Create accurate microsecond timer without interrupts

Question

Hey guys,

I want to create an accurate timer without interrupts. My timer has to be very accurate because I have to send HIGH and LOW signals for 1us. I have read this post to know that I need high-frequency clock source (HFCLK, 16 MHz), which means better resolution (62.5 ns) and higher power consumption (typ. 5 or 70 uA depending on HFCLK source). But I don't know how to program my code so I followed this post and this post. But I have still some problems with it. I post my code below.

My timer should only replace the unaccurate nrf_delay_us.

#include <stdint.h>

#include "nrf_delay.h"
#include "app_error.h"
#include "nrf_gpio.h"
#include "nrf_drv_timer.h"

#define OUTPUT_PIN  18


/** @brief Function for Timer 1 initialization.
 */
void TIMEOUT_TIMER1_Init(void)
{
    NRF_TIMER1->TASKS_STOP = 1;           /**< Stop Timer 1 */
    NRF_TIMER1->TASKS_CLEAR = 1;          /**< Clear Timer 1 */

    NRF_TIMER1->MODE      = 0;            /**< Mode 0: Timer */
    NRF_TIMER1->BITMODE   = 3;            /**< Bitmode 3: 32 bit (for what?, What happens if I choose 16 bit?) */
    NRF_TIMER1->PRESCALER = 4;            /**< Prescaler 4: 1us Timer -> f(TIMER) = 16MHz / (2^PRESCALER) = 1MHz -> t(TIMER) = 1us */
    NRF_TIMER1->INTENCLR = TIMER_INTENCLR_COMPARE0_Disabled
                            | TIMER_INTENCLR_COMPARE1_Disabled
                            | TIMER_INTENCLR_COMPARE2_Disabled
                            | TIMER_INTENCLR_COMPARE3_Disabled
                            | TIMER_INTENCLR_COMPARE4_Disabled
                            | TIMER_INTENCLR_COMPARE5_Disabled; /**< Disable all interrupts */
}


void TIMEROUT_TIMER1_Start(uint32_t timeout_us)
{
    NRF_TIMER1->TASKS_STOP = 1;
    NRF_TIMER1->TASKS_CLEAR = 1;

    // this line: set TIMER1 to 0 (start value)
    // this line: hand over the time "timeout_us" to TIMER1 (end time)
    // this line: stop TIMER1 if end time ("timeout_us") is reached

    NRF_TIMER1->TASKS_START = 1;
}


void Write_Byte(uint8_t byte)
{
    int walk;

    for (walk = 0; walk < 8; ++walk)
    {
        nrf_gpio_pin_clear(OUTPUT_PIN);    
        if (byte & 0x01)
        {
            TIMEROUT_TIMER1_Start(1);       //instead of nrf_delay_us()                      
            nrf_gpio_pin_set(OUTPUT_PIN);                
            TIMEROUT_TIMER1_Start(60);      //instead of nrf_delay_us()                   
        }
        else
        {
            TIMEROUT_TIMER1_Start(60);       //instead of nrf_delay_us()                      
            nrf_gpio_pin_set(OUTPUT_PIN);                 
            TIMEROUT_TIMER1_Start(1);        //instead of nrf_delay_us()                     
        }
        byte >>= 1;                                       
    }
}


/**
 * @brief Function for application main entry.
 */
int main(void)
{
    uint32_t err_code;
    TIMEOUT_TIMER1_Init;
    
    uint8_t AA = 0xAA;
    nrf_gpio_cfg_output(OUTPUT_PIN);

    while (true)
    {
        Write_Byte(AA);
        nrf_delay_us(20);
    }
}

In line 19: (Init) Why should I use 32 bits and not 16 bits? where is the difference?

In lines 35 - 37: (time function) I hope you can help me here. I don't know how I can hand over my timeout value to my function and to my timer, so that the timer can work with it...

I hope, you can help me.

Thanks in advance,

Christoph

Håkon Alseth · Accepted Answer

Hi,

I see what you mean. I get a delay of ~2.3 us when using the timer, and approx. 1.2 us when using nrf_delay_us().

When dealing with such low delays, the overhead with handling the actual timer tasks and events will give a significant error in your GPIO toggling.

For the 1 us specific delay, I think you have to use a busy-loop, and not the timer.

I have played a bit around and tried to create a function that can give a more reliable delay.

1 us input:

2 us input:

10 us input:

Could you try this and see if it works similarly on your end?

Fullscreen main.c Download

/**
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 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, this
 *    list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form, except as embedded into a Nordic
 *    Semiconductor ASA integrated circuit in a product or a software update for
 *    such product, must reproduce the above copyright notice, this list of
 *    conditions and the following disclaimer in the documentation and/or other
 *    materials provided with the distribution.
 *
 * 3. Neither the name of Nordic Semiconductor ASA nor the names of its
 *    contributors may be used to endorse or promote products derived from this
 *    software without specific prior written permission.
 *
 * 4. This software, with or without modification, must only be used with a
 *    Nordic Semiconductor ASA integrated circuit.
 *
 * 5. Any software provided in binary form under this license must not be reverse
 *    engineered, decompiled, modified and/or disassembled.
 *
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 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */
/** @file
 *
 * @defgroup blinky_example_main main.c
 * @{
 * @ingroup blinky_example
 * @brief Blinky Example Application main file.
 *
 * This file contains the source code for a sample application to blink LEDs.
 *
 */

#include <stdbool.h>
#include <stdint.h>
#include "nrf_delay.h"
#include "boards.h"

#define CC_USED 0

void timer_setup(void)
{
    NRF_TIMER1->BITMODE = TIMER_BITMODE_BITMODE_32Bit;
    NRF_TIMER1->SHORTS = TIMER_SHORTS_COMPARE0_CLEAR_Enabled << TIMER_SHORTS_COMPARE0_CLEAR_Pos| TIMER_SHORTS_COMPARE0_STOP_Enabled << TIMER_SHORTS_COMPARE0_STOP_Pos;
    NRF_TIMER1->PRESCALER = 0;
}

/* Just a test function*/
static inline void sleep_us(uint32_t timeout_us)
{
    for (register uint32_t i = 0; i < timeout_us; i++)
    {
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
    }
}

#pragma push
#pragma O3
void timer_sleep(uint32_t timeout_us)
{

    if (timeout_us == 0) 
    {
        return;
    }
    if (timeout_us == 1)
    {
        /* Adjust the amount of NOPs here */
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop();
        return;
    }

    NRF_TIMER1->CC[CC_USED] = (timeout_us << 4) - 18;
    NRF_TIMER1->TASKS_START = 1;
    while(NRF_TIMER1->EVENTS_COMPARE[CC_USED] == 0) 
    {
      /* Call __WFE() if loop is long */
    }
    NRF_TIMER1->EVENTS_COMPARE[CC_USED] = 0;
}
#pragma pop

/**
 * @brief Function for application main entry.
 */
int main(void)
{
    /* Configure board. */
    bsp_board_init(BSP_INIT_LEDS);
    NRF_CLOCK->TASKS_HFCLKSTART = 1;
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);
    NRF_NVMC->ICACHECNF = NVMC_ICACHECNF_CACHEEN_Msk;
    timer_setup();
    /* Toggle LEDs. */
    while (true)
    {
        timer_sleep(2);
        NRF_P0->OUT ^= (1 << LED_1);

    }
}

/**
 *@}
 **/

Christoph_I said:
For example I set timeout_us = 4400 seconds. This timeout_us is never reached because after 4294 seconds there will happen an overflow and the timer will restart at 0seconds, right? This will be an endless loop, right?

a uint32_t will overflow in this scenario, resulting in a lower timeout. If you set the timer to 0x 12 0000 1000, the timer will count to 0x1000, and give an event.

Kind regards,

Håkon

Håkon Alseth · Answer

Hi Christoph,

Christoph_I said:
Now I'm getting nearly the 1us! :)

That is good news!

Christoph_I said:
To read from input: NRF_P0->IN = (1 << INPUT_PIN);

Try this: uint32_t pin = (NRF_P0->IN & (1 << INPUT_PIN));

Others look OK.

Christoph_I said:
nrf_gpio_pin_set() / clear() ?

Because they are function calls, although static inlined, which use stacked memory to access the register. If you look at the assembly output, you'll likely see more instructions than just directly accessing the register. The output depends on the compiler used as well.

For GCC, you need to force the optimization like this:

#pragma GCC push_options
#pragma GCC optimize ("O3")
void timer_sleep(uint32_t timeout_us)
{

    if (timeout_us == 0) 
    {
        return;
    }
    if (timeout_us == 1)
    {
        /* Adjust the amount of NOPs here */
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop(); __nop(); __nop(); 
        __nop(); __nop();
        return;
    }

    NRF_TIMER1->CC[CC_USED] = (timeout_us << 4) - 18;
    NRF_TIMER1->TASKS_START = 1;
    while(NRF_TIMER1->EVENTS_COMPARE[CC_USED] == 0) 
    {
      /* Call __WFE() if loop is long */
    }
    NRF_TIMER1->EVENTS_COMPARE[CC_USED] = 0;
}
#pragma GCC pop_options

Your overall settings for optimization will be overridden for this specific function only.

*edit*

Tested with "O3" optimization on SES v4.22, and got:

1 us input = 1.05 to 1.1 us

2 us input = 1.9 us

10 us input = 9.9 us

Please check with these pragma's for GCC optimization and see if you also see similar behavior.

Kind regards,

Håkon

Create accurate microsecond timer without interrupts

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