Design idea - will it work?

picker · Joined: 07 Sep 2003 Posts: 19 Location: South Africa

Hello everyone,

I am in the early stages of a design I am interested in, which will have an "intelligent" device connected to a PIC18F8720's one serial port and the PIC connected to the PC via the other serial port. The intelligent device communicates at 115200 bps and my question is whether the PIC can efficiently handle this rate and whether I can use the same baud rate from the PC?

I have always got away with 9600bps, but this application needs a high throughput. From what I understand I can run the PIC from a 40Mhz crystal to allow me to get everything done. Will this increase in baud rate affect the cable length from the PC and are there any pitfalls I should look out for?

Regards
Picker

rrb011270. · Guest

TSchultz · Joined: 08 Sep 2003 Posts: 66 Location: Toronto, Canada

The idea sounds OK, I prefer to use an RS-485 interface instead of RS-232. The differential signal(s) are more reliable and less sensitive to noise is properly terminated and handled. I like the Maxim fail safe drivers and normally use the MAX3089.

I would comment that you carefully consider the PIC selection. There seems to be some problem issues with the PIC18Fxx20 series parts. There are numerous erattas and it does not look like the problems are resolved yet. There are some additional problem with the PIC18LFxx20 series parts escecially with the config bits and part erasure and programming.

This family of parts have a lot of growing pains, we are using them in a design because of the 128K FLASH, of which we are using 85%. If we had not already invested so much time and effort we would change to another processor altogether.

This is first PIC series I have had major problems with and I am not inpressed with Microchip's handling of things. They seem to not want to admit to silicon problems and try and blame everything else. Once they get things right these should be nice parts, but for now they are a real "pain" to use sometimes.

RKnapp · Joined: 23 Feb 2004 Posts: 51

I am a newbie to the use of these chips, but I think the 8720 can only run from a 20Mhz clock -- NOT a 40Mhz.

Do you have info about this? Could you pls point to it?

Thanks,

Smile

Robert

Neutone · Joined: 08 Sep 2003 Posts: 839 Location: Houston

mpfj · Joined: 09 Sep 2003 Posts: 95 Location: UK

I think to clarify it should be pointed out that the 20-40MHz clock range is only achievable using the 4xPLL mode (when, say, a 10MHZ clock is used externally, and some internal PLL logic turns it in 40MHz).

That's why the datasheet might say 20MHz only at the max freq.