PIC output

[S. Hackwelder]

Hey!
Just lurking about and had a question. How would you have a microcontroller output a voltage that corresponds to a measured value? Say your measure 20 (units) with your PIC, I want 20 millivolts or milliamps to output to a multimeter.I would like a range of zero to about 200.

Thanks

[Spockie-Tech]

Use a Pulse Width Modulation output powered by a interrupt-driven timer (or inbuilt PWM if the micro has it), then a low-pass filter (a resistor and a capacitor) to convert the variable width pulse into a variable DC voltage.

Sounds complicated, but its 2 components and some code. :)

If you need to drive any sort of load with the voltage, you will need an op-amp/buffer to prevent ripple

Exact values of the R/C circuit will depend on your PWM frequency.

For more details, google for "pwm to voltage filter". heres one example page
http://www.ontrak.net/pwm.htm

[xorcise]

You can use an 8-pin 12-bit DAC like the MCP4921. It has a very simple SPI interface and works a charm. Set it up with a 2.5V reference and you can get voltage divisions at 610uV (0.61 mV) each. I'm using the MCP4922 12-bit Dual-DAC in an application and have code written to drive it here (it has buffers and more built-in features):
MCP4922 12-BIT DUAL DAC DRIVER & TEST DEMO (http://www.circuit-ed.com/forums/viewforum.php?f=8)

[S. Hackwelder]

Hey;
Thanks for the suggestions guys. I am working on the pwm code now. I hope I can get that to work.

Thanks

[Comatose]

PWM works okay, but some sort of serial interface is really the best way to go for moving data out of a PIC. Its very easy to get a PIC talking to hyperterminal.

[Spockie-Tech]

Assuming the goal is just to get the informaiton on a display and dont mind running a program to do so, then serial output is a good option.

If however, the application is a portable display (like a Multimeter) or some other tiny 7-seg display (there are lots of units that require a 20-200mv input), the PWM->voltage output is the go

[S. Hackwelder]

Ya,I intend for the multimeter to display the results of my measurement. I am building as device that will mount to my plane measure max. speed reached, and after I land I want to display the results. Ya, serial output would be good, but it is more overhead than I am willing to have(computer,rs232 chip,etc).


Thanks!

P.S. Yes , 200 is wishful thinking!