topic Pulse not counting properly using LPTMR in Kinetis Microcontrollers

Pulse not counting properly using LPTMR

shabana — Sat, 15 Nov 2014 10:36:54 GMT

Hi,

In using LPTMR to count the pulse.

here is the snip shot of the code

void lptmr_pulse_count(uint32 count_val, int pc_src, int psg_src)

{

// Reset LPTMR module

lptmr_clear_registers();

// For interupt handler

lptmr_mode = LPT_CNTR_MODE;

// Enable LPT Interrupt in NVIC

enable_irq(LPTMR_IRQ_NUM); // LPTMR Vector is 101. IRQ# is 101-16=85

// Set up the interrupt vector for counter == compare val

// Set the compare value

LPTMR0_CMR = count_val;

// Set up LPTMR to bypass prescaler clock

LPTMR0_PSR = (LPTMR_PSR_PBYP_MASK | LPTMR_PSR_PCS(psg_src));

// select pulse counter mode, pulse counter clk src, and interrupt enable.

LPTMR0_CSR |= LPTMR_CSR_TMS_MASK | LPTMR_CSR_TPS(pc_src) | LPTMR_CSR_TIE_MASK;

LPTMR0_CSR |= LPTMR_CSR_TEN_MASK;

lptmr_intrpt = 0;

return;

}

count_value = 0xFFFF

clock source is LPO.

in oscilloscope I counted the pulse it is 22 but in CNR register the value is 69.. please see the image for the pulse

Re: Pulse not counting properly using LPTMR

yasuhikokoumoto — Sat, 15 Nov 2014 20:07:27 GMT

Hi Shabana m,

do you mean the glitch filter clock is LPO?
However you disabled (or bypassed) the glitch filer.
I think the error came from the input signal glitch.
Therefore you should better set the glitch filter as the following
because your input signal is 2.27MHz and LPO is 1kHz.

[before]
LPTMR0_PSR = (LPTMR_PSR_PBYP_MASK | LPTMR_PSR_PCS(psg_src));

[after]
LPTMR0_PSR = (LPTMR_PSR_PRESCALE(0x6) | LPTMR_PSR_PCS(psg_src));
// PBYP=0 PCS=psg_src=0b01(?) PRESCALE=0b0110=128cycles(220cycles would be best but not available)

This means that the input low level should keep longer than 128ms and the high level should keep longer than 128ms.

I hope this will help you.

Best regards,
Yasuhiko Koumoto.

Re: Pulse not counting properly using LPTMR

shabana — Mon, 17 Nov 2014 06:17:59 GMT

Hi Yasuhiko koumoto,

Thank u sooooo much...Its working... :smileyhappy:

Best regards,

Shabana