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From: "Matt Foster"
Subject: Re: RS232 newbie - in need of assistance!
X-Newsreader: Microsoft Outlook Express 6.00.2720.3000
Date: Fri, 3 Jan 2003 21:27:40 -0000
NNTP-Posting-Date: Fri, 03 Jan 2003 21:27:36 GMT
Organization: blueyonder (post doesn't reflect views of blueyonder)
Hi Michael, A E
Everyone's looking at those resistors in disgust - so I shall adapt them, I
don't want to rely on internal pullups just because I like soldering ;-)
I'm going to take a look at MAX233, a few people have suggested using this -
but the thing about my crystal and nice even baud rates just scared me so
I'll ignore it.
You're right in the sense I only need half-duplex (one way) communication -
from PIC to PC. Not the other way round - so I'll take out the relevant
I'll check back with a new schematic!
"Michael F. Coyle" wrote in message
> "Matt Foster" wrote in message
> > Ok
> > Here's my first attemot at a schematic - please ruthlessly rip it to
> > laugh at my incompetence and then give me a better one :-D
> > http://matthewfoster.myby.co.uk/lousyattempt.jpg
> Hi Matt,
> It's pretty good but I would suggest a few changes:
> 1. The switches should not be in series with the resistors -- when a
> is open, the input pin does not go down to zero volts. Instead it floats
> around between 0 and 5V because MOS inputs are high impedance.
> Instead, connect each input to a resistor to 5V and a switch to ground.
> When the switch is open the resistor pulls the input to 5V; when the
> is closed, the pin is grounded (and as a side effect, current flows
> the resistor to ground, which needn't concern us here).
> A further simplification is to eliminate the resistors altogether! (Just
> have switches to ground.) Port B on PICs have internal pullup resistors
> that can be turned on within the program. This saves you the five
> 2. If you havent bought the 16F84 yet, consider using the PIC16F628
> It's a newer part, has more memory, and has a built-in UART which makes
> programming this application a snap. You would have to do your serial
> transmission in software on the 84.
> If you go for the 628, you may have to use a different crystal frequency
> make the built-in baud rate generator to work correctly. Most standard
> rates are unrealizable with a 4 MHz oscillator. A better value to use is
> 3.6864 MHz. Then all standard baud rates from 300 to 230,400 are
> See the 628 data sheet for UART and baud rate generator details.
> 3. Check your MAX232 data sheet -- mine shows 1uF capacitors, not 10uF.
> think some older data sheets used 10uF.)
> 4. I haven't checked your serial port pinouts -- you're on your own there
> But you're definitely on the right track.
> Hope this helps. Good Luck!
> - Michael
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