Internal vs. external osc for comms

[rmteo]

I have used the internal oscillator for Asynch Serial Comms at speeds from 300 to 115200 baud successfully. Some things to look out for. The newer PIC's have better tolerences (those released in the last 2-3 years or so). Operating temperature is the main consideration. Most of my apps have been indoor types at typical room temperatures. If there are wide temperature variations during operation, an external oscillator may be more appropriate. Here are some numbers from a PIC16F690.

[polymer52]

I've use the 12f629,675,683 and the 16f628a all at 2400 and all with just a plain twisted pair wire up to 25 feet with great results.
I just send data to serial data lcd's so I don't need high speeds. These are mounted outside and I've had the temp to range from 15 to 80 deg F.

[Bruce]

rmteo is spot-on.

I've used a ton of newer rev PICs' with internal osc specs @1% for serial communications, and never had a single glitch - as long as they were used in a controlled environment where the temparature remained failry constant.

Step outside this, where the temerature can change quickly, and you're just asking for trouble. In this case .. for sure go with a good quality external crystal, or even a quality resonator. Crystals beat resonators hands-down, and are a bit more expensive, but are worth the expense for peace of mind.

If you need the absolute best precision, then go with an external TTL oscillator. These beat all of the above, but they for sure will increase current demand on your power supply.

I have tested all-of-the-above in very harsh conditions here in Colorado, and I guarantee it does make a HUGE difference.

[mtripoli]

Thanks very much for the replies. This is exactly what I was looking for. I did look at the specs in the data sheet and saw the numbers; I really needed some feedback from those that have done it and what they've found. In this application the transmit/recieve units are always used indoors (its a "party atmosphere") so I'm comfortable with using the internal osc.

B'rgds,

Mike Tripoli