Hardware flow control of UART

[wanli]

Hi folks,

I am trying to do some communication between two devices(two microcontroller) via UART Port. Since the speeds of both devices are different with each other (one is 70 MHz, the other one is 20MHz), there are some bytes lost when I send a string from faster device to slower device. and even worse, sometimes some wrong characters are received.

I think I must take a flow control mechanism to control the transport. I have tried some methods, using a IO pin, but it seems nothing helpful.

Did someone ever take such hardware flow control to make UART working stable? Please tell me your experience.

Thanks a lot.
wanli

[dyeatman]

What two processors are you using? Obviously at least one is NOT a PIC.

Basically the clock speed of the processors makes no difference, the baud rate is the only real consideration and they must match exactly for error free commuincations.

However, if processor clock speed is far enough away from the baud rate multiple, baud rate "error" could be your problem, especially if they are off by say 1.5% in opposite directions. This documented in the PIC datasheets in ths oscillator/UART sections.


For example I use an 18.432MHZ oscillator instead of 20MHz so baud rate multiple is exactly on and I have never encountered a problem with processor to processor communications even at 115K baud.

[wanli]

Thank you first!

I use a Java executable microcontroller (ajile 80 on JStamp) which working at 73.3MHz to
communicate with a Microchip MCU PIC18F6720, which is built in on a
board, and driven by a 20MHz Oscillator. I can't change the oscillator
because it is also built in on the board.

Before two weeks I posted an article in which I descriped my problem.

http://www.ccsinfo.com/forum/viewtopic.php?p=32051&highlight=#32051

Some friends (include you) replied me and give me some suggestion. I've tried all possibilities, but there are still problems.

My serial port code are as following:

#use RS232 (baud = 115200, XMIT = PIN_SERCON_TX, RCV = PIN_SERCON_RX, BITS = 8, PARITY = N, ERRORS)


char buffer[BUFFER_SIZE];
int next_in = 0;
int next_out = 0;

#define bkbhit (next_in!=next_out)

//interrupt routine
#int_RDA
RDA_isr()
{

int t;

buffer[next_in]=getc();
t=next_in;
next_in=(next_in+1) % BUFFER_SIZE;
if(next_in==next_out)
next_in=t; // Buffer full !!

}

//get one char from the receive buffer
char bgetc()
{
BYTE c;

while(!bkbhit) ;
c=buffer[next_out];
next_out=(next_out+1) % BUFFER_SIZE;
return(c);
}

//Main routine goes here
void main()
{

char value;

int i;
char buff;


//enable the serial port rx interrupt
enable_interrupts(INT_RDA);
enable_interrupts(global);

do
{

//here process the received data
while(bkbhit)
{
buff = bgetc();
printf( "%c", buff );
}

}
while(TRUE);
}

When I send a string from Java device to PIC, it is only right when I
insert enough delay (e.g. 20 ms) between each byte. Otherwise some bytes will be lost or even wrong.

I have tested, from PIC to the Java device, all transmit are all right. So I
think it is the problem of different speed.

Have you any idea?

I am appreciate for your reading such a long reply.
Wanli

[drh]

Here's a guess.
You have the watch dog timer enabled and it is reseting every 18ms.
_________________
David

[davidpk]

I had this exact same problem with an 18F452 running at 40MHz and my desktop PC. The communication from the PC to the PIC was only single characters but the PIC would respond with a lot of data (100's of bytes). I would get about half the data then it would turn into junk.

I was trying to run at 115.2K and the baud-rate error would accumulate through the whole transmission. When I changed the baud to 57.6K the problem cleared up.

It would show up again if I had longer strings of data. The best way to fix the problem is to use a baud-rate crystal.

[Ttelmah]

[Guest]

[wanli]

sorry, I forgot to login for above reply.

[drh]

That's why it was a guess. The post from Ttelmah is very good!
_________________
David