.5 bits. (R/W)

Defines stop bits for the URAT to be 1,1.5, and 2

this also aligns with the arduino drivers

typedef enum {
UART_STOP_BITS_1 = 0x1, /*!< stop bit: 1bit*/
UART_STOP_BITS_1_5 = 0x2, /*!< stop bit: 1.5bits*/
UART_STOP_BITS_2 = 0x3, /*!< stop bit: 2bits*/
UART_STOP_BITS_MAX = 0x4,
} uart_stop_bits_t;

However, 3 stop bits should be supported

13.3.5 UART Data Frame
Figure 13-3 shows the basic data frame structure. A data frame starts with a START condition and ends with a
STOP condition. The START condition requires 1 bit and the STOP condition can be realized using 1/1.5/2/3-bit
widths (as set by UART_STOP_BIT_NUM, UART_DL1_EN, and UAR_DL0_EN). The START is low level, while the
STOP is high level.

What are the register settings for 3 uart stop bits?

Thank you!

Looks like, as the text implies, you can use UART_DL1_EN and UAR_DL0_EN to add extra bits to the stop bit amount defined there. It's not super-well documented as devices rarely need 2 stop bits, let alone 3.
EDIT: This is wrong, look at the reply of my colleague below instead.

Hi there, I'm sorry for the trouble caused by the unclear register descriptions.

UART_STOP_BIT_NUM, which is used to set the length of stop bits, has 4 valid values. 0: 1 bits; 1: 2 bits; 2: 1.5 bits; 3: 3 bits.
UART_DL1_EN and UART_DL0_EN are used to set the turnaround delay as required by the RS485 protocol. It has nothing to do with the length of stop bits. This function is documented in Section 26.4.2 Turnaround Delay https://www.espressif.com/sites/default ... n#uart.pdf

I will update the descriptions in TRM in the next release.

Thank you, but this doesn't seem to align with the code.

From your post

UART_STOP_BIT_NUM, which is used to set the length of stop bits, has 4 valid values. 0: 1 bits; 1: 2 bits; 2: 1.5 bits; 3: 3 bits.

From the code I see
https://github.com/espressif/arduino-es ... rt_types.h (line 56) Which translates to 0: undefined, 1: 1 bits; 2: 1.5 bits; 3: 2 bit. There is agreement that 2 means 1.5 stop bits, but the values for 0,1, and 3 are not in agreement.

Additionally, the current documentation on page 520
https://www.espressif.com/sites/default ... ual_en.pdf
says:

UART_STOP_BIT_NUM This register is used to set the length of stop bit. 1: 1 bit. 2: 1.5 bits. 3: 2 bits. (R/W)

This also agrees with the code.

Is it possible 0 is used to represent 3 stop bits?

I am new the forum. Where should this be cross posted to hopefully get data structure updated to include 3 stop bits and updated to handle it.

Thank you!

Hi @ESP_wangning, do you have any thoughts or suggestions on the best next step to figure this out?

Thank you!

Sorry that I just noticed your reply!
It seems that there is some inconsistency between hardware and software. I'm checking and will keep you updated.

Sorry that my previous explanation is wrong.

The length of stop bits can be 1, 1.5 or 2, configured by UART_STOP_BIT_NUM.
0: Invalid. No effect
1: 1 bits
2: 1.5 bits
3: 2 bits

In RS485 mode, you may add a turnaround delay of a bit before the start bit or after the stop bit, configured by UART_DL0_EN or UART_DL1_EN respectively. If you add one turnaround delay, this is actually what we mean by "3 stop bits".
You can add two turnaround delays before the start bit and after the stop bit at the same time, or in other words the "stop bits" can be 4 bits long at most.

We will make the descriptions clearer in the next TRM release.

Explanation of why 2 stop bits were needed for a mechanical TTY https://retrocomputing.stackexchange.com/a/631
Extra stop bit(s) might be needed to slow down a datastream, but it would be pretty unusual for hardware less than about 30 years old.

If you need this, possibly you can use the RMT peripheral to implement a semi-software UART?

Thank you! Sorry I missed your reply. I will look into the RMT. I am trying to use this to connect into a DMX data stream of a lighting console. The MTBF I think can be handled as an extra stop bit.


https://www.theatreartlife.com/staying- ... mystified/