如何在Arduino Due中创建准确的计时器

Question

I'm trying to create some PWM waves using a TC timer in Arduino Due. My problem is that I cannot generate accurate frequencies using this kind of timers.

Here's a simple code for what I'm trying to do :

static void setupTimer(uint32_t freq_desired)
{

uint32_t ul_div;
uint32_t ul_tcclks;
uint32_t ul_sysclk = sysclk_get_cpu_hz();
uint32_t counts;

// Configure PMC
pmc_enable_periph_clk(ID_TC);

// Configure TC for the given frequency and trigger on RC compare.
tc_find_mck_divisor(
    (uint32_t)freq_desired, // The desired frequency as a uint32.
    ul_sysclk,              // Master clock freq in Hz.
    &ul_div,                // Pointer to register where divisor will be stored.
    &ul_tcclks,             // Pointer to reg where clock selection number is stored.
    ul_sysclk);             // Board clock freq in Hz.

tc_init(TC0, CHANNEL, ul_tcclks | TC_CMR_CPCTRG);

// Find the best estimate of counts, then write it to TC register C.
counts = (ul_sysclk/ul_div)/freq_desired;

tc_write_rc(TC0, 0, counts);

// Enable interrupts for this TC, and start the TC.
tc_enable_interrupt(TC0, CHANNEL0, TC_IER_CPCS);                // Enable interrupt.

tc_start(TC0,CHANNEL0);         // Start the TC.

NVIC_DisableIRQ(TC_IRQn);
NVIC_ClearPendingIRQ(TC_IRQn);
NVIC_SetPriority(TC_IRQn,configMAX_PRIORITIES);
NVIC_EnableIRQ(TC_IRQn);
}

Then on the Timer's Handle all what I do is triggering an output pin with :

ioport_toggle_pin_level(OUT_PIN);

The problem is that when I try for instance to generate a 1000Hz wave (giving 2000Hz for the timer of course), it works fine. But when I try, like 3427Hz, then it generate only 3420Hz or something like that.

Do you have please any idea how to fix that ? I tried to add round() for calculating the 'counts' variable value, it helped a bit, but still not extremely accurate.

Thanks in advance.

Answer 1

I'm guessing that TC0 is an 8-bit timer? You won't get good precision at "unusual" intervals with only 256 choices.

Try using a 16-bit timer. I've seen several "Due Timer"s on GitHub if you need them.

From my calculations:

Get an 8-bit divisor with a prescale by 256:

84MHz / 3427 / 256 =  95.75 divisor, round to 96

which generates

84MHz / 256 / 96 = 3,417.96 Hz

But with 16 bits, no prescale:

84MHz / 3427 =  24,511.23 divisor, truncate to 24511

which generates

84MHz / 24511 = 3,427.03 Hz

Since the calculated divisor of 24511 fits inside a 16-bit value, you've gotten a lot closer to your chosen rate.

And, of course, if TC0 is in fact a 16-bit timer, then there's something else wrong! ;-) Good luck!

Answer 2

TC0 is 24 bit. all timer period registers on the due are 24 bit. if using pwm the counter period registers are 16 bit