elua/doc/en/arch_platform.html

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<h3>The platform interface</h3>
<p>The platform interface is the part of <b>eLua</b> that makes it easily portable between different hardware platforms by grouping the common elements
  of all platforms supported by <b>eLua</b> in a common interface. For more details about the platform interface and the overall structure of 
  <b>eLua</b> in general, check <a href="arch_overview.html">this link</a>.</p>
  <p>The platform interface is defined in the <i>inc/platform.h</i> header file from the <b>eLua</b> source distribution. It is a collection of various 
  components (UART, SPI, timers ...), each of them is detailed in the next subsections. Each such component has an <b>id</b> which is a number that 
  identifies that component in <b>eLua</b>. Generally, numbers are assigned to components in their "natural" order; for example, PORTA will have the id
  0, PORTB will have 1 and so on. Similarly, the second SPI interface (SPI1) of the MCU will probably have an id equal to 1. However, this is not a strict
  rule. The implementation of the platform interface might choose to expose only some of the peripherals (components) of the MCU, thus this rule might be 
  broken. For example, if a board has 3 UARTs, but for some reason the second UART (UART1) is dedicated and can't be touched by <b>eLua</b>, then UART0 will have the id 0 and UART2 will
  have the id 1, so UART1 won't ever be accesible to the code. </p>
  <p>With some exceptions (most notably the low-level support functions), the different modules supported by the platform interface are 
  mirrored more or less accurately in separate Lua modules that can be used directly from <b>eLua</b>. Check the reference manual for a 
  complete description of these modules.</p>
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