Low Power Ticker

Low level interface to the low power ticker of a target. More...

Modules
	Low Power Ticker tests
	Tests to validate the proper implementation of the low power 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_lp_ticker_irq_handler (ticker_irq_handler_type ticker_irq_handler)
	Set low power ticker IRQ handler.
const ticker_data_t *	get_lp_ticker_data (void)
	Get low power ticker's data.
void	lp_ticker_irq_handler (void)
	The wrapper for ticker_irq_handler, to pass lp ticker's data.
void	lp_ticker_init (void)
	Initialize the low power ticker.
void	lp_ticker_free (void)
	Deinitialize the lower power ticker.
uint32_t	lp_ticker_read (void)
	Read the current tick.
void	lp_ticker_set_interrupt (timestamp_t timestamp)
	Set interrupt for specified timestamp.
void	lp_ticker_disable_interrupt (void)
	Disable low power ticker interrupt.
void	lp_ticker_clear_interrupt (void)
	Clear the low power ticker interrupt.
void	lp_ticker_fire_interrupt (void)
	Set pending interrupt that should be fired right away.
const ticker_info_t *	lp_ticker_get_info (void)
	Get frequency and counter bits of this ticker.

Detailed Description

Low level interface to the low power ticker of a target.

Defined behavior

• Has a reported frequency between 4KHz and 64KHz - verified by lp_ticker_info_test
• Has a counter that is at least 12 bits wide - verified by lp_ticker_info_test
• Continues operating in deep sleep mode - verified by lp_ticker_deepsleep_test
• All behavior defined by the ticker specification

Undefined behavior

• See the ticker specification
• Calling any function other than lp_ticker_init after calling lp_ticker_free

Potential bugs

• Glitches due to ripple counter - Verified by lp_ticker_glitch_test

See also

Low Power Ticker tests

Compile-time optimization macros

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

LP_TICKER_PERIOD_NUM, LP_TICKER_PERIOD_DEN: These denote the ratio (numerator, denominator) of the ticker period to a microsecond. For example, a 64kHz ticker would have NUM = 125, DEN = 8; a 4kHz ticker would have NUM = 250, DEN = 1; a 32.768kHz ticker would have NUM = 15625, DEN = 512. Both numerator and denominator must be 32 bits or less. They do not need to be fully simplified, so 32.768kHz could also be NUM = 1000000, DEN = 32768, but more simplification may be a minor speed optimisation, as can matching numerator or denominator with US_TICKER.

LP_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 lp_ticker_get_info by test lp_ticker_info_test.

Function Documentation

set_lp_ticker_irq_handler()

ticker_irq_handler_type set_lp_ticker_irq_handler

(

ticker_irq_handler_type

ticker_irq_handler

)

Set low power 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_lp_ticker_data()

const ticker_data_t * get_lp_ticker_data

(

void

)

Get low power ticker's data.

Returns

The low power ticker data

lp_ticker_irq_handler()

void lp_ticker_irq_handler

(

void

)

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

lp_ticker_init()

void lp_ticker_init

(

void

)

Initialize the low power ticker.

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

Pseudo Code:

void lp_ticker_init()
{
    // Enable clock gate so processor can read LPTMR registers
    POWER_CTRL |= POWER_CTRL_LPTMR_Msk;
 
    // Disable the timer and ensure it is powered down
    LPTMR_CTRL &= ~(LPTMR_CTRL_ENABLE_Msk | LPTMR_CTRL_COMPARE_ENABLE_Msk);
 
    // Configure divisors - no division necessary
    LPTMR_PRESCALE = 0;
    LPTMR_CTRL |= LPTMR_CTRL_ENABLE_Msk;
 
    // Install the interrupt handler
    NVIC_SetVector(LPTMR_IRQn, (uint32_t)lp_ticker_irq_handler);
    NVIC_EnableIRQ(LPTMR_IRQn);
}

lp_ticker_free()

void lp_ticker_free

(

void

)

Deinitialize the lower power ticker.

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

After calling this function no other ticker functions should be called except lp_ticker_init(). Calling any function other than init after freeing is undefined.

Note

This function stops the ticker from counting.

lp_ticker_read()

uint32_t lp_ticker_read

(

void

)

Read the current tick.

If no rollover has occurred, the seconds passed since lp_ticker_init() was called can be found by dividing the ticks returned by this function by the frequency returned by lp_ticker_get_info.

Returns

The current timer's counter value in ticks

Pseudo Code:

uint32_t lp_ticker_read()
{
    uint16_t count;
    uint16_t last_count;
 
    // Loop until the same tick is read twice since this
    // is ripple counter on a different clock domain.
    count = LPTMR_COUNT;
    do {
        last_count = count;
        count = LPTMR_COUNT;
    } while (last_count != count);
 
    return count;
}

lp_ticker_set_interrupt()

void lp_ticker_set_interrupt

(

timestamp_t

timestamp

)

Set interrupt for 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 lp_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, lp_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 lp_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 lp_ticker_set_interrupt(timestamp_t timestamp)
{
    lp_ticker_clear_interrupt();
    LPTMR_COMPARE = timestamp;
    LPTMR_CTRL |= LPTMR_CTRL_COMPARE_ENABLE_Msk;
}

lp_ticker_disable_interrupt()

void lp_ticker_disable_interrupt

(

void

)

Disable low power ticker interrupt.

Pseudo Code:

void lp_ticker_disable_interrupt(void)
{
    // Disable the compare interrupt
    LPTMR_CTRL &= ~LPTMR_CTRL_COMPARE_ENABLE_Msk;
}

lp_ticker_clear_interrupt()

void lp_ticker_clear_interrupt

(

void

)

Clear the low power 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 lp_ticker_clear_interrupt(void)
{
    // Write to the ICR (interrupt clear register) of the LPTMR
    LPTMR_ICR = LPTMR_ICR_COMPARE_Msk;
}

lp_ticker_fire_interrupt()

void lp_ticker_fire_interrupt

(

void

)

Set pending interrupt that should be fired right away.

Pseudo Code:

void lp_ticker_fire_interrupt(void)
{
    NVIC_SetPendingIRQ(LPTMR_IRQn);
}

lp_ticker_get_info()

const ticker_info_t * lp_ticker_get_info

(

void

)

Get frequency and counter bits of this ticker.

Pseudo Code:

const ticker_info_t* lp_ticker_get_info()
{
    static const ticker_info_t info = {
        32768,      // 32KHz
        16          // 16 bit counter
    };
    return &info;
}