54L15 Timer Interrupt cost too high. How to reduce interrupt cost?

RE: 54L15 Timer Interrupt cost too high. How to reduce interrupt cost?

Vidar Berg — Wed, 01 Jul 2026 06:11:03 GMT

Hi Austin,

If you use GPIOs to measure the timing it is important that you use pins from the fast P2 port and that you use direct register access to set or clear the pin to minimise latency and jitter introduced by your measurement.

#include <hal/nrf_gpio.h>

...

nrf_gpio_cfg_output(NRF_GPIO_PIN_MAP(2, 9))

// Code to clear P2.09

NRF_P2->OUTCLR = (1 << 9);

// Code to set P2.09

NRF_P2->OUTSET = (1 << 9);

Another alternative is to use the timing functions available in Zephyr and print the measured result out on UART: https://docs.zephyrproject.org/latest/kernel/timing_functions/index.html

[quote user="Austin (S.F.)"] It is a real-time application. Because that, I have some calculate and response immidiatly.[/quote]

I understand that timing is critical but in some situations you can achieve shorter latencies by not using interrupts.

[quote user="Austin (S.F.)"]It seems zero latency for IRQs no work.[/quote]

"zero latency" will not make any difference for your test as there aren't any other interrupts that overlap with your TIMER interrupt anyway.

RE: 54L15 Timer Interrupt cost too high. How to reduce interrupt cost?

Austin (S.F.) — Tue, 30 Jun 2026 10:55:02 GMT

Hi Vidar

I added "CONFIG_ZERO_LATENCY_IRQS=y".

And the code as below

nrfx_timer_t timer00_inst = NRFX_TIMER_INSTANCE(NRF_TIMER_INST_GET(TIMER00_IDX));

static void timer00_handler(nrf_timer_event_t event_type, void *p_context)
{
    if (event_type == NRF_TIMER_EVENT_COMPARE0)
    {
        LED_R_toggle;
        test_bit = GET_AD_IN_1_DATA;
    }
}

ISR_DIRECT_DECLARE(timer00_direct_isr)
{
    nrfx_timer_irq_handler(&timer00_inst);  
    return 0;  
}

int8_t Timer00_init(void)
{
    //pick 1 or 2 to test the difference
    //1. use this
    IRQ_CONNECT(NRFX_IRQ_NUMBER_GET(NRF_TIMER_INST_GET(TIMER00_IDX)), 
                0,
                nrfx_timer_irq_handler,
                &timer00_inst,
                0);


    //2. or this
    IRQ_DIRECT_CONNECT(NRFX_IRQ_NUMBER_GET(NRF_TIMER_INST_GET(TIMER00_IDX)),
                       0,
                       timer00_direct_isr,   
                       0);                   

    uint32_t base_frequency = NRF_TIMER_BASE_FREQUENCY_GET(timer00_inst.p_reg);
    nrfx_timer_config_t config = NRFX_TIMER_DEFAULT_CONFIG(base_frequency);
    config.bit_width = NRF_TIMER_BIT_WIDTH_32;
    config.p_context = "4us";

    nrfx_timer_init(&timer00_inst, &config, timer00_handler);
    nrfx_timer_clear(&timer00_inst);

    uint32_t desired_ticks = nrfx_timer_us_to_ticks(&timer00_inst, TIME_TO_INT);
    desired_ticks = desired_ticks + 8;

    nrfx_timer_extended_compare(&timer00_inst, NRF_TIMER_CC_CHANNEL0, desired_ticks,
                                NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK, true);

    nrfx_timer_enable(&timer00_inst);
    printk("Timer status: %s\r\n",
           nrfx_timer_is_enabled(&timer00_inst) ? "enabled" : "disabled");

    return 0;
}

When using 1. and remark 2., the result as below. The Timer 00 cost 1.53us .

When using 2. remak 1., the result as below. The Timer 00 cost 1.595us .

Do you have better way to reduce the cost?

It seems zero latency for IRQs no work.

Austin

RE: 54L15 Timer Interrupt cost too high. How to reduce interrupt cost?

Austin (S.F.) — Mon, 29 Jun 2026 08:18:07 GMT

Hi Vidar

Yes, I really need the timer interrupt to preempt my code.

It is a real-time application. Because that, I have some calculate and response immidiatly.

I will try this " direct ISA". After I testing it , I will let you know the result.

Austin

RE: 54L15 Timer Interrupt cost too high. How to reduce interrupt cost?

Vidar Berg — Mon, 29 Jun 2026 07:42:45 GMT

This is a continuation of this thread here: https://devzone.nordicsemi.com/f/nordic-q-a/128563/nrf54l15-sb_config_bootloader_mcuboot-y-makes-main-gpio-toggle-stop.

[quote user=""]How to reduce inrerrupt cost from 1.04us to around 200ns?[/quote]

To reduce the overhead you can define your ISR as "direct" with no rescheduling on return: https://nrfconnectdocs.nordicsemi.com/ncs/latest/zephyr/kernel/services/interrupts.html#defining-a-direct-isr. Building with CONFIG_SPEED_OPTIMIZATIONS=y may also help shave off a few CPU cycles. But what I was trying to ask in the previous thread is whether you really need the timer interrupt to preempt your code. Given the short interval, it may make more sense to poll the TIMER counter register instead.

RE: 54L15 Timer Interrupt cost too high. How to reduce interrupt cost?

Turbo J — Sat, 27 Jun 2026 11:37:07 GMT

Smells like an X/Y problem. What are you actually trying to do?

Interrupt timing is horrible when you turn on the radio/BT stack.

If you need that strict of a timing, try to do the work in hardware (PWM peripherial or FLPR).