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'[PIC]: RS232 baud rates'
2001\02\13@202027 by Drew Vassallo

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Interestingly, I've found a number of serial routines that use 9600 baud
(nice 104.167us per bit) and 19200 (nice 52.08us per bit), but none for
38400 (even nicer 26.04us per bit).  I wonder why nobody uses this.

I changed my half-bit delay routines for my 19200 baud to 38400, and guess
what?  It works better than my 19200 baud transfer, as expected.

Anyone care to elaborate on the mystical 19200 barrier?  Of course, it's
really only practical for cases where you are sending/receiving
synchronously with the rest of your code.  Asynchronous at high speeds is,
for all practical reasons, impossible.

--Andrew
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2001\02\13@212739 by Dan Michaels

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Drew Vassallo wrote:
>Interestingly, I've found a number of serial routines that use 9600 baud
>(nice 104.167us per bit) and 19200 (nice 52.08us per bit), but none for
>38400 (even nicer 26.04us per bit).  I wonder why nobody uses this.
>
>I changed my half-bit delay routines for my 19200 baud to 38400, and guess
>what?  It works better than my 19200 baud transfer, as expected.
>
>Anyone care to elaborate on the mystical 19200 barrier?  Of course, it's
>really only practical for cases where you are sending/receiving
>synchronously with the rest of your code.  Asynchronous at high speeds is,
>for all practical reasons, impossible.
>

38400 is basically the only thing I use anymore. But then, I
pretty much always use 20mhz xtals.

People who use 4/8 mhz xtals are probably used to using 9600/19200
- that's my guess. Also, when using the PIC UART with lower freq
xtals, choosing 38400 may translate into SPBRG values that produce
large baudrate errors. Third guess is inertia.

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2001\02\13@214022 by Dale Botkin

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On Tue, 13 Feb 2001, Dan Michaels wrote:

{Quote hidden}

I typically use 9600 because it's pretty much the default rate for almost
everything - PC (well, Windoze) comm programs, Cisco routers &
switches, most other devices I use.  Saves me from having to change things
to talk to the PIC, and most of the data transfer I do is not terrily
demanding for speed anyway.

Dale
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2001\02\13@221020 by Bob Ammerman

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> Anyone care to elaborate on the mystical 19200 barrier?  Of course, it's
> really only practical for cases where you are sending/receiving
> synchronously with the rest of your code.  Asynchronous at high speeds is,
> for all practical reasons, impossible.

Never say never.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\13@221857 by Robert Shady
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My guess is that most people develop on 4mhz,
which means you need to check the serial every
25 cycles @ 38,400 baud and in most situations
that's not practical? :) Doesn't leave much time
for anything else.  At 20mhz this isn't an issue.

-- Rob

--- Drew Vassallo <spam_OUTsnurpleTakeThisOuTspamHOTMAIL.COM> wrote:
{Quote hidden}

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2001\02\13@222328 by Mitch Miller

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Why impossible?  I've implemented full asynch. serial send and receive at
38.4k using an SX (hope that's not a forbidden term on the PIC list <G>).

--Mitch

{Original Message removed}

2001\02\13@231403 by Drew Vassallo

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>Why impossible?  I've implemented full asynch. serial send and receive at
>38.4k using an SX (hope that's not a forbidden term on the PIC list <G>).

++sarcasm

Well, gee whiz, I'm using a 50+ MIPS chip and *somehow* managed to find the
cycle time to fit in the 38.4k async routines.

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2001\02\14@011108 by Bill Westfield

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Why impossible?  I've implemented full asynch. serial send and receive at
   38.4k using an SX (hope that's not a forbidden term on the PIC list <G>).

It's not forbidden, but it's also not a fair comparison, since you're
talking about a cpu that's 4 to 20 times faster...  There's a ram-based
microcodable engine inside a cd2481 that easilly handles 4 ports at
115200bps, plus dma to a host processor, but that wouldn't be a fair
comparison either.

I think a faster-than-19200, half-duplex, semi-synchronous async bit-banging
driver tends to need to come with so many warnings and caveats that no one
is particularly proud of having written one.  You'd think there would be
enough uses that there would be more of them, though...

BillW

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2001\02\14@025131 by PIC development

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I have a bit-banged 38400 Rx routine for a 12c509 @ 4 MHz.
see http://www.beowulf.demon.co.uk/serial-stepper.html

regards
Pete

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On Tue, 13 Feb 2001, Drew Vassallo wrote:

> Interestingly, I've found a number of serial routines that use 9600 baud
> (nice 104.167us per bit) and 19200 (nice 52.08us per bit), but none for
> 38400 (even nicer 26.04us per bit).  I wonder why nobody uses this.

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2001\02\14@034023 by Bill Westfield

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You know, bit-banged serial in microcontrollers might be a good excuse to
revive split-speed async communitcations.  1200bps TO the PIC, 38400 FROM
the PIC - there would be enough time to check for a received character IN
BETWEEN each transmitted character, since receive bits are longer than
transmitted bytes...

(split speed modems (1200/150, FSK, usually) were popular (in limitted
circles) back before fancier modulation schemes were implemented...)

BillW

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2001\02\14@104044 by Olin Lathrop

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> You know, bit-banged serial in microcontrollers might be a good excuse to
> revive split-speed async communitcations.  1200bps TO the PIC, 38400 FROM
> the PIC - there would be enough time to check for a received character IN
> BETWEEN each transmitted character, since receive bits are longer than
> transmitted bytes...

Yes, receiving is the hard part.  Not only do you have to be looking when
the start bit happens, but there is a 1 1/2 cycle uncertainty about the
start bit's actual time.  This is because of you don't know where the start
bit started within the 3 cycle loop.  Any error on the start bit time adds
directly to errors on the data bit times because everything is relative to
the leading edge of the start bit.

I once did a 9600 baud software UART on a PIC with a 150KHz oscillator.  I
guess that would scale up to 256Kbaud at 4MHz and 1.28Mbaud at 20MHz.  The
transmitter worked fine, but the receiver mangled or dropped about 1 in 10
bytes.  This was an output-mostly application so it was relatively easy to
work around the receiver error rate in the protocol.


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