11 Mar 2012
@CodingBadly, I modified your code a bit to explore the phenomenon further. It seems that millis() increments by 2 every 41 or 42 steps. My code is below. Let me know if I have misrepresented your ideas. It could be interesting to try to figure out what give rise to the 2 step, and whether the frequency varies.
@PeterH, Based on the data from that sketch I think I can see the problem you refer to. For example, suppose the required interval is 41 millisecs. Let's also assume that on the previous iteration millis() was 40 (so the event did not happen). On the next iteration millis() will be 42 (because it jumped 2 places) and then the event will be triggered a millisecond late. If I use that new value of millis() as the base for the next event it should happen when millis() becomes 83. Whereas if I calculated from the originally intended event time (41) I would schedule the next event for 82. On the other hand if the required interval doesn't coincide with the millis() jump there won't be any error.
I think what has been confusing me (maybe still does) is your phrase "when there is any latency between the timed condition becoming true, and the code to detect that executing". I believe that by "saving" millis() at the start of each iteration of loop() it doesn't matter when the code actually gets around to executing the event. And the timing of the subsequent event is not linked to the actual time of execution of the previous event. Instead it is linked to the value of millis() at the time the loop() began its iteration.
Bearing in mind the limited purpose of my sketch and its intended audience I still think the problem is small enough that the balance of advantage lies with being consistent with the usage in Blink-Without-Delay rather than trying to explain this problem to newbies.
As I said earlier, I am thinking of writing a short additional note to cover this point and the two others that have been raised. Your comment has helped to clarify my understanding, and may make a description easier to write. Do feel free to write a few explanatory sentences/paragraphs on the subject yourself to save me the bother.
@Msquare, your post just came in as I finished this. Sorry if I was too thick to see your point. You, also, are invited to write a suitable explanation for newbies.
...R
static unsigned long Mark;
static unsigned long Previous;
static unsigned short Histogram[256];
static unsigned short OverOne[256];
void setup()
{
Serial.begin( 115200 );
Serial.println( F( "Five seconds to lift off..." ) );
unsigned long Current;
unsigned long Delta;
int count = 0;
while (count < 256) {
Current = millis();
Delta = Current - Previous;
if (Delta > 0) {
Previous = Current;
Histogram[count] = (unsigned char)Delta;
if (Delta > 1) {
OverOne[count] = count;
}
count++;
}
}
for ( int i = 1; i < 256; ++i )
{
Serial.print( i );
Serial.write( '\t' );
Serial.print( Histogram[i] );
Serial.write( '\t' );
Serial.println( OverOne[i] );
}
Serial.println();
}
void loop()
{
}