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'Changing baudrates'
2000\05\02@050123 by James

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Has anyone ever tried to transmit/receive at different
baudrates...basically changing baudrates between pieces of data?

I'm having problems and basically have narrowed it down to the receive
module on the PIC.  I'm using a 16c74a which has a double buffer in the
receive module.

Does anyone know what happens to the receive module when baud rates are
changed (i.e. changing the SPBRG).

Thanks.

-James

2000\05\02@110130 by Dan Michaels

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At 05:00 AM 5/2/00 -0400, you wrote:
>Has anyone ever tried to transmit/receive at different
>baudrates...basically changing baudrates between pieces of data?
>
>I'm having problems and basically have narrowed it down to the receive
>module on the PIC.  I'm using a 16c74a which has a double buffer in the
>receive module.
>
>Does anyone know what happens to the receive module when baud rates are
>changed (i.e. changing the SPBRG).
>
>Thanks.
>
>-James
>

You might take a look at the Mchp errata notes - receive errors
were common with the UART h.w. in older '74s, in certain operating
modes.

2000\05\02@113546 by Chris Eddy

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James, I have two thoughts;

1    I have uncovered some flaws in Microchip and other vendors silicon
where changing a register on the fly introduces some form of error.
(Especially ones that one would not think of changing often).  Not that you
will see this here, but certainly keep your eyes open for a flaw.
2    Recieving is a challenge, but sending is relatively easy.  Why not set
the reciever to work right, and simply bit bang the send?  If the baud rate
is not too high, you can in line it.  If it is real slow, you could even
have the timer int space it out.

Chris Eddy

James wrote:

> Has anyone ever tried to transmit/receive at different
> baudrates...basically changing baudrates between pieces of data?

2000\05\02@120740 by Martin SchŠfer

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Dan Michaels wrote:

> You might take a look at the Mchp errata notes - receive errors
> were common with the UART h.w. in older '74s, in certain operating
> modes.

To be more precise: The 74A definitely has the problem with SPBRG = 1. The
problem was solved with the 74B die.

martin

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>

2000\05\02@190416 by Dwayne Reid

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<x-flowed>At 05:00 AM 5/2/00 -0400, James wrote:
>Has anyone ever tried to transmit/receive at different
>baudrates...basically changing baudrates between pieces of data?
>
>I'm having problems and basically have narrowed it down to the receive
>module on the PIC.  I'm using a 16c74a which has a double buffer in the
>receive module.
>
>Does anyone know what happens to the receive module when baud rates are
>changed (i.e. changing the SPBRG).

Yes.  Maybe.  I'm not sure.

The PIC 16c74 and 16c74A have a silicon bug that causes receive errors when
the BRGH bit is HI (divide 16).  However, I do NOT know what happens if you
change just the SPBRG register but leave BRGH alone (and set LO).

I'm assuming that you are not attempting to receive data at the instant you
change baud rates.  What you may want to do is make sure that you have
pulled all the data from the receive fifo before you change baud
rates.  Then change the baud rate and flush the receive fifo in case you
did get garbage.  Note: just read the receive register 3 times to flush the
fifo.

dwayne



Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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