9 years, 8 months ago.

I suggest a more detailed dive into Timer.read_us and read_ms.

There is a basic claim that every SW timer should fulfill: The Overflow occurs in an unsigned form, i.e. from 2^N-1 (=max. type value) to 0. N is the full bit range of the unsigned function type. (The reason for this is that elapsed time is often measured by (ActualTime - LastTime) f. e.)

This is not the case for read_us because int Timer::read_us() is signed instead of unsigned. This is Problem #1.

Additionally I cannot say if the HW-timer behind read_us is a full 32 bit timer always. If not we have Problem #2.

With respect to read_ms I can say clearly that the overflow will not occur at the variable's max value because of

int Timer::read_ms() {
    return read_us() / 1000;
}

This is Problem #3.

Solving Problems 1&2 I expect the following. After 2^32 microseconds read_us overflows correctly to 0. Ok so far. Unfortunately due to Problem 3 read_ms overflows from (2^32-1)/1000 to 0. This happens roughly after 1100 45 days. Few people will have systems that will run up to this crash but some will.

Please confirm. If desired I may help to patch mbed sources since I have long time experience with µs resolution timing control combining HW and SW timers. Cheers. Edited on 2015-04-31

11 years, 5 months ago.

Thanks for this - no I'd not found this and it does give an excuse for the apparent limitation. It does seem a bit shortsighted to limit the utility of timer. On the LPC11U24 there's no realtime clock so this is quite a useful feature.

At present I've got a bit of a bodge that gets round this using type conversion to remove the negative and a seond variable to account for the rollovers. This method works fine as long as the calling routine polls the counter at least twice per 70 minute run.

Comparing to the rtc on a 1768 this goes no more than 300ms out after 4 hours...

#include "mbed.h"
#include "mbed.h"
//  Expanding the TIMER function to allow timing to very long times 
// Demo written for lpc1768 to compare to onboard realtime clock.
// uses highusecs to encode the top 32 bits of the timer output
// and an int to uint conversion to get rid of the annoying negative.

Timer t;

 
int main() {
set_time(1256729737);  // Set RTC time to Wed, 28 Oct 2009 11:35:37
float umax=0,seconds=0;
unsigned int realusecs=0,oldusecs=0,highusecs=0,minutes=0;;
int dtime,usecs=0;

    t.start();
    usecs=t.read_us();
    realusecs=(unsigned int)usecs;
    time_t rtcstart = time(NULL);
    printf("\n\n\r");
    while(1)
    {
    wait(1);
    oldusecs=realusecs;
    usecs=t.read_us();
    realusecs=(unsigned int)usecs;
    //if(realusecs==0) t.reset(); // timer counts back up to 0 and finishes there
   if(realusecs<oldusecs) { // its rolled over
    highusecs++; // increment highbyte
   }
   seconds=((float)realusecs/1000000.0)+4294.967296*(float)highusecs;
   minutes=(int)(seconds/60);
    
    if(umax<realusecs) umax=realusecs;
    time_t rtcsecs = time(NULL);
    dtime=rtcsecs-rtcstart;
    printf("Seconds=%6.2f  mins=%d   realus=%u     hus=%u     us=%d       rtcs=%d         oldusecs=%u          \r",seconds,minutes,realusecs,highusecs,usecs,dtime,oldusecs);
   
    }
}