pulseIn Timeout

Projects Programming
1

I have a Mega 2560, with an LED on 13 and 12 hard-wired to ground.

unsigned long temp;
void setup(){
  pinMode(13,OUTPUT);
  pinMode(12,INPUT);
}

void loop(){
   digitalWrite(13,HIGH);
   temp = pulseIn(12,LOW,1000000L);
   digitalWrite(13,LOW);
   temp = pulseIn(12,LOW,1000000L);
}

In the above code, if I understand the pulseIn function correctly, I'm expecting to see the LED on for a second and off for a second. But it flashes much faster (on for ~250 ms and off for ~250 ms).

Any thoughts? Thanks!

EDIT - I'm using Arduino 0022.

2

Looks like a bug in the library. The code for pulseIn calls the following macro, with the timeout you pass as the parameter:

#define microsecondsToClockCycles(a) ( ((a) * (F_CPU / 1000L)) / 1000L )

With F_CPU = 16000000L, the expression becomes (a * 16000L) / 1000L. The multiplication will overflow if 'a' (the timeout value you are passing) exceeds 268435. So a timeout of one million is too much for it. [The multiplication is done using unsigned arithmetic, because that's the type of the 'timeout' parameger to pulseIn.]

3

LolluPaandi:
In the above code, if I understand the pulseIn function correctly, I'm expecting to see the LED on for a second and off for a second. But it flashes much faster (on for ~250 ms and off for ~250 ms).

Any thoughts? Thanks!

dc42 is probably right. Also when a parameter like "timeout" is specified in microseconds, you should be wary of supplying a million of them. The implication is that you are expecting a pulse within a "few" microseconds, and if you don't get it you timeout.

Also, a "counted" loop (like pulseIn does) is unlikely to be accurate at such high intervals (interrupts would throw the interval out).

4

dc42 is right. I changed the define in wiring.h to

#define microsecondsToClockCycles(a) ( ((a) * (F_CPU / 10000L)) /100L )

and now my LED flashes every ~1 second.

I see your point Nick. Which is why my LED flashes approximately every one second, and not accurately. But my application can tolerate this - I'm trying to detect a light pattern from an external device (flashing 'fast' or flashing 'slow' or steady) where 'fast' is every second and 'slow' is every 3 seconds.

Also, in the documentation of pulseIn, it is mentioned that the default value of timeout is one second. But I don't see how this is implemented. Is that a bug as well?

I'm fairly new to Arduino. Is there a place to log bugs?

Thanks. I love this community.

5

LolluPaandi:
Also, in the documentation of pulseIn, it is mentioned that the default value of timeout is one second. But I don't see how this is implemented. Is that a bug as well?

Yes, I think it is. This illustrates well the perils of untested, unverified code.

6

LolluPaandi:
Also, in the documentation of pulseIn, it is mentioned that the default value of timeout is one second. But I don't see how this is implemented. Is that a bug as well?

It's implemented in the function prototype for pulseIn in the file WProgram.h:

unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout = 1000000L);

It's pretty standard in C++ to put defaults on (trailing) arguments to functions in this fashion.

7

See also - http://arduino.cc/forum/index.php/topic,74813.0.html -

I posted a bug report yesterday for pulseIn() - Google Code Archive - Long-term storage for Google Code Project Hosting. -

Updated it today with reference to both threads as backgrounder and a fixed version of pulsein().

8

I posted the same bug back in September (http://arduino.cc/forum/index.php/topic,72657.0.html)

A better fix was suggested that only requires a change to the pulsein function and won't affect other code that uses microsecondsToClockCycles and wiring.h

This seems not have made it to 023 or 1.0