Cheap Processors with Fast USARTs for RS485?

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I want to implement low cost rs485-connected data collection nodes, and need
to choose a microcontroller for them that will support baud rates of more
than say 400 kbps, preferably up to 1 mbps.  I know there are some
PIC18Fxxxx's that have an "Advanced UART" that are claimed do this.  Other
alternatives?

- Chris



Re: Cheap Processors with Fast USARTs for RS485?
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  Almost any uC with a HW uart will do those Baud rates, so you
should select more by pin count, and Analog performance.
  GoalAsic do a C51 variant with RS485 HW inbuilt, but most
solutions have an external RS485 driver.

  For small uC with good analog resources, see the Silicon Labs
series - many have 2% Oscillators, so you can avoid Xtals.

  The Philips LPC family have medium analog performances,
and are available in  8/10/14/16/20/28/32 pins.

-jg


Re: Cheap Processors with Fast USARTs for RS485?
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Having the terminals handle the rates is not the major problem.
However getting a clean signal from point to point is another
matter.  You should first do some experiments with the actual
wiring and line terminations etc, put some signals on and see how
they appear at various points.  Haul out the scopes.  Watch out for
how the lines get turned around.

The protocols you use will also make a major difference.

--
Chuck F ( snipped-for-privacy@yahoo.com) ( snipped-for-privacy@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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Re: Cheap Processors with Fast USARTs for RS485?
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Thanks for the information, to all who replied.  What characteristics of
the protocol affect the rate?  What are examples of fast ones?  Ones
that are simple to parse?

I imagine a usart with a large buffer would also help, especially for
short bursty messages?

- Chris

Re: Cheap Processors with Fast USARTs for RS485?

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If you can limit the speed below 250 kbit/s, the slew-rate limited
RS-485 transceivers could be used, with a reduced risk of all kinds of
EMC problems.

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Profibus-DP is essentially asynchronous characters over RS-485, thus
following those wiring rules should give good results on any RS-485
bus. Profibus-DP is not very picky about the hardware up to about 1.5
Mbit/s, but running it at 12 Mbit/s @ 100 m requires quite a lot
attention to the details (minimal intermediate node stub length,
series inductors in each node connector to compensate for the stub
stray capacitance etc.).

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The critical parameter is the transmitter turn off. Preferably, this
should be handled directly by the UART hardware, if not, an interrupt
should at least be available, when the last character has _actually_
been transmitted. It should be noted, that on 16550 UART clones, an
interrupt is available only when the last byte has been inserted into
the Tx-shift register and you have to constantly poll for a separate
status bit to check when the last stop bit of the last byte has
actually been sent, before turning off the transmitter.

Depending of the kind of application, constant polling at the last
character may or may not be an option. Thus, it is very important to
check the behaviour of the internal UART before selecting the micro
controller.

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In any high speed half-duplex system, the Tx/Rx and Rx/Tx turnaround
delays are critical for any decent throughput. In a multidrop network,
each node listens to all the traffic, but is only interested in
messages addressed to it, thus, some processing power is lost
monitoring the line in order to skip uninteresting data, so that the
next addressing information can be detected. Thus, the protocol should
be designed in such a way that nodes that are not addressed, can skip
whole messages without too much processing.

Paul
 

Re: Cheap Processors with Fast USARTs for RS485?

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need


The AT91SAM7S32 Series has dual UART with some nice features for RS-485.
* DMA support for transmit and receive
* 8 kB RAM for buffers
* 32 kB Flash
* 40-50 MIPS @3.3V.
* RS-485 Enable single.
   You do not have to get an interrupt to enable/disable the RS-485
transceiver.
* Receiver idle interrupt, when a package is ready.
* It should be possible to run the parts at such a speed that 1 ;bit is
possible.
* Free GCC compiler (and IAR C compiler up to 32 kB)

Production parts have entered the fab, but will only be available in a few
months
- Takes 3-4 months normally,


--
Best Regards,
Ulf Samuelsson   ulf@a-t-m-e-l.com
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