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| <section id="changing-the-system-clock-configuration"> |
| <h1>Changing the System Clock Configuration<a class="headerlink" href="#changing-the-system-clock-configuration" title="Permalink to this heading"></a></h1> |
| <div class="admonition warning"> |
| <p class="admonition-title">Warning</p> |
| <p>Migrated from: |
| <a class="reference external" href="https://cwiki.apache.org/confluence/display/NUTTX/Changing+the+System+Clock+Configuration">https://cwiki.apache.org/confluence/display/NUTTX/Changing+the+System+Clock+Configuration</a></p> |
| </div> |
| <section id="question"> |
| <h2>Question<a class="headerlink" href="#question" title="Permalink to this heading"></a></h2> |
| <p><cite>Is an STM32 configuration booting with the internal 16 MHz clock, then |
| switching later (on command) to an external 25 MHz xtal doable? I don’t think |
| so, but would you mind confirming that?</cite></p> |
| </section> |
| <section id="answer"> |
| <h2>Answer<a class="headerlink" href="#answer" title="Permalink to this heading"></a></h2> |
| <p>Of course, that is what always happens: The STM32 boots using an internal clock |
| and switches to the external crystal source after booting. But I assume that |
| you mean MUCH later on, after initialization.</p> |
| <p>Yes that can be done too. There are only a few issues and things to be aware of:</p> |
| <section id="custom-clock-configuration"> |
| <h3>Custom Clock Configuration<a class="headerlink" href="#custom-clock-configuration" title="Permalink to this heading"></a></h3> |
| <p>The <code class="docutils literal notranslate"><span class="pre">configs/vsn/</span></code> configuration does something like you say. It skips the |
| initial clock configuration by defining |
| <code class="docutils literal notranslate"><span class="pre">CONFIG_ARCH_BOARD_STM32_CUSTOM_CLOCKCONFIG=y</span></code>. Then the normal clock |
| configuration logic in <code class="docutils literal notranslate"><span class="pre">arch/arm/src/stm32/stm32_rcc.c</span></code> is not executed. |
| Instead, the “custom” clock initialization at <code class="docutils literal notranslate"><span class="pre">configs/vsn/src/sysclock.c</span></code> |
| is called:</p> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span><span class="w"> </span><span class="nf">stm32_clockconfig</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> |
| <span class="p">{</span> |
| <span class="w"> </span><span class="cm">/* Make sure that we are starting in the reset state */</span> |
| |
| <span class="w"> </span><span class="n">rcc_reset</span><span class="p">();</span> |
| |
| <span class="cp">#if defined(CONFIG_ARCH_BOARD_STM32_CUSTOM_CLOCKCONFIG)</span> |
| |
| <span class="w"> </span><span class="cm">/* Invoke Board Custom Clock Configuration */</span> |
| |
| <span class="w"> </span><span class="n">stm32_board_clockconfig</span><span class="p">();</span> |
| |
| <span class="cp">#else</span> |
| |
| <span class="w"> </span><span class="cm">/* Invoke standard, fixed clock configuration based on definitions in board.h */</span> |
| |
| <span class="w"> </span><span class="n">stm32_stdclockconfig</span><span class="p">();</span> |
| |
| <span class="cp">#endif</span> |
| |
| <span class="w"> </span><span class="cm">/* Enable peripheral clocking */</span> |
| |
| <span class="w"> </span><span class="n">rcc_enableperipherals</span><span class="p">();</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>Doing things that way, you can have complete control over when the crystal |
| clock source is used. The initial “custom” clock configuration can use an |
| internal source, then other custom clock configuration logic can change the |
| clock source later.</p> |
| <p>NOTE: Since this original writing, the VSN configuration has been retired and |
| is no long in at config/vsn. The retired code can still be found in the |
| <a class="reference external" href="https://bitbucket.org/patacongo/obsoleted/src/master/nuttx/configs/vsn">Obsoleted repository</a>.</p> |
| </section> |
| <section id="peripheral-clocks"> |
| <h3>Peripheral Clocks<a class="headerlink" href="#peripheral-clocks" title="Permalink to this heading"></a></h3> |
| <p>The peripheral clock used by many devices to set up things like the SPI |
| frequency and UART bard rates. Currently, those peripheral clock frequencies |
| are hardcoded in the board.h header file. So you have two options:</p> |
| <ol class="arabic simple"> |
| <li><p><strong>Fixed Peripheral Clocking</strong>. Ideally, you would like to keep the peripheral |
| clock frequencies the same in either case. Then life is simple. You could |
| probably use an internal RC clock source as input to a PLL and set up |
| dividers so that you get the same peripheral clocks. Then, I think, from |
| the standpoint of the peripherals, nothing happened.</p></li> |
| <li><p><strong>Variable Peripheral Clocking</strong>. You can make the peripheral clocking |
| variable. I had to do this for the SAMA5Dx family. Look at |
| <code class="docutils literal notranslate"><span class="pre">boards/arm/stm32/sama5d4-ek/include/board_sdram.h</span></code> for example. Notice |
| that the frequencies are not constants, but function calls:</p></li> |
| </ol> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#define BOARD_MAINCK_FREQUENCY BOARD_MAINOSC_FREQUENCY</span> |
| <span class="cp">#define BOARD_PLLA_FREQUENCY (sam_pllack_frequency(BOARD_MAINOSC_FREQUENCY))</span> |
| <span class="cp">#define BOARD_PLLADIV2_FREQUENCY (sam_plladiv2_frequency(BOARD_MAINOSC_FREQUENCY))</span> |
| <span class="cp">#define BOARD_PCK_FREQUENCY (sam_pck_frequency(BOARD_MAINOSC_FREQUENCY))</span> |
| <span class="cp">#define BOARD_MCK_FREQUENCY (sam_mck_frequency(BOARD_MAINOSC_FREQUENCY))</span> |
| </pre></div> |
| </div> |
| <p>Given that I know that XTAL oscillator frequency I can derive the frequency of |
| other clocks. This turns out to be more work than you would think, however, |
| because there are probably C pre-processor tests that will now fail. Like:</p> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#if BOARD_MCK_FREQUENCY > 16000000</span> |
| <span class="p">...</span><span class="w"> </span><span class="k">do</span><span class="w"> </span><span class="n">something</span><span class="w"> </span><span class="p">...</span> |
| <span class="cp">#endif</span> |
| </pre></div> |
| </div> |
| <p>Such logic would have to be converted from a compile time decision to a |
| run-time decision, perhaps like this:</p> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">BOARD_MCK_FREQUENCY</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="mi">16000000</span><span class="p">)</span> |
| <span class="p">{</span> |
| <span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="k">do</span><span class="w"> </span><span class="n">something</span><span class="w"> </span><span class="p">...</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>The SAMA5D4-EK case was intended for the case where the software is running out |
| of SDRAM and the clocking cannot be reconfigured. Rather, it must derive the |
| clocking as it was left by the bootloader. But you could do something like what |
| was done for the SAMA5D4-EK when you change the frequency too. You could also |
| make the peripheral clocks variable.</p> |
| </section> |
| <section id="reinitializing-peripherals"> |
| <h3>Reinitializing Peripherals<a class="headerlink" href="#reinitializing-peripherals" title="Permalink to this heading"></a></h3> |
| </section> |
| <section id="variable-peripheral-clocking"> |
| <h3>Variable Peripheral Clocking<a class="headerlink" href="#variable-peripheral-clocking" title="Permalink to this heading"></a></h3> |
| <p>If you did something like what was done for the SAMA5D4-EK when you change the |
| frequency, then the peripheral clocks would be variable. The main problem would |
| then be that you would have to re-initialize the peripherals when the |
| peripheral clocking changes. If, for example, the UART was initialized at |
| the initial peripheral clock, then you would have to recalculate the BAUD |
| divisor if the peripheral clock changes.</p> |
| <p>But this is not really be a big issue. You can force the UARTs to recalculate |
| the BAUD divisor with TERMIOS ioctl calls. You could use the setfrequency() |
| methods to recalculate I2C and SPI BAUD divisors. But there are also memory |
| card frequencies and more.</p> |
| </section> |
| <section id="systick-timer"> |
| <h3>Systick Timer<a class="headerlink" href="#systick-timer" title="Permalink to this heading"></a></h3> |
| <p>If the CPU frequency changes, you would have to change the Systick timer |
| configuration: It is always driven by the CPU clock</p> |
| </section> |
| <section id="up-mdelay"> |
| <h3>up_mdelay<a class="headerlink" href="#up-mdelay" title="Permalink to this heading"></a></h3> |
| <p>up_mdelay() provides a low level timing loop and must be re-calibrated for |
| anything that causes change in the rate of execution of that timing loop. |
| This calibration is not critical and fairly large errors in the calibration |
| are tolerable. Hopefully, you could keep the execution rate close enough that |
| up_mdelay() would not be grossly in error.</p> |
| </section> |
| <section id="power-management"> |
| <h3>Power Management<a class="headerlink" href="#power-management" title="Permalink to this heading"></a></h3> |
| <p>This is also the same kind of thing that you would have to do if you wanted to |
| switch clocking for power management reasons. NuttX does have a power |
| management system and perhaps making use of the power management system |
| to manage system clocking changes might be possible. For example, when the |
| clocking changes, you could force some power management state change. That |
| state change would notify all drivers and, in response, the drivers could |
| recalculate their frequency related settings.</p> |
| <p>Here is some Power Management documentation:</p> |
| <div class="toctree-wrapper compound"> |
| <ul> |
| <li class="toctree-l1"><a class="reference internal" href="../components/drivers/special/power/pm/index.html">Power Management</a></li> |
| </ul> |
| </div> |
| </section> |
| </section> |
| </section> |
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