topic Capture problem with SCT on LPC824 in LPC Microcontrollers

Capture problem with SCT on LPC824

remco2 — Tue, 04 Apr 2017 10:38:36 GMT

Hi,

I'm trying to use the SCT for decoding the RC-5 (remote control) protocol.

I've configured the SCT to count the low and high bit times, using an input pin and two events. Unfortunately, in my interrupt handling code, the captured times are often (not always) reported to be 0. If they are non-zero they usually have a value that can be expected. Any help would be highly appreciated.

I've added a drawing of a shorted RC-5 pulse train and where I expect the events.

Event 2 is configured to capture on a rising edge, start the counter and limit the counter

Event 3 is configured to capture on a falling edge, limit the counter and request an interrupt

Event 4 is configured to stop and limit the counter after a match condition (to catch a timeout).

My configuration is as follows:

Chip_Clock_EnablePeriphClock(SYSCTL_CLOCK_SWM);

//Enable the SCT Timer for PWM and RC-5 decoder

Chip_Clock_EnablePeriphClock(SYSCTL_CLOCK_SCT);

Chip_SCT_Init(LPC_SCT);

//Configure the input pin for input 0
Chip_SWM_MovablePinAssign(SWM_SCT_IN0_I, 17);
Chip_INMUX_SetSCTInMux(LPC_INMUX,SCT_INMUX_0,SCT_INP_IN0);

//Regs 2 and 3 are capture, reg 4 is match (0,1 are in use by the L counter as match regs for PWM)

LPC_SCT->REGMODE_H=(1<<2)|(1<<3);

//Set the capture regs to capture on the events

LPC_SCT->CAPCTRL[2].H=(1<<2);

LPC_SCT->CAPCTRL[3].H=(1<<3);

//Set the match reg for the timeout

LPC_SCT->MATCH[4].H=2*MAX_FULL_BIT;

LPC_SCT->MATCHREL[4].H=2*MAX_FULL_BIT;

//Configure the events

LPC_SCT->EV[2].CTRL=(1<<4) | (0x1 << 10) | (0x2 << 12); //Select H timer, Rising input only

LPC_SCT->EV[3].CTRL=(1<<4) | (0x2 << 10) | (0x2 << 12); //Select H timer, falling input only

LPC_SCT->EV[4].CTRL=(4) | (1<<4) | (0x1 << 12); //Select H timer, match reg 4 only

//Enable the events in state 0 (the only state used)

LPC_SCT->EV[2].STATE=0x1; //Enable in state 0

LPC_SCT->EV[3].STATE=0x1; //Enable in state 0

LPC_SCT->EV[4].STATE=0x1; //Enable in state 0

//Configure the start, stop and limit behaviour

LPC_SCT->LIMIT_H=(1<<2)|(1<<3))|(1<<4); //All events limit

LPC_SCT->START_H=(1<<2); //Start when a rising edge is detected

LPC_SCT->STOP_H =(1<<4); //Stop after the timeout

//Enable the interrupts

LPC_SCT->EVEN=(1<<3)|(1<<4);

LPC_SCT->CTRL_H = (31<<5) | 0x2; //Divide by 32 for microsecond resolution, set counter in stop mode

The interrupt code is as follows:

void SCT_IRQHandler(void) {
    if (LPC_SCT->EVFLAG&SCT_EVT_4) {
       //Timeout, check received bits
       Chip_SCT_ClearEventFlag(LPC_SCT, SCT_EVT_4);
    } else {
        Chip_SCT_ClearEventFlag(LPC_SCT, SCT_EVT_3);
        int lowTime=LPC_SCT->CAP[2].H;
        int highTime=LPC_SCT->CAP[3].H;

      //Do actual decoding of bit(s). But lowTime and highTime are often 0....
    }
}

In the debugger the capture registers are never 0, but between the time the breakpoint is hit and the IDE is updated, a lot of additional events occur I assume...

I've no idea on how to proceed from here...

Regards,

Remco Poelstra

Re: Capture problem with SCT on LPC824

kerryzhou — Fri, 07 Apr 2017 09:16:05 GMT

Hi Remco Poelstra,

I share a LPC824 SCT capture code with you.

You can find it from the attachment.

This code has been test on my side, the captured data is changing.

Please check my attached project, this is the test result on my side:

Wish it helps you!

Have a great day,
Kerry

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Re: Capture problem with SCT on LPC824

remco2 — Fri, 07 Apr 2017 15:10:22 GMT

Hi,

Thanks! I'll check it out!

Regards,

Remco Poelstra

Re: Capture problem with SCT on LPC824

kerryzhou — Mon, 10 Apr 2017 04:51:05 GMT

Hi Remco Poelstra,

You are welcome.

Please check it at first, any question, just let me know!

Have a great day,
Kerry

Re: Capture problem with SCT on LPC824

athmesh_n — Thu, 17 Jan 2019 18:02:20 GMT

Hi kerryzhou‌,

How can we count the frequency using capture, my capture is working. What is the basic of Timer Capture

Re: Capture problem with SCT on LPC824

kerryzhou — Fri, 18 Jan 2019 01:13:31 GMT

Hi ATHMESH NANDAKUMAR,

Capture will get the counter, then use the counter can calculate the frequency, as you know, each counter time is you own defined.

Wish it helps you!

Have a great day,
Kerry