eLua platform interface - timers

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 this section for details. Keep in mind that in the following paragraphs a timer id can reffer to both a hardware timer or a virtual timer.

Data structures, constants and types

typedef u32 timer_data_type;

This defines the data type used to specify delays and time intervals (which are always specifide in microseconds).

// 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
};

This enum lists all the operations that can be executed on a given timer.

Functions

platform_timer_exists platform_timer_delay platform_s_timer_delay platform_timer_op platform_s_timer_op platform_timer_get_diff_us platform_timer_set_match_int

int platform_timer_exists( unsigned id );

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 here for details) and the virtual timer configuration (here for details). For example:

#define NUM_TIMER   2      // The platform has 2 hardware timers

Arguments: id - the timer ID

Returns: 1 if the timer exists, 0 otherwise

void platform_timer_delay( unsigned id, u32 delay_us );

Waits on a timer, then returns. This function is "split" in two parts: a platform-independent part implemented in src/common.c (that handles virtual timers) and a platform-dependent part that must be implemented by each platform in a function named platform_s_timer_delay. This function handles both hardware timer IDs and virtual timer IDs.
IMPORTANT NOTE: 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 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.

Arguments:

  • id - the timer ID
  • delay_us - the delay time (in microseconds)

Returns: nothing.

void platform_s_timer_delay( unsigned id, u32 delay_us );

This function is identical in functionality to 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, only hardware timer IDs. It has the same limitations as platform_timer_delay.

Arguments:

  • id - the timer ID
  • delay_us - the delay time (in microseconds)

Returns: nothing.

u32 platform_timer_op( unsigned id, int op, u32 data );

Executes an operation on a timer. This function is "split" in two parts: a platform-independent part implemented in src/common.c (that handles virtual timers) and a platform-dependent part that must be implemented by each platform in a function named platform_s_timer_op. This function handles both hardware timer IDs and virtual timer IDs.

Arguments:

  • id - the timer ID
  • op - the operation. op can take any value from the this enum, as follows:
    • PLATFORM_TIMER_OP_START: start the specified timer by setting its counter register to a predefined value.
    • PLATFORM_TIMER_OP_READ: get the value of the specified timer's counter register.
    • 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.
    • PLATFORM_TIMER_GET_CLOCK: get the clock of the specified timer.
    • PLATFORM_TIMER_OP_GET_MAX_DELAY: get the maximum achievable timeout on the specified timer (in us).
    • PLATFORM_TIMER_OP_GET_MIN_DELAY: get the minimum achievable timeout on the specified timer (in us).
  • data - used to specify the timer clock value when op = PLATFORM_TIMER_SET_CLOCK, ignored otherwise

Returns:

  • 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

u32 platform_s_timer_op( unsigned id, int op, u32 data );

This function is identical in functionality to 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, only hardware timer IDs.

Arguments:

  • id - the timer ID
  • op - the operation. op can take any value from the this enum, as follows:
    • PLATFORM_TIMER_OP_START: start the specified timer by setting its counter register to a predefined value.
    • PLATFORM_TIMER_OP_READ: get the value of the specified timer's counter register.
    • 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.
    • PLATFORM_TIMER_GET_CLOCK: get the clock of the specified timer.
    • PLATFORM_TIMER_OP_GET_MAX_DELAY: get the maximum achievable timeout on the specified timer (in us).
    • PLATFORM_TIMER_OP_GET_MIN_DELAY: get the minimum achievable timeout on the specified timer (in us).
  • data - used to specify the timer clock value when op = PLATFORM_TIMER_SET_CLOCK, ignored otherwise

Returns:

  • 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

u32 platform_timer_get_diff_us( unsigned id, timer_data_type end, timer_data_type start );

Return the time difference (in us) betweeen two timer values. This function is generic for all platforms, thus it is implemented in src/common.c.

Arguments:

  • id - the timer ID
  • end - the first timer value
  • start - the second timer value

Returns: the time difference (in microseconds)

int platform_timer_set_match_int( unsigned id, u32 period_us, int type );

Setup the timer match interrupt. Only available if interrupt support is enabed, check here for details.

Arguments:

  • 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

Returns:

  • PLATFORM_TIMER_INT_OK if the operation was successful.
  • PLATFORM_TIMER_INT_TOO_SHORT if the specified period is too short.
  • PLATFORM_TIMER_INT_INVALID_ID if the specified timer cannot handle this operation.

Virtual timers

Virtual timers were added to eLua to overcome some limitations:

  • there are generally few hardware timers available, some of which might be dedicated (thus not usable directly by eLua).
  • 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).

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:

  • 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.
  • the number of virtual timers and their base frequency are fixed at compile time.
  • virtual timers are generally used for large delays with low accuracy, since their base frequency should be fairly low (see below).

To enable virtual timers:

  1. edit platform_conf.h (see 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
  2. 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();
}

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.

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.

Virtual timers are capable of generating timer match interrupts just like regular timers, check here for details.