Microsecond Ticker

Low level interface to the microsecond ticker of a target. More...

Modules
	Microsecond Ticker tests
	Tests to validate the proper implementation of the microsecond ticker.
	Ticker Hal
	Low level interface to the ticker of a target.
	Ticker Tests
	Tests to validate the proper implementation of a ticker.

Functions
ticker_irq_handler_type	set_us_ticker_irq_handler (ticker_irq_handler_type ticker_irq_handler)
	Set ticker IRQ handler.
const ticker_data_t *	get_us_ticker_data (void)
	Get ticker's data.
void	us_ticker_irq_handler (void)
	The wrapper for ticker_irq_handler, to pass us ticker's data.
void	us_ticker_init (void)
	Initialize the ticker.
void	us_ticker_free (void)
	Deinitialize the us ticker.
uint32_t()	us_ticker_read (void)
	Read the current counter.
void	us_ticker_set_interrupt (timestamp_t timestamp)
	Set interrupt to fire at specified timestamp.
void	us_ticker_disable_interrupt (void)
	Disable us ticker interrupt.
void	us_ticker_clear_interrupt (void)
	Clear the us ticker interrupt.
void	us_ticker_fire_interrupt (void)
	Set pending interrupt that should be fired right away.
const ticker_info_t *	us_ticker_get_info (void)
	Get frequency and counter bits of this ticker.

Detailed Description

Low level interface to the microsecond ticker of a target.

Defined behavior

• Has a reported frequency between 250KHz and 8MHz for counters which are less than 32 bits wide - Verified by test us_ticker_info_test
• Has a reported frequency up to 100MHz for counters which are 32 bits wide - Verified by test us_ticker_info_test
• Has a counter that is at least 16 bits wide - Verified by test us_ticker_info_test
• All behavior defined by the ticker specification

Undefined behavior

• See the ticker specification

See also

Microsecond Ticker tests

Compile-time optimization macros

To permit compile-time optimization, particularly of wait_us, the following macros should be defined by a target's device.h:

US_TICKER_PERIOD_NUM, US_TICKER_PERIOD_DEN: These denote the ratio (numerator, denominator) of the ticker period to a microsecond. For example, an 8MHz ticker would have NUM = 1, DEN = 8; a 1MHz ticker would have NUM = 1, DEN = 1; a 250kHz ticker would have NUM = 4, DEN = 1. Both numerator and denominator must be 16 bits or less, but need not be fully simplified.

US_TICKER_MASK: The value mask for the ticker - eg 0x07FFFFFF for a 27-bit ticker.

If any are defined, all 3 must be defined, and the macros are checked for consistency with us_ticker_get_info by test us_ticker_info_test.

Function Documentation

set_us_ticker_irq_handler()

ticker_irq_handler_type set_us_ticker_irq_handler

(

ticker_irq_handler_type

ticker_irq_handler

)

Set ticker IRQ handler.

Parameters

ticker_irq_handler

IRQ handler to be connected

Returns

previous ticker IRQ handler

Note

by default IRQ handler is set to ticker_irq_handler

this function is primarily for testing purposes and it's not required part of HAL implementation

get_us_ticker_data()

const ticker_data_t * get_us_ticker_data

(

void

)

Get ticker's data.

Returns

The microsecond ticker data

us_ticker_irq_handler()

void us_ticker_irq_handler

(

void

)

The wrapper for ticker_irq_handler, to pass us ticker's data.

us_ticker_init()

void us_ticker_init

(

void

)

Initialize the ticker.

Initialize or re-initialize the ticker. This resets all the clocking and prescaler registers, along with disabling the compare interrupt.

Implementations must tolerate this function being called multiple times, and subsequent calls after the first one shall clear any configured interrupt.

Note

Initialization properties tested by ticker_init_test

Pseudo Code:

void us_ticker_init()
{
    // Enable clock gate so processor can read TIMER registers
    POWER_CTRL |= POWER_CTRL_TIMER_Msk;
 
    // Disable the timer and ensure it is powered down
    TIMER_CTRL &= ~(TIMER_CTRL_ENABLE_Msk | TIMER_CTRL_COMPARE_ENABLE_Msk);
 
    // Configure divisors
    uint32_t prescale = SystemCoreClock / 1000000;
    TIMER_PRESCALE = prescale - 1;
    TIMER_CTRL |= TIMER_CTRL_ENABLE_Msk;
 
    // Install the interrupt handler
    NVIC_SetVector(TIMER_IRQn, (uint32_t)us_ticker_irq_handler);
    NVIC_EnableIRQ(TIMER_IRQn);
}

us_ticker_free()

void us_ticker_free

(

void

)

Deinitialize the us ticker.

Powerdown the us ticker in preparation for sleep, powerdown, or reset.

After this function is called, no other ticker functions should be called except us_ticker_init(), calling any function other than init is undefined.

Note

This function stops the ticker from counting.

Pseudo Code:

uint32_t us_ticker_free()
{
    // Disable timer
    TIMER_CTRL &= ~TIMER_CTRL_ENABLE_Msk;
 
    // Disable the compare interrupt
    TIMER_CTRL &= ~TIMER_CTRL_COMPARE_ENABLE_Msk;
 
    // Disable timer interrupt
    NVIC_DisableIRQ(TIMER_IRQn);
 
    // Disable clock gate so processor cannot read TIMER registers
    POWER_CTRL &= ~POWER_CTRL_TIMER_Msk;
}

us_ticker_read()

uint32_t() us_ticker_read

(

void

)

Read the current counter.

Read the current counter value without performing frequency conversions. If no rollover has occurred, the seconds passed since us_ticker_init() was called can be found by dividing the ticks returned by this function by the frequency returned by us_ticker_get_info.

Returns

The current timer's counter value in ticks

Pseudo Code:

uint32_t us_ticker_read()
{
    return TIMER_COUNT;
}

us_ticker_set_interrupt()

void us_ticker_set_interrupt

(

timestamp_t

timestamp

)

Set interrupt to fire at specified timestamp.

Parameters

timestamp

The time in ticks to be set. Guaranteed to be between 0 and 2^bits-1, where bits is the number of bits returned by us_ticker_get_info()

Note

If timestamp is less than the current time read by the ticker, then the intention is to set an interrupt for once the ticker rolls over and reaches this time.

Upper level Mbed OS code is responsible for ensuring that if we try to set a wake-up for a time in the past, us_ticker_fire_interrupt() is called instead. It will also ensure that if we want to wake up far in the future, we will instead set a wakeup for about (rollover period/2) ticks in the future, then reschedule the timer for the correct time.

Calling this function with timestamp of more than the supported number of bits returned by us_ticker_get_info results in undefined behavior.

If the timer interrupt is pending when this function is called (e.g. due to the ticker being set, but the interrupt being disabled before it could fire), this function shall clear the pending interrupt before reenabling and setting it to prevent a spurious execution of the interrupt.

Pseudo Code:

void us_ticker_set_interrupt(timestamp_t timestamp)
{
    us_ticker_clear_interrupt();
    TIMER_COMPARE = timestamp;
    TIMER_CTRL |= TIMER_CTRL_COMPARE_ENABLE_Msk;
}

us_ticker_disable_interrupt()

void us_ticker_disable_interrupt

(

void

)

Disable us ticker interrupt.

Pseudo Code:

void us_ticker_disable_interrupt(void)
{
    // Disable the compare interrupt
    TIMER_CTRL &= ~TIMER_CTRL_COMPARE_ENABLE_Msk;
}

us_ticker_clear_interrupt()

void us_ticker_clear_interrupt

(

void

)

Clear the us ticker interrupt.

This is called from Mbed's interrupt handler and should clear the interrupt flag in the peripheral to stop the interrupt from being executed again after it returns. This does not do anything if called before the interrupt fires (e.g. it doesn't cancel the interrupt if it's set in the future).

Pseudo Code:

void us_ticker_clear_interrupt(void)
{
    // Write to the ICR (interrupt clear register) of the TIMER
    TIMER_ICR = TIMER_ICR_COMPARE_Msk;
}

us_ticker_fire_interrupt()

void us_ticker_fire_interrupt

(

void

)

Set pending interrupt that should be fired right away.

The ticker should be initialized prior calling this function.

Note: This function might be called from the ticker ISR, in which case it should schedule the ticker ISR to be executed again as soon as it returns.

Pseudo Code:

void us_ticker_fire_interrupt(void)
{
    NVIC_SetPendingIRQ(TIMER_IRQn);
}

us_ticker_get_info()

const ticker_info_t * us_ticker_get_info

(

void

)

Get frequency and counter bits of this ticker.

Pseudo Code:

const ticker_info_t* us_ticker_get_info()
{
    static const ticker_info_t info = {
        1000000,    // 1 MHz
        32          // 32 bit counter
    };
    return &info;
}