MicroC and 8051 pin input

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Hi. I'm trying to count the number of times P3.3 on the 8051 goes low.
The C code I've generated doesn't give me any errors but it isn't
working either. Can any of you offer some pointers? The code is as

#include <8051io.h>
#include <8051bit.h>            /* Bit set/clear macros */
#include <8051reg.h>

main() {
  int i
  i = 0;
      if(!(P3 & 0xCD))
        WriteLCD( "*" );

Thanks a lot,

Re: MicroC and 8051 pin input

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How "not working" - does it fill the screen with * when you press a button ?


Re: MicroC and 8051 pin input

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If you're only trying to monitor P3.3, why the 0xCD mask - as opposed to
If you're counting '*''s on the screen, why do you need to increment 'i'?
If you're keeping the count in 'i', how do you get out of the loop to read
it - debugger?
You don't mention which 8051 variant you're using, but did you initialise
the ports properly? Do you need pullup resistors? Watchdog timer enabled?


Re: MicroC and 8051 pin input
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It seems that this is one of your very first projects. The first
embedded project is to turn on a led. Try that first, forget the
LCD for the moment. Change the program to turn it off, test it,
change it again to turn on the led, and test it.

The next step is to read an input that has a pushbutton connected.
Try to turn on a led when the butten is pressed, and goes off when
the button is released.

The third step is to forget the led and switch, and print a text
on the LCD, see if you get 'Hello World' on the LCD.

If you have done those 3 experiments, you are ready to combine
what you have learnt, and make that counter.

Thanks, Frank.
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Re: MicroC and 8051 pin input

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In addition to the incorrect mask value (0xCD) I see a couple of problems
with this program.  First, rather than counting the number of times the
port goes low, ie, transitions from the high to low state, it counts the
number of times it finds the port pin low.  Asterisks will be written to
the display and 'i' will be incremented at a rate determined by the time
required to traverse the infinite loop rather than by the number of times
the pin goes low.  To solve this you must also wait for the port to be
high before waiting for it to go low.

Second, short pulses from high to low and back to high can be missed if
they occur while setbit(), WriteLCD(), delay(), and "i++" are executing.
To solve this you would do better to use an interrupt input and count
the number of interrupts that occur.  There would still be a window of
pulses that could be missed but it would be quite a bit shorter.

Oh, and you should probably move the call to setbit() outside the infinite
loop, too.

          Michael Kesti            |  "And like, one and one don't make
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Re: MicroC and 8051 pin input
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I don't know the 8051, but the delay doesn't look good to me.  What if
the pin goes low and high again, you'll have missed a count.  You don't
give enough information to give a sensible decision, like are these
keypresses or what frequency the pulses come in.  E.g. key presses need
debouncing -like 3 counts with 15mS gaps.

Are you a student?

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