STM32F1 UART TX idles after every byte

[jlpoltrack]

Describe the bug
When using UART1 on STM32F1, TX seem to idle after every byte sent. Don't expect this idle period.

To Reproduce
Source from Arduino IDE:

void setup() {
  Serial.begin(1000000);
}

void loop() {
  Serial1.printf("Test");
  Serial1.flush();
  delay(100);
}

Screenshot from Scope:

Expected behavior
Expect to see continuous output from TX, such as:

Screenshots
Screenshots above

Desktop (please complete the following information):

• OS: Win 10
• Arduino IDE version: 1.8.13
• STM32 core version: 1.9.0
• Tools menu settings if not the default: Default
• Upload method: Serial

Board (please complete the following information):

• Name: STM32F103C Blue Pill

[ABOSTM]

Hi @jlpoltrack,
As I understand, there is no real issue, you just don't get the full performances.
I don't consider this behavior has a bug, it works the way it has been implemented:
Current implementation send each bytes one by one, even if a multi character string is provided as argument.
And the consequence is that there is interrupt and CPU activity between each bytes (to prepare and send the next bytes).

Nevertheless, I agree that it is not really optimized.
This is why I propose a slightly different implementation in PR #1313

[jlpoltrack]

@ABOSTM
Thanks for the response and explaining the current implementation. As I didn't see this mentioned in the API documentation, thought I had something wrong with my setup. I noticed when I saw a regression in packet size at higher UART speeds.
I look forward to trying out the new implementation in the future.