Lpc2000

varuzhandanielyan wrote:

>I use UART1 to connect to RS485.  For it
>I need to enable RS485 transmitter before sending
>and again desible it when sending is over.
>Below is my code:
>void Transmit1(int ch)
>{
>  U32 enabled_interrupts;
>  // wait for transmission completed
>  while (!(U1LSR & TEMT));
>
>  enabled_interrupts = VICIntEnable;  //disable all interrupts
>  VICIntEnClr        = enabled_interrupts;
>
>  U1THR = ch;
>//while (IO0PIN & Bit(15));
>  T1EMR = Bit(2);		// ExtMatch2 = 1
>  T1MR2 = T1TC + 10*60000000/115200;
>  T1EMR |= Bit(8);      // ClearOnExtMatch2
>
>  VICIntEnable = enabled_interrupts;
>}
>
>Problem:
>the U1THR = ch; operator does not start sending
>immediatelly.  There is a random delay in range
>approximatelly from 0 to 1 bit duration.
>I made different experiments to reset the baudrate
>counter or other means but without result.  Only
>way I found to solve the problem was an extra pin
>(P0.15 in my case) usage to detect the real start
>of trasmittion, this is done by commented while.
>So one needs to use an extra pin and accept, that
>interruptions could be desible by up to one bit
>duration.
>Maybe somebody knows how to force the UART1 start
>to transmit the character immediatelly after writing
>to U1THR register.
>  
>
Ha!  Welcome to RS485.  It is a bit more complex than it looks.

I don't know why you believe that you needs such precise control over 
the relationship of TXD/RXD and the DIRection?  By it's very nature, 
RS485 communications over twisted pair require that the line be 
precharged before it can be used.

Think about it, the wire is floating electrically, any stray magnetic or 
electrical field can induce a blip into the "dead" wire.  By first 
turning the direction to Transmit, then hold that for a few TX char 
times, it allows the uart at the RX side to settle down.  The uart that 
the RX side may have heard a blip and is in the process of receiving a 
erroneous char.  Once you precharge the line, send your data, then HOLD 
the carrier (direction) until the last char has been sent and then flip 
the line back to RX mode.

Most of the systems I've seen / worked on have a "quiet" line floating 
as the units are all listening for some other unit to speak.

TomW



-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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