Scott Pierce wrote:
> I'm trying to use the CCP module on a 16F628 to generate a 40Khz pulse
> in order to communicate via IR.  The following test program I wrote
> using the PICC compiler should set up the CCP module for a free running
> 40Khz, 50%
> Duty pulse.  RB3, the output of the CCP module on the F628 is connected
> to
> the Anode of an IR LED.  The Cathode of the IR LED is connected to
> RB5.

Not a good idea.  First, there is no need for a separate pin to
enable/disable the 40KHz pulse output.  You can do that just as easily in
software by enabling/disabling the CCP module.  Second, you need a resistor
in series with the LED if running it from 5V.  The forward voltage of an IR
LED is nowhere near 5V, so without a resistor something will have to give.
That will probably be one or both of the PIC pins.  Third, IR LEDs can take
a lot more current than most visible LEDs of the same size.  Check the data
sheet.  Most likely you can put at least 100mA thru it when it's on, for an
average current of 50mA.  Fourth, you can't drive that much current with a
PIC pin.  Connect the CCP output pin to a resistor, other end of resistor to
base of NPN transistor, emitter to ground, collector to LED cathode, LED
anode to another resistor, and the other end of that resistor to 5V.  Size
the LED resistor to get 100mA (depending on your LED) thru the LED when the
transistor is on.  Size the base resistor to make sure the transistor is
saturated when sinking the LED current.  With a jellybean transistor like
2N4401 you can count on a gain of at least 50.  That means you need at least
2mA base drive to support 100mA LED current.  A 2Kohm base resistor will get
you a little more than that, which is probably a good answer.

> a 40khz pulse should be output on the IR LED for a short period of time
> (about a second).

That's not a short period of time.  The IR detector will probably want to
see pulses from 10 to 50 cycles.  Much beyond that and it's automatic gain
circuitry will decide the pulse is ambient and set its threshold to filter
it out.

> I have included a simple Excel spreadsheet I made to help calculate
> the settings for the CCP Module for the PIC 16F628, which I used to
> come up with my values for the program.

That's silly.  What happens when you change the PIC oscillator frequency or
want to use a different IR modulation frequency?  These should be parameters
directly in your code, probably a top level include file, and everything
else should be computed from that at assembly time.


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