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| <h1 id="using-hal-in-your-libraries">Using HAL in Your Libraries</h1> |
| <p>This page describes the recommended way to implement libraries that |
| utilize HAL functionality.</p> |
| <p>Consider the light switch example from the <a href="../hal/">HAL overview</a> page.</p> |
| <p>The light switch module needs a GPIO pin to set |
| the light switch to on or off. The module declares that the GPIO to |
| control the light switch is declared elsewhere in one of two methods.</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code> extern const int light_switch_gpio; |
| </code></pre></div> |
| |
| <p>or</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code> extern int bsp_light_app_get_light_gpio(); |
| </code></pre></div> |
| |
| <p>Which method a library uses |
| (extern const versus function) depends on what functionality the library |
| might wish to declare. The <code>extern const</code> model uses the smallest code |
| footprint, but since its constant cannot be changed at runtime (based on |
| the configuration of the device say). The <code>extern function</code> call requires |
| the BSP to implement a function which allows flexibility to assign the |
| GPIO for the light switch to be determined at runtime with a small expense of |
| slightly more code.</p> |
| <p>Then the <code>libs/light</code> would go on to use the specified GPIO during its runtime |
| execution. For example this shows both ways the library could interface to the |
| bsp to get the right GPIO:</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code> void light_on(void) { |
| . . . |
| /* finally time to turn the light on */ |
| hal_gpio_set(light_switch_gpio); |
| } |
| |
| void light_on(void) { |
| . . . |
| int pin = bsp_light_app_get_light_gpio(); |
| /* finally time to turn the light on */ |
| hal_gpio_set(pin); |
| } |
| </code></pre></div> |
| |
| <p>When you library user includes <code>libs/light</code> as a dependency and then builds |
| with <strong>newt</strong>, they will get an unresolved external for either <code>light_switch_gpio</code> |
| or <code>bsp_light_app_get_light_gpio()</code> and will need to add to their BSP.</p> |
| <p>In their BSP, they can add </p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code> const int light_switch_gpio = 5; |
| </code></pre></div> |
| |
| <p>or</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code> int bsp_light_app_get_light_gpio(void) { |
| return my_configured_lightswitch_io(); |
| } |
| </code></pre></div> |
| |
| <p>So where did the number <code>5</code> come from. Their BSP already includes the specific |
| MCU that they are using. And their <code>mcu/xxx/hal_gpio.c</code> already defines |
| the mapping between physical pins and the virtual device ids. </p> |
| <p>The user can determine (when writing code) from their boards physical pinmap |
| and from the <code>hal_gpio.c</code> MCU mapping which virtual device_id corresponds |
| to the correct physical pin in their system.</p> |
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