elua/doc/eluadoc/arch_platform_timers.lua

-- eLua platform interface - timers

--[[
// The next 3 functions need to be implemented only if the generic system timer mechanism
// (src/common.c:cmn_systimer*) is used by the backend
u64 platform_timer_sys_raw_read();
void platform_timer_sys_enable_int();
void platform_timer_sys_disable_int();
--]]

data_en = 
{
  -- Title
  title = "eLua platform interface - timers",

  -- Menu name
  menu_name = "Timers",

  -- Overview
  overview = [[This part of the platform interface groups functions related to the timers of the MCU. It also makes provisions for using $virtual timers$ on any platform, see @#virtual@this section@
  for details. Keep in mind that in the following paragraphs a $timer id$ can refer to both a hardware timer or a virtual timer.]],

  -- Data structures, constants and types
  structures = 
  {
    { text = "typedef u64/u32 timer_data_type;",
      name = "Timer data type",
      desc = [[This defines the data type used to specify delays and time intervals (which are always specified in $microseconds$). The choice between u64 and u32 for the timer data type depends
on the build type, check ^#the_system_timer^here^ for mode details.]]
    },

    { text = [[// Timer operations
enum
{
  PLATFORM_TIMER_OP_START,
  PLATFORM_TIMER_OP_READ,
  PLATFORM_TIMER_OP_SET_CLOCK,
  PLATFORM_TIMER_OP_GET_CLOCK,
  PLATFORM_TIMER_OP_GET_MAX_DELAY,
  PLATFORM_TIMER_OP_GET_MIN_DELAY,
  PLATFORM_TIMER_OP_GET_MAX_CNT
};]], 
      name = "Timer operations",
      desc = "This enum lists all the operations that can be executed on a given timer."
    }
  },

  -- Functions
  funcs = 
  {
    { sig = "int #platform_timer_exists#( unsigned id );",
      desc = [[Checks if the platform has the timer specified as argument. Implemented in %src/common.c%, it uses the $NUM_TIMER$ macro that must be defined in the
  platform's $platform_conf.h$ file (see @arch_overview.html#platforms@here@ for details) and the virtual timer configuration (@#virtual@here@ for details). For example:</p>
  ~#define NUM_TIMER   2      $// The platform has 2 hardware timers$~<p>]],
      args = "$id$ - the timer ID",
      ret = "1 if the timer exists, 0 otherwise"
    },

    { sig = "void #platform_timer_delay#( unsigned id, timer_data_type delay_us );",
      desc = [[Waits on a timer, then returns. This function is "split" in two parts: a platform-independent part implemented in %src/common_tmr.c% (that
  handles virtual timers and the system timer) and a platform-dependent part that must be implemented by each platform in a function named @#platform_s_timer_delay@platform_s_timer_delay@. This function handles both
  hardware timer IDs and virtual timer IDs.<br>
  <a name="limitations" /><span class="warning">IMPORTANT NOTE</span>: the real delay after executing this functions depends on a number of variables, most notably the base clock of the timer 
  and the size of the timer counter register (32 bits on some platforms, 16 bits on most platforms, other values are less common). To ensure that the delay you're requesting is achievable, use 
  @#platform_timer_op@platform_timer_op@ with $PLATFORM_TIMER_OP_GET_MAX_DELAY$ and $PLATFORM_TIMER_OP_GET_MIN_DELAY$ to obtain the maximum and the minimum 
  achievable wait times on your timer, respectively. Even if your delay is within these limits, the $precision$ of this function still varies a lot, mainly as a function of 
  the timer base clock.]],
      args = 
      {
        "$id$ - the timer ID",
        "$delay_us$ - the delay time (in microseconds)"
      }
    },

    { sig = "void #platform_s_timer_delay#( unsigned id, timer_data_type delay_us );",
      desc = [[This function is identical in functionality to @#platform_timer_delay@platform_timer_delay@, but this is the function that must actually be implemented by a platform port 
  and it must never handle virtual timer IDs or the system timer ID, only hardware timer IDs. It has the same @#limitations@limitations@ as @#platform_timer_delay@platform_timer_delay@.]],
      args = 
      {
        "$id$ - the timer ID",
        "$delay_us$ - the delay time (in microseconds)"
      }
    },

    { sig = "timer_data_type #platform_timer_op#( unsigned id, int op, timer_data_type data );",
      desc = [[Executes an operation on a timer. This function is "split" in two parts: a platform-independent part implemented in %src/common_tmr.c% (that handles virtual timers and the system timer) and a 
  platform-dependent part that must be implemented by each platform in a function named @#platform_s_timer_op@platform_s_timer_op@. This function handles both hardware timer IDs and virtual 
  timer IDs.]],
      args = 
      {
        "$id$ - the timer ID",
        [[$op$ - the operation. $op$ can take any value from the @#timer_operations@this enum@, as follows:
  <ul>
    <li>$PLATFORM_TIMER_OP_START$: start the specified timer by setting its counter register to a predefined value.</li>
    <li>$PLATFORM_TIMER_OP_READ$: get the value of the specified timer's counter register.</li> 
    <li>$PLATFORM_TIMER_SET_CLOCK$: set the clock of the specified timer to $data$ (in hertz). You can never set the clock of a virtual timer, which is set at compile time.</li>  
    <li>$PLATFORM_TIMER_GET_CLOCK$: get the clock of the specified timer.</li>  
    <li>$PLATFORM_TIMER_OP_GET_MAX_DELAY$: get the maximum achievable timeout on the specified timer (in us).</li>  
    <li>$PLATFORM_TIMER_OP_GET_MIN_DELAY$: get the minimum achievable timeout on the specified timer (in us).</li>
    <li>$PLATFORM_TIMER_OP_GET_MAX_CNT$: get the maximum value of the timer's counter register.</li>
  </ul>]],
        "$data$ - used to specify the timer clock value when $op = PLATFORM_TIMER_SET_CLOCK$, ignored otherwise",
      },
      ret = 
      {
        "the predefined value used when starting the clock if $op = PLATFORM_TIMER_OP_START$",
        "the timer's counter register if $op = PLATFORM_TIMER_OP_READ$",
        "the actual clock set on the timer, which might be different than the request clock depending on the hardware if $op = PLATFORM_TIMER_SET_CLOCK$",
        "the timer clock if $op = PLATFORM_TIMER_GET_CLOCK$", 
        "the maximum achievable delay (in microseconds) if $op = PLATFORM_TIMER_OP_GET_MAX_DELAY$",
        "the minimum achievable delay (in microseconds) if $op = PLATFORM_TIMER_OP_GET_MIN_DELAY$",
        "the maximum value of the timer's coutner register if $op == PLATFORM_TIMER_OP_GET_MAX_CNT$",
      }
    }, 

    { sig = "timer_data_type #platform_s_timer_op#( unsigned id, int op, timer_data_type data );",
      desc = [[This function is identical in functionality to @#platform_timer_op@platform_timer_op@, but this is the function that must actually be implemented by a platform port and it must 
  never handle virtual timer IDs or the system timer, only hardware timer IDs.]],
      args = 
      {
        "$id$ - the timer ID",
        [[$op$ - the operation. $op$ can take any value from the @#opval@this enum@, as follows:
  <ul>
    <li>$PLATFORM_TIMER_OP_START$: start the specified timer by setting its counter register to a predefined value.</li>
    <li>$PLATFORM_TIMER_OP_READ$: get the value of the specified timer's counter register.</li> 
    <li>$PLATFORM_TIMER_SET_CLOCK$: set the clock of the specified timer to $data$ (in hertz). You can never set the clock of a virtual timer, which is set at compile time.</li>  
    <li>$PLATFORM_TIMER_GET_CLOCK$: get the clock of the specified timer.</li>  
    <li>$PLATFORM_TIMER_OP_GET_MAX_DELAY$: get the maximum achievable timeout on the specified timer (in us).</li>  
    <li>$PLATFORM_TIMER_OP_GET_MIN_DELAY$: get the minimum achievable timeout on the specified timer (in us).</li>
    <li>$PLATFORM_TIMER_OP_GET_MAX_CNT$: get the maximum value of the timer's counter register.</li>
  </ul>]],
        "$data$ - used to specify the timer clock value when $op = PLATFORM_TIMER_SET_CLOCK$, ignored otherwise",
      },
      ret = 
      {
        "the predefined value used when starting the clock if $op = PLATFORM_TIMER_OP_START$",
        "the timer's counter register if $op = PLATFORM_TIMER_OP_READ$",
        "the actual clock set on the timer, which might be different than the request clock depending on the hardware if $op = PLATFORM_TIMER_SET_CLOCK$",
        "the timer clock if $op = PLATFORM_TIMER_GET_CLOCK$", 
        "the maximum achievable delay (in microseconds) if $op = PLATFORM_TIMER_OP_GET_MAX_DELAY$",
        "the minimum achievable delay (in microseconds) if $op = PLATFORM_TIMER_OP_GET_MIN_DELAY$",
        "the maximum value of the timer's coutner register if $op == PLATFORM_TIMER_OP_GET_MAX_CNT$",
      }
    }, 

    { sig = "timer_data_type #platform_timer_get_diff_us#( unsigned id, timer_data_type start, timer_data_type end );",
      desc = [[Return the time difference (in us) between two timer values (as returned by calling @refman_gen_tmr.html#platform_timer_op@platform_timer_op@ with $PLATFORM_TIMER_OP_READ$ or $PLATFORM_TIMER_OP_START$. This function 
is generic, thus it is implemented in %src/common.c%. <span class="warning">NOTE</span>: the order of $start$ and $end$ is important. $end$ must correspond to a moment in time which came after $start$. The function knows how to deal 
with $a single$ timer overflow condition ($end$ is less than $start$); if the timer overflowed 2 or more times between $start$ and $end$ the result of this function will be incorrect.]],
      args = 
      {
        "$id$ - the timer ID",
        "$start$ - the initial counter value.",
        "$end$ - the final counter value.",
      },
      ret = "the time difference (in microseconds)"
    },

    { sig = "int #platform_timer_set_match_int#( unsigned id, timer_data_type period_us, int type );",      
      desc = [[Setup the timer match interrupt. Only available if interrupt support is enabled, check @inthandlers.html@here@ for details.This function is "split" in two parts: a platform-independent part 
implemented in %src/common_tmr.c% (that handles virtual timers and the system timer) and a platform-dependent part that must be implemented by each platform in a function named 
@#platform_s_timer_set_math_int@platform_s_timer_set_match_int@. This function handles both hardware timer IDs and virtual timer IDs. <span class="warning">NOTE</span>: the @#the_system_timer@system timer@ can't
generate interrupts.]],
      args = 
      {
        "$id$ - the timer ID",
        "$period_us$ - the period (in microseconds) of the timer interrupt. Setting this to 0 disables the timer match interrupt.",
        [[$type$ - $PLATFORM_TIMER_INT_ONESHOT$ for an interrupt that occurs only once after $period_us$ microseconds, or $PLATFORM_TIMER_INT_CYCLIC$ for an interrupt that occurs every
$period_us$ microseconds]]        
      },
      ret = 
      {
        "$PLATFORM_TIMER_INT_OK$ if the operation was successful.",
        "$PLATFORM_TIMER_INT_TOO_SHORT$ if the specified period is too short.",
        "$PLATFORM_TIMER_INT_TOO_LONG$ if the specified period is too long.",
        "$PLATFORM_TIMER_INT_INVALID_ID$ if the specified timer cannot handle this operation."
      }
    },
  
    { sig = "int #platform_s_timer_set_match_int#( unsigned id, timer_data_type period_us, int type );",
      desc = [[This function is identical in functionality to @#platform_timer_set_match_int@platform_timer_set_match_int@, but this is the function that must actually be implemented by a platform port and it must 
  never handle virtual timer IDs or the system timer, only hardware timer IDs.]],
      args = 
      {
        "$id$ - the timer ID",
        "$period_us$ - the period (in microseconds) of the timer interrupt. Setting this to 0 disables the timer match interrupt.",
        [[$type$ - $PLATFORM_TIMER_INT_ONESHOT$ for an interrupt that occurs only once after $period_us$ microseconds, or $PLATFORM_TIMER_INT_CYCLIC$ for an interrupt that occurs every
$period_us$ microseconds]]        
      },
      ret = 
      {
        "$PLATFORM_TIMER_INT_OK$ if the operation was successful.",
        "$PLATFORM_TIMER_INT_TOO_SHORT$ if the specified period is too short.",
        "$PLATFORM_TIMER_INT_TOO_LONG$ if the specified period is too long.",
        "$PLATFORM_TIMER_INT_INVALID_ID$ if the specified timer cannot handle this operation."
      }
    },

    {
      sig = "timer_data_type #platform_timer_read_sys#();",
      desc = "Returns the current value of the system timer, see @#the_system_timer@here@ for more details.",
      ret = "The current value of the system timer."
    },

    {
      sig = "int #platform_timer_sys_available#();",
      desc = [[Used to check the availability of the system timer. This function is platform independent and is implemented in %src/common_tmr.c%. It returns the value of the $PLATFORM_HAS_SYSTIMER$ macro, check
@#the_system_timer@here@ for more details.]],
      ret = "1 if the system timer is implemented, 0 otherwise."
    },
    
    {
      sig = "u64 #platform_timer_sys_raw_read#();",
      desc = [[Return the counter of the timer used to implement the system timer. Needs to be implemented only if eLua's generic system timer mechanism is used, check @#the_system_timer@here@ for details.]],
      ret = "The counter of the timer used to implement the system timer."
    },

    {
      sig = "void #platform_timer_sys_enable_int#();",
      desc = [[Enable the overflow/match interrupt of the timer used to implement the system timer. Needs to be implemented only if eLua's generic system timer mechanism is used, check @#the_system_timer@here@ for details.]],
    },
    
    {
      sig = "void #platform_timer_sys_disable_int#();",
      desc = [[Disable the overflow/match interrupt of the timer used to implement the system timer. Needs to be implemented only if eLua's generic system timer mechanism is used, check @#the_system_timer@here@ for details.]],
    },

  },


  auxdata = 
  {
    { title = "Virtual timers",
      desc = 
  [[$Virtual timers$ were added to eLua to overcome some limitations:</p>
  <ul>
    <li>there are generally few hardware timers available, some of which might be dedicated (thus not usable directly by eLua).</li>
    <li>many times it is difficult to share a hardware timer between different parts of an application because of conflicting requirements. Generally it's not possible to have timers that can
        achieve long delays and high accuracy at the same time (this is especially true for systems that have 16 bit or even smaller timers).</li>
  </ul>
  <p>In this respect, $virtual timers$ are a set of timers that share a single hardware timer. It is possible, in this way, to have a hardware timer that can implement 4, 8 or more virtual/software
  timers. There are a few drawbacks to this approach:</p>
  <ul>
    <li>the hardware timer used to implement the virtual timers must generally be dedicated. In fact it can still be used in "read only mode", which means that the only operations that can
        be executed on it are $PLATFORM_TIMER_OP_READ$, $PLATFORM_TIMER_GET_CLOCK$, $PLATFORM_TIMER_OP_GET_MAX_DELAY$ and $PLATFORM_TIMER_OP_GET_MIN_DELAY$. However,
        since the "read only mode" is not enforced by the code, it is advisable to treat this timer as a dedicated resource and thus make it invisible to eLua by not associating it with 
        an ID.</li>
    <li>the number of virtual timers and their base frequency are fixed at compile time.</li>
    <li>virtual timers are generally used for large delays with low accuracy, since their base frequency should be fairly low (see below).</li>
  </ul>
  <p>To $enable$ virtual timers:</p>
  <ol>
    <li>edit $platform_conf.h$ (see @arch_overview.html#platforms@here@ for details) and set $VTMR_NUM_TIMERS$ to the number of desired virtual timers and 
       $VTMR_FREQ_HZ$ to the base frequency of the virtual timers (in hertz). For example:
  ~#define VTMR_NUM_TIMERS       4 // we need 4 virtual timers
#define VTMR_FREQ_HZ          4 // the base clock for the virtual timers is 4Hz~</li>
    <li>in your platform port setup a hardware timer to fire an interrupt at $VTMR_FREQ_HZ$ and call the $cmn_virtual_timer_cb$ function (defined in %src/common.c%) in the 
       timer interrupt handler. For example, if the the interrupt handler is called $timer_int_handler$, do this:
  ~void timer_int_handler( void )
{
  // add code to clear the timer interrupt flag here if needed
  cmn_virtual_timer_cb();
}~</li>
  </ol>
  <p>Note that because of step 2 above you are limited by practical constraints on the value of $VTMR_FREQ_HZ$. If set too high, the timer interrupt will fire too often, thus taking too much
  CPU time. The maximum value depends largely on the hardware and the desired behaviour of the virtual timers, but in practice values larger than 10 might visibly change the behaviour of your 
  system.</p>
  <p>To $use$ a virtual timer, identify it with the constant $VTMR_FIRST_ID$ (defined in %inc/common.h%) plus an offset. For example, $VTMR_FIRST_ID+0$ (or simply
  $VTMR_FIRST_ID$) is the ID of the first virtual timer in the system, and $VTMR_FIRST_ID+2$ is the ID of the third virtual timer in the system.</p>
  <p>Virtual timers are capable of generating timer match interrupts just like regular timers, check @#platform_timer_set_match_int@here@ for details.
  ]]
    },
    { title = "The system timer",
    desc = 
  [[The system timer was introduced in eLua 0.9 as a simpler alternative to the traditional eLua timers. Working with regular timers in eLua might be challenging for a number of reasons:</p>
  <ul>
    <li>depending on the hardware, the timers might have a limited range. Because of this, they might not be able to timeout in the interval requested by the user.</li>
    <li>the timers might have different ranges even on the same platform (they might have a different base clock, for example). The problem is further aggravated when switching platforms.</li>
    <li>the timers might be shared with other hardware resources (for example PWMs or ADC triggers) so using them might have unexpected side effects.</li>
    <li>manual timer management is error prone. The user needs to keep into account the timers he's using, their base frequencies and wether they are shared or not with the C code.</li>
  </ul>
  <p>The ^#virtual_timers^virtual timers^ can fix some of the above problems, but their resolution is fairly low and they still require manual management.</p>
  <p>The $system timer$ attemps to fix (at least partially) these issues. It is a timer with fixed resolution (1us) %on all platforms% and large counters:</p>
  <ul>
    <li>if eLua is compiled in floating point mode (default) the counter is 52 bits wide. It will overflow after more than 142 %years%.</li>
    <li>if eLua is compiled in 32 bit integer-only mode (lualong) the counter is 32 bits wide. It will overflow after about one hour.</li>
    <li>if eLua is compiled in 64 bit integer-only mode (lualonglong, new in 0.9) the counter is again 52 bits wide and it will also overflow after more than 142 years.</li>
  </ul>
  <p>The eLua API was partially modified to take full advantage of this new timer:</p>
  <ul>
    <li>all the functions that can operate with a timeout (for example @refman_gen_uart.html#uart.read@uart.read@ or @refman_gen_net.html#net.accept@net.accept@) 
will default to the system timer is a timer ID is not specified explicitly.</li>
    <li>all the function in the @refman_gen_tmr.html@timer module@ will default to the system timer if a timer ID is not specified explicitly.</li>
    <li>timeouts are specified in a more unified manner across the eLua modules as a $[timeout], [timer_id]$ pair:
  <table class="table_center" style="margin-top: 10px; margin-bottom: 4px;">
  <tbody>
  <tr>
    <th>timeout</th>
    <th>timer_id</th>
    <th>Result</th>
  </tr>
  <tr>
    <td>not specified</td>
    <td>any value</td>
    <td>infinite timeout (the function blocks until it completes).</td>
  </tr>
  <tr>
    <td>0</td>
    <td>any value</td>
    <td>no timeout (the function returns immediately).<//td>
  </tr>
  <tr>
    <td>a positive value</td>
    <td>not specified</td>
    <td>the system timer will be used to measure the function's timeout.</td>
  </tr>  
  <tr>
    <td>a positive value</td>
    <td>a timer ID</td>
    <td>the specified timer will be used to measure the function's timeout.</td>
  </tr>
  </tbody>
  </table>
  </li>  
  </ul>
  <p>Using the system timer as much as possible is also encouraged with C code that uses the eLua C api, not only with Lua programs. The C code can use the system timer by specifying 
$PLATFORM_TIMER_SYS_ID$ as the timer ID.</p>
  <p>From an implementation stand point, the system timer is built around a hardware timer with a base clock of at least 1MHz that can generate an interrupt when the timer counter overflows
or when it reaches a certain value. The interrupt handler updates the upper part of the system timer counter (basically an overflow counter). eLua has a generic mechanism that can be used
to implement a system timer on any platform using this method. To take advantage of this mechanism follow the steps below:</p>
<ol>
  <li>define the $PLATFORM_HAS_SYSTIMER$ macro in your %platform_conf.h% file.</li>
  <li>implement @#platform_timer_sys_raw_read@platform_timer_sys_raw_read@, @#platform_timer_sys_enable_int@platform_timer_sys_enable_int@ and @#platform_timer_sys_disable_int@platform_timer_sys_disable_int@.</li>
  <li>include the %common.h% header.</li>
  <li>setup your hardware timer and its associated interrupt. This should happen an initialization time (for example in %platform_init%).</li>
  <li>call %cmn_systimer_set_base_freq% with the base frequency of your timer in Hz.</li>
  <li>call %cmn_systimer_set_interrupt_freq% with the frequency of the timer's overflow/match interrupt in Hz. Alternatively you can call %cmn_systimer_set_interrupt_period_us% to set the timer's overflow/match
interrupt %period% (in microseconds) instead of its frequency. Use the latter form if the frequency is not an integer.</li>
  <li>call %cmn_systimer_periodic% from your timer's overflow interrupt handler.</li>
  <li>use this implementation for @#platform_timer_read_sys@platform_timer_read_sys@:
 ~timer_data_type platform_timer_read_sys()
{
  return cmn_systimer_get();
}~</li></ol>
  <p>Note that the above mechanism is optional. A platform might have a different method to implement the system timer; this is OK as long as the system timer requirements are respected.</p>
  <p><span class="warning">IMPORTANT NOTE</span>: although system timer support in eLua is optional, implementing the system timer is highly recommended. As already specified, all the timer IDs
in various eLua modules default to the system timer. This means that any code that was written under the assumption that a system timer is present (which is a fair assumption) will fail on 
platforms that don't actually have a system timer. Check @status.html#systmr@here@ for a list of platforms that implement the system timer. If your platform doesn't implement the
system timer, you'll get this warning at compile time:</p>
 ~#warning This platform does not have a system timer. Your eLua image might not work as expected.~
]]
  }
 }
}