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| <h1 id="peripherals-as-state-machines"><a class="header" href="#peripherals-as-state-machines">Peripherals as State Machines</a></h1> |
| <p>The peripherals of a microcontroller can be thought of as set of state machines. For example, the configuration of a simplified <a href="https://en.wikipedia.org/wiki/General-purpose_input/output">GPIO pin</a> could be represented as the following tree of states:</p> |
| <ul> |
| <li>Disabled</li> |
| <li>Enabled |
| <ul> |
| <li>Configured as Output |
| <ul> |
| <li>Output: High</li> |
| <li>Output: Low</li> |
| </ul> |
| </li> |
| <li>Configured as Input |
| <ul> |
| <li>Input: High Resistance</li> |
| <li>Input: Pulled Low</li> |
| <li>Input: Pulled High</li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| <p>If the peripheral starts in the <code>Disabled</code> mode, to move to the <code>Input: High Resistance</code> mode, we must perform the following steps:</p> |
| <ol> |
| <li>Disabled</li> |
| <li>Enabled</li> |
| <li>Configured as Input</li> |
| <li>Input: High Resistance</li> |
| </ol> |
| <p>If we wanted to move from <code>Input: High Resistance</code> to <code>Input: Pulled Low</code>, we must perform the following steps:</p> |
| <ol> |
| <li>Input: High Resistance</li> |
| <li>Input: Pulled Low</li> |
| </ol> |
| <p>Similarly, if we want to move a GPIO pin from configured as <code>Input: Pulled Low</code> to <code>Output: High</code>, we must perform the following steps:</p> |
| <ol> |
| <li>Input: Pulled Low</li> |
| <li>Configured as Input</li> |
| <li>Configured as Output</li> |
| <li>Output: High</li> |
| </ol> |
| <h2 id="hardware-representation"><a class="header" href="#hardware-representation">Hardware Representation</a></h2> |
| <p>Typically the states listed above are set by writing values to given registers mapped to a GPIO peripheral. Let's define an imaginary GPIO Configuration Register to illustrate this:</p> |
| <div class="table-wrapper"><table><thead><tr><th style="text-align: right">Name</th><th style="text-align: right">Bit Number(s)</th><th style="text-align: right">Value</th><th style="text-align: right">Meaning</th><th style="text-align: right">Notes</th></tr></thead><tbody> |
| <tr><td style="text-align: right">enable</td><td style="text-align: right">0</td><td style="text-align: right">0</td><td style="text-align: right">disabled</td><td style="text-align: right">Disables the GPIO</td></tr> |
| <tr><td style="text-align: right"></td><td style="text-align: right"></td><td style="text-align: right">1</td><td style="text-align: right">enabled</td><td style="text-align: right">Enables the GPIO</td></tr> |
| <tr><td style="text-align: right">direction</td><td style="text-align: right">1</td><td style="text-align: right">0</td><td style="text-align: right">input</td><td style="text-align: right">Sets the direction to Input</td></tr> |
| <tr><td style="text-align: right"></td><td style="text-align: right"></td><td style="text-align: right">1</td><td style="text-align: right">output</td><td style="text-align: right">Sets the direction to Output</td></tr> |
| <tr><td style="text-align: right">input_mode</td><td style="text-align: right">2..3</td><td style="text-align: right">00</td><td style="text-align: right">hi-z</td><td style="text-align: right">Sets the input as high resistance</td></tr> |
| <tr><td style="text-align: right"></td><td style="text-align: right"></td><td style="text-align: right">01</td><td style="text-align: right">pull-low</td><td style="text-align: right">Input pin is pulled low</td></tr> |
| <tr><td style="text-align: right"></td><td style="text-align: right"></td><td style="text-align: right">10</td><td style="text-align: right">pull-high</td><td style="text-align: right">Input pin is pulled high</td></tr> |
| <tr><td style="text-align: right"></td><td style="text-align: right"></td><td style="text-align: right">11</td><td style="text-align: right">n/a</td><td style="text-align: right">Invalid state. Do not set</td></tr> |
| <tr><td style="text-align: right">output_mode</td><td style="text-align: right">4</td><td style="text-align: right">0</td><td style="text-align: right">set-low</td><td style="text-align: right">Output pin is driven low</td></tr> |
| <tr><td style="text-align: right"></td><td style="text-align: right"></td><td style="text-align: right">1</td><td style="text-align: right">set-high</td><td style="text-align: right">Output pin is driven high</td></tr> |
| <tr><td style="text-align: right">input_status</td><td style="text-align: right">5</td><td style="text-align: right">x</td><td style="text-align: right">in-val</td><td style="text-align: right">0 if input is < 1.5v, 1 if input >= 1.5v</td></tr> |
| </tbody></table> |
| </div> |
| <p>We <em>could</em> expose the following structure in Rust to control this GPIO:</p> |
| <pre><code class="language-rust ignore">/// GPIO interface |
| struct GpioConfig { |
| /// GPIO Configuration structure generated by svd2rust |
| periph: GPIO_CONFIG, |
| } |
| |
| impl GpioConfig { |
| pub fn set_enable(&mut self, is_enabled: bool) { |
| self.periph.modify(|_r, w| { |
| w.enable().set_bit(is_enabled) |
| }); |
| } |
| |
| pub fn set_direction(&mut self, is_output: bool) { |
| self.periph.modify(|_r, w| { |
| w.direction().set_bit(is_output) |
| }); |
| } |
| |
| pub fn set_input_mode(&mut self, variant: InputMode) { |
| self.periph.modify(|_r, w| { |
| w.input_mode().variant(variant) |
| }); |
| } |
| |
| pub fn set_output_mode(&mut self, is_high: bool) { |
| self.periph.modify(|_r, w| { |
| w.output_mode.set_bit(is_high) |
| }); |
| } |
| |
| pub fn get_input_status(&self) -> bool { |
| self.periph.read().input_status().bit_is_set() |
| } |
| }</code></pre> |
| <p>However, this would allow us to modify certain registers that do not make sense. For example, what happens if we set the <code>output_mode</code> field when our GPIO is configured as an input?</p> |
| <p>In general, use of this structure would allow us to reach states not defined by our state machine above: e.g. an output that is pulled low, or an input that is set high. For some hardware, this may not matter. On other hardware, it could cause unexpected or undefined behavior!</p> |
| <p>Although this interface is convenient to write, it doesn't enforce the design contracts set out by our hardware implementation.</p> |
| |
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