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<a href="https://gitlab.rtems.org/mohamedayman23">Mohamed Ayman</a> created a merge request: <a href="https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1143">!1143</a>
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Project:Branches: mohamedayman23/rtems:fix-USART-mathematical-bug-stm32f4 to rtems/rtos/rtems:main
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Author: Mohamed Ayman
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Assignees:
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Reviewers:
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<h2 id="user-content-summary" dir="auto" style="margin-top: 0; margin-bottom: 10px;" align="initial">Summary<a href="#summary" aria-label="Link to heading 'Summary'" data-heading-content="Summary" class="anchor" style="margin-top: 0;"></a>
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">stm32f4/usart: fix BRR calculation logical issues</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">The USART BRR calculation in usart_get_bbr() contains two issues that can lead to incorrect baud rate configuration.</p>
<ol dir="auto" style="text-align: initial; margin: 0 0 1rem; padding: 0;">
<li style="margin-top: 0; line-height: 1.6em; margin-left: 25px; padding-left: 3px;">Incorrect fractional divisor formula</li>
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial"><a class="no-attachment-icon gfm" href="https://gitlab.rtems.org/-/project/26/uploads/cf211e30bffcbffedd38219176ae33ed/Screenshot_from_2026-03-17_04-59-01.png" target="_blank" rel="noopener noreferrer" data-canonical-src="/uploads/cf211e30bffcbffedd38219176ae33ed/Screenshot_from_2026-03-17_04-59-01.png" data-link="true" style="margin-top: 0;"><img src="https://gitlab.rtems.org/-/project/26/uploads/cf211e30bffcbffedd38219176ae33ed/Screenshot_from_2026-03-17_04-59-01.png" alt="Screenshot from 2026-03-17 04-59-01.png" width="900" height="488" data-canonical-src="/uploads/cf211e30bffcbffedd38219176ae33ed/Screenshot_from_2026-03-17_04-59-01.png" class="gfm" style="max-width: 100%; height: auto; margin-top: 0; vertical-align: baseline; object-fit: contain; object-position: top;"></a></p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">The current implementation computes the fractional part as:</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">the <strong style="font-weight: 600; margin-top: 0;">important part</strong>: USARTDIV = mantissa + fraction / a</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Putting them together From:</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">USARTDIV = mantissa + fraction / a</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">and: USARTDIV = pclk / (a × baud)</p>
<ul dir="auto" style="text-align: initial; list-style-type: disc; margin: 0 0 1rem; padding: 0;">
<li style="margin-top: 0; line-height: 1.6em; margin-left: 25px; padding-left: 3px;">multiply both sides by <code style="font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 0.125rem 0.25rem;">a</code>: pclk / baud = a × mantissa + fraction</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">solve for <code style="font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 0.125rem 0.25rem;">fraction</code>: fraction = (pclk / baud) - a × mantissa</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">convert to integer-safe form: fraction = (pclk - baud × a × mantissa) / baud</li>
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">This implementation is both mathematically correct and avoids <strong style="font-weight: 600; margin-top: 0;">division by zero</strong>.</p>
<ol start="2" dir="auto" style="text-align: initial; margin: 0 0 1rem; padding: 0;">
<li style="margin-top: 0; line-height: 1.6em; margin-left: 25px; padding-left: 3px;">Incorrect mantissa update for high approximation</li>
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">When computing the higher approximation of USARTDIV, the current code assigns: div_mantissa_high = div_fraction_low;</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">This appears to be a copy-paste error. The mantissa must be derived from div_mantissa_low, not from the fractional part.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">As written, this produces completely invalid divisor values (mixing mantissa and fraction domains), which can lead to large baud rate errors.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">This properly computes the next representable USARTDIV value.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Together, these fixes ensure that BRR values are computed correctly, avoid undefined behavior, and prevent potential baud rate misconfiguration.</p>
<h2 id="user-content-" dir="auto" style="margin-top: 20px; margin-bottom: 10px;" align="initial"><a href="#" aria-label="Link to heading ''" data-heading-content="" class="anchor" style="margin-top: 0;"></a></h2>
<p dir="auto" style="color: #3a383f; margin: 0;" align="initial">AI usage: yes<br style="margin-top: 0;">
Prompt used: "why the baud rate is missed in the main equation, make a lot of computations with random numbers and give me the the predicted output and the actual one"<br>
AI model: claude sonnet 4.6<br>
How AI was used: for extensive computations and checking the accuracy</p>
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