[ECOS] Re: Half Duplex RS485

wangcui iucgnaw@msn.com
Fri Jan 25 04:16:00 GMT 2008

Here is the patch, shows all modifications in my working folder.
Acutally, I did more than FIFO stuff, I also made it support 2 diffent 16x5x chips on my target(different register-offset-step), and something else.

BTW, I prefer to solve half-duplex problem on hardware level, which avoid lots of effort on software level.

> From: laurie.gellatly@netic.com
> To: iucgnaw@msn.com; grante@visi.com; ecos-discuss@sources.redhat.com
> Subject: RE: [ECOS] Re: Half Duplex RS485
> Date: Fri, 25 Jan 2008 12:59:54 +1100
> Hi Wang,
> I'd be interested to see what you had to modify to make the FIFO work.
> The eCos source does not know about the fractional divider.
> Did you enable fix that as well?
> ...Laurie:{)
> -----Original Message-----
> From: wangcui [mailto:iucgnaw@msn.com]
> Sent: Friday, 25 January 2008 12:33 PM
> To: Laurie Gellatly; Grant Edwards; ecos-discuss@sources.redhat.com
> Subject: RE: [ECOS] Re: Half Duplex RS485
> Just FYI, the LPC2XXX's UART0 is not fully functional. So when the 16x5x
> serial driver check FIFO(in serial_config_port()), it will fail, thus FIFO
> is disabled for UART0. But UART1 works fine.
> To resolve it, I have to modified 16x5x driver code, force enable FIFO for
> UART0 and UART1.
>> From: laurie.gellatly@netic.com
>> To: grante@visi.com; ecos-discuss@sources.redhat.com
>> Date: Thu, 24 Jan 2008 08:12:17 +1100
>> Subject: RE: [ECOS] Re: Half Duplex RS485
>> Wang/Grant, thanks for the replies.
>>>> My project has RS485 half duplex driven by UART0 of an
>>>> LPC2112. At present I've modified pc_serial_start_xmit and
>>>> pc_serial_stop_xmit to change a pin state so that the same
>>>> wires can be used for transmit and receive.
>>>> This does not see to work when the FIFO is enabled.
>>>Then you probably did it wrong. :)
>>>I imagine that RTS is shutting off too soon. The problem is
>>>that pc_serial_stop_xmit() is called when the driver has no
>>>more data to send _to_ the UART. That's not when you need to
>>>shut off RTS. You need to shut off RTS when the UART is done
>>>sending data and both the FIFO and shift register are empty.
>>>You're probably shutting off RTS while the UART still has data
>>>in the tx FIFO and the tx shift register.
>> Funny thing is that it appears that the first part of the
>> transmission is lost.
>>>> Has anyone else done RS485 half duplex?
>>>Many, many times.
>>>> Did you modify these routines or write your own?
>>>I usually pick a UART that supports half-duplex operation, and
>>>then just enabled that feature in the UART. [I use a custom
>>>eCos serial driver that supports quite a few more advanced UART
>>>features than the standard driver (e.g. flow control,
>>>half-duplex, inter-byte timeouts, 9-bit modes, FIFO control,
>>>If you don't have a proper UART, you need to enable the tx
>>>shift register empty interrupt and use that to trigger code
>>>that de-asserts RTS. If you're using a broken UART that
>>>doesn't have a tx shift register empty interrupt, then you'll
>>>have to poll for the tx shift register empty status. If you're
>>>using a really broken UART that doesn't have a _working_
>>>shift-register empty status[1], then you may have to start a
>>>timer that will wake you up at the point in time where RTS
>>>needs to be changed.
>>>> Did you get the FIFO to work?
>>>When there was one, yes.
>>>[1] There are broken UARTs (including a few PC chipsets) whose
>>> shift-register empty bit gets set _before_ the stop bit has
>>> been sent. In that case, you may need to use some sort of
>>> time-delay to wait until after the stop bit has been sent
>>> to toggle RTS. On a properly implimented RS-485 bus, there
>>> should be pull-up and pull-down resisters so that the bus
>>> idles in the mark state (same value as a stop bit), but to
>>> be on the safe side you should leave the bus driver on
>>> until after the stop bit has been sent.
>> Grant, do you know if the LPC series have 'broken' UARTS?
>> Specifically LPC2212 and LPC2103?
>> U0TSR looks promising as an indication of when the bits
>> 'have left the building' - is it accurate or do I
>> need to add a time delay?
>> Really appreciate the help. ...Laurie:{)
>> --
>> Before posting, please read the FAQ: http://ecos.sourceware.org/fom/ecos
>> and search the list archive: http://ecos.sourceware.org/ml/ecos-discuss
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