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

Kmax18
Senior

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 ...................


_legacyfs_online_stmicro_images_0693W00000binIeQAI.png
_legacyfs_online_stmicro_images_0693W00000binHvQAI.png

1 ACCEPTED SOLUTION

Accepted Solutions

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..​

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View solution in original post

3 REPLIES 3
AA1
Senior III

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
Senior

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.
_legacyfs_online_stmicro_images_0693W00000biy78QAA.png

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..​

Tips, Buy me a coffee, or three.. PayPal Venmo
Up vote any posts that you find helpful, it shows what's working..