STM8S105 UART (async), data frame with 1 unknown bit after 2 stop bits

[Kmax18]

I am analyzing the UART data frame of an outsourced device that uses the STM8S105 MCU. The data frame specification is:

* 1 start bit,

* 8 data bits,

* 1 parity bit,

* 2 stop bits.

Therefore I expect a data frame as in the below Figure 115 from the STM8 reference manual RM0016. However, both the logic analyzer and oscilloscope show an extra bit after the 2 stop bits. Then the next data frame begins. This is illustrated in the second figure below.

Question: what feature creates the extra bit? This is unexpected.

Is it possibly related to the break character feature?

Thank you for your response.

Figures 1 and 2 ...................

[AA1]

As the extra bit have level '1', so is the same as have 3 stop bits, And so no problem with communication.

How did you detect this extra bit?

[Kmax18]

Hello @Arsénio Almeida​, thank you for your response. I found the extra but while troubleshooting a malfunctioning UART communication link. The extra bit is easy to see in the below oscilloscope screenshot when comparing it with my own UART comm.

[Tesla DeLorean · Accepted solution]

Extra intersymbol gaps should​ not break async comms, and may be the result of not responding quickly to a TC interrupt, or not using TXE to tightly pack output stream.

Could be a 9-bit mode or parity..​