Questions on ultrasonic distance finder

[Kamikaze47]

TMR1H increments when TMR1L overflows at 256. So its 4x256+23.

Keep in mind that the timer will run at 1/4 the frequency of your main oscillator.

edit: also, pauseus 1 will not result in a 1us pause because pauseus has a minimum pause time. have a look at pauseus in the PBP manual.

[Ioannis]

Do you really have to built it yourself?

Look at the http://www.robot-electronics.co.uk/a...c_Rangers.html

Once I had to built that ranger in 1990 but then I could not find ready made modules.

After a few year I tested the modules of the above link and was surprised.

First of the idea to drive the Tx US transducer by the well known MAX232 chip (it can produce fast +/- 10 volts of pulses!) and second by its simplicity and performance.

Ioannis

[lilimike]

Well the main reason why I want to build it myself is to learn everything about it, I really like the challenge and when I go to bed just about every other day or so I sleep well because I like the feeling of having learned something new.

Now trying to calculate and figure how I got this result on my display is probably going to keep me up for a few days because even after Kamikaze's response I am still confused. Maybe it's all about the timing differences between the speed my brain works and the speed of a PIC at 4MHz.

Or maybe I am trying to calculate something that is uncalculatable!

[Kamikaze47]

OK, if you are running your PIC at 4MHz, then the timer will run at 1MHz, which means that TMR1L will increment every 1uS (when the prescale is set to 1:1).

timing the command PAUSEUS 1 will not give you a result of 1 becuase of 2 reasons. First, PAUSEUS has a minimum pause time and 1uS is so fast that the time it takes to check the timer value comes into play.

Try this:

temp VAR word

TMR1H=0 ' Reset timer value
TMR1L=0
T1CON.0=1 ' Start the timer
PAUSEUS 1000 ' Pause 1000uS
T1CON.0=0 ' Stop the timer
temp.HighByte=TMR1H
temp.LowByte=TMR1L
LCDOUT $FE,$80,"TMR1 = ", dec temp

The LCD should display somewhere around 1000 - Probably a little more because of the time it takes to start and stop the timer.

[lilimike]

you are absolutely right, 1001 is what I get.

I am sorry if I ask again about the timing but are you able to explain this?

if I keep the timer running, I removed the PAUSEUS and I do something like this:

LCDOUT $FE,$80,"H",dec TMR1H,"L",dec TMR1L,"H",dec TMR1H,"L",dec TMR1L

The first set of H/L I get 401
The second set I get 4604

Why it isn't the second set taking arround 800?
Is it because the actual calculation is based on the assembly and when it is compiling it is doing funny things?

[Kamikaze47]

Its possible that the LCDOUT routine really is taking that long.

But ive found that its generally best to stop the timer to take a reading, and usually best to store that reading in a variable before displaying it, or doing anything else with it.

See what happens if you do it like this:


temp VAR word

TMR1H=0 ' Reset timer value
TMR1L=0
T1CON.0=1 ' Start the timer
pauseus 1000
T1CON.0=0 ' Stop the timer
temp.HighByte=TMR1H
temp.LowByte=TMR1L
T1CON.0=1 ' Restart the timer
LCDOUT $FE,$80,"TMR1 = ", dec temp
T1CON.0=0 ' Stop the timer
temp.HighByte=TMR1H
temp.LowByte=TMR1L
LCDOUT $FE,$C0,"TMR1 = ", dec temp

The difference between the 2 results will tell you how long the first LCDOUT routine is taking.

[lilimike]

...well I am getting closer to the numbers I am expecting but...

My values were 1001 and 4206 so it took 3205 to perform LCDOUT.

Then I restarted the timer just before the last LCDOUT and added this to your code and got a value of 7405

T1CON.0=0 ' Stop the timer
temp.HighByte=TMR1H
temp.LowByte=TMR1L
LCDOUT $FE,$80,"TMR1 = ", dec temp

So the second time LCDOUT executed it took 3199
I did it a third time and found a different number again.

So LCDOUT executed 3 times and it took less time everytime it executed.

1st time took 3205
2nd time took 3199
3rd time took 3187

I figured writting to the LCD on a blank line took 3205
overwriting on the first line of the LCD took 3199
and overwritting on the second line took 3187

But I did it a fourth time (overwrote on the first line) and it took 3312
This is the code I ended up with:

temp VAR word
TMR1H=0 ' Reset timer value
TMR1L=0
T1CON.0=1 ' Start the timer
pauseus 1000
T1CON.0=0 ' Stop the timer
temp.HighByte=TMR1H
temp.LowByte=TMR1L
T1CON.0=1 ' Restart the timer
LCDOUT $FE,$80,"TMR1 = ", dec temp
T1CON.0=0 ' Stop the timer
temp.HighByte=TMR1H
temp.LowByte=TMR1L
T1CON.0=1 ' Restart the timer
LCDOUT $FE,$C0,"TMR1 = ", dec temp
T1CON.0=0 ' Stop the timer
temp.HighByte=TMR1H
temp.LowByte=TMR1L
T1CON.0=1 ' Restart the timer
LCDOUT $FE,$80,"TMR1 = ", dec temp
T1CON.0=0 ' Stop the timer
temp.HighByte=TMR1H
temp.LowByte=TMR1L
T1CON.0=1 ' Restart the timer
LCDOUT $FE,$C0,"TMR1 = ", dec temp
T1CON.0=0 ' Stop the timer
temp.HighByte=TMR1H
temp.LowByte=TMR1L
LCDOUT $FE,$80,"TMR1 = ", dec temp

This beeing said, if I start over from the begining, no matter how many times I am always getting the same results so this rules out a bad resonator or voltage noise or any othe hardware issue.

So there is something in the process that is hapenning which is irregular and I wish I could understand what it is!

I am really sorry to be such a pain and I appreciate your patience but I am still a little confused.

Mike