16550 max baud rate
Robert Scott
When driven with the standard 1.8432 MHz XTAL, the 16550
UART can be set to 115.2 kBaud only if the divisor is
loaded with 0001. But the National Semiconductor datasheet
says the valid divisor range is 2 to 65535.
Is 115.2 kBaud a valid configuration under these conditions?
Robert Scott
Ann Arbor, Michigan (rscott {at sign} wwnet {period} net is my real email address.)
(My automatic return address is intentionally invalid to foil spammers.)
David Emrich
Robert Scott <see_...@dk37cv.com> wrote in message
news:38aa24e1...@news.wwnet.net...
>
> When driven with the standard 1.8432 MHz XTAL, the 16550
> UART can be set to 115.2 kBaud only if the divisor is
> loaded with 0001. But the National Semiconductor datasheet
> says the valid divisor range is 2 to 65535.
>
> Is 115.2 kBaud a valid configuration under these conditions?
>
Works for me! (I didn't check the data sheet that closely, just programmed
the divide to 1 and off it went!).
David.
Michael Akers
I used to work for National and I just took a look at the datasheet for the
16550. The datasheet says that a code of 0000 is disallowed, thus allowing
0001 to whatever. Were you looking at the app note perhaps? The app notes
can be several device revisions behind.
--
Michael Akers
M. Akers Enterprises
BASCOM US Sales and Support
----
"Trust the computer industry to shorten 'Year 2000' to Y2K ....
It was this kind of thinking that caused the problem in the first
place."
Robert Scott <see_...@dk37cv.com> wrote in message
news:38aa24e1...@news.wwnet.net...
>
> When driven with the standard 1.8432 MHz XTAL, the 16550
> UART can be set to 115.2 kBaud only if the divisor is
> loaded with 0001. But the National Semiconductor datasheet
> says the valid divisor range is 2 to 65535.
>
> Is 115.2 kBaud a valid configuration under these conditions?
>
>
>
Robert Scott
wrote:
>Hi Robert,
>I used to work for National and I just took a look at the datasheet for the
>16550. The datasheet says that a code of 0000 is disallowed, thus allowing
>0001 to whatever. Were you looking at the app note perhaps? The app notes
>can be several device revisions behind.
Just yesterday I downloaded the file PC16550D.pdf from the
National web site and in section 8.3 PROGRAMMABLE BAUD
GENERATOR it says:
"The UART contains a programmable Baud Generator that
is capable of taking any clock input from DC to 24 MHz
and dividing it by any divisior from 2 to 2^16-1."
In any case, enough real people have confirmed that a
divisor of 1 is OK so I'm just going to use it and
not worry about it any more.
m...@here.com
> Hi Robert,
> I used to work for National and I just took a look at the datasheet for the
> 16550. The datasheet says that a code of 0000 is disallowed,
I believe the 16550 will still work (no divide by zero error here)
though it will be set to some funny baud rate.
Marc Warden
new parts no longer work with that divisor of 1?
Sincerely,
MarcW.
Robert Scott wrote in message <38aa977...@news.wwnet.net>...
>On Wed, 16 Feb 2000 05:25:51 GMT, "Michael Akers" <mwa...@home.com>
>wrote:
>
>>Hi Robert,
>>I used to work for National and I just took a look at the datasheet for
the
>>16550. The datasheet says that a code of 0000 is disallowed, thus allowing
>>0001 to whatever. Were you looking at the app note perhaps? The app notes
>>can be several device revisions behind.
>
Mark Borgerson
Marc Warden wrote:
>
> Use it and what happens if NS revises the mask or changes fab labs and the
> new parts no longer work with that divisor of 1?
>
I suspect that it is the documents that are in error. AFAIK, 115200Baud has
always been the max baud rate available with the 1.832MHz clock---which
implies that a divisor of 1 has been valid for quite a while. Certainly,
Windows allows that rate. Can a 70Billion dollar company be wrong???
(come on---quit laughing!)
Mark Borgerson
Declan Hughes
>On Wed, 16 Feb 2000 05:25:51 GMT, "Michael Akers" <mwa...@home.com>
>wrote:
>
>>Hi Robert,
>>I used to work for National and I just took a look at the datasheet for the
>>16550. The datasheet says that a code of 0000 is disallowed, thus allowing
>>0001 to whatever. Were you looking at the app note perhaps? The app notes
>>can be several device revisions behind.
>
>Just yesterday I downloaded the file PC16550D.pdf from the
>National web site and in section 8.3 PROGRAMMABLE BAUD
>GENERATOR it says:
>
>"The UART contains a programmable Baud Generator that
>is capable of taking any clock input from DC to 24 MHz
>and dividing it by any divisior from 2 to 2^16-1."
>
>In any case, enough real people have confirmed that a
>divisor of 1 is OK so I'm just going to use it and
>not worry about it any more.
>
>Robert Scott
Sorry to flog an almost dead horse, but, the above quote is what is written
in the June 1995 datasheet for the PC16550D. The June 1995 datasheet for the
PC16552D (the dual UART version) reads slightly differently,
"The DUART contains two independently programmable Baud Generators. Each is
capable of taking a common clock input from DC to 24 MHz and dividing it by
any divisior from 1 to 2^16-1. The highest input clock frequency
reccommended with a divisor = 1 is 24 MHz."
In both datasheets the following line comes at the end of the quoted
sections (8.3 in each case),
"Using a divisor of zero is not reccommended" !
If a divisor of 1 were a problem in the PC16550D then one might expect that
it would have been emphasized here. Recently I called NS in Arlington/Texas to
ask some questions about the PC1552D (for instance, do both VDD pins need to
be powered and decoupled, and do both the VSS pins need to be grounded, and
the answer = yes in both cases) and I was able to talk with a design engineer
who knew all about these UARTs.
Regards,
Declan Hughes
dhu...@aero.tamu.edu