Power PIC from RS232? PIC16F876

runtime · Joined: 15 Sep 2003 Posts: 36

Hello,

Does anyone know if it is possible to power a PIC from the serial port? what should I need? maybe someone has a circuit layout...

Thank you in advance
Peter

Eric Minbiole · Joined: 10 Jun 2004 Posts: 16 Location: USA

It is definitely possible to power a PIC from the serial port, but there are some limitations. RS232 doesn't provide dedicated power lines, but you can 'steal' power from the RTS and DTR lines. I did some work on this a few years ago, making a hardware "dongle" that was powered from the serial port. In my case, there was no other I/O (such as LEDs, etc) needed, so power drain could be kept low. Of course, your success will depend on what other (if any) I/O you need to power.

The amount of power available varies greatly from port to port, with laptops often having the least amout available. If you want to make something that will work on <I>most</I> serial ports, then it's not too difficult, as you can often get 8+mA @ 6+V: Plenty to drive a PIC, a regulator, etc.

If you want your device to work on <I>all</I> serial ports, then it's harder. I don't remeber the exact numbers, but I think I figured I could rely on having around 3 or 4 mA at a few Volts. To minimize consumption, I ran the PIC at 3.3V, and selected the lowest power periperals I could find: I used the LTC1385 to convert TTL <-> RS232 levels. It draws only 200uA. I also used a low power regulator, LT1121.

This page has some excellent tips: http://www.hut.fi/Misc/Electronics/circuits/rspower.html

Good luck!

Guest · Posted: Thu Jun 10, 2004 4:36 pm

Here's a link to a (blurry) picture of my serial port powered PIC. I was able to house everything neatly inside a small project box, with a built-in D9 connector. I just plug the box directly onto the computer's serial port:

http://www.pisymbol.com/images/dongle1.jpg

SteveS · Joined: 27 Oct 2003 Posts: 126

As mentioned it is very possible, and it depends on what you need it to do. My entry in the Microchip low power contest last year (honorable mention, dang!) was powered solely off RS232 and converted RS232 to TTY Baudot (and back). The winner (he got the $10000) had a mode where he ran off RS232 power. His design is published. I like the DS275 or DS276 for the RS232 interface. I diode-or-ed RTS and DTR and used a low power linear regulator. You can squeeze out a bit more if you use a high efficiency switcher. Run the PIC at 3.3V and use all the power saving features. I used the 18F1320 which has a neat feature where the CPU is off but peripherals still run. I used that to capture the TTY edges and wake from RS232.

Good luck,

SteveS