Size of UART buffer

Andrey Savinkov

Hi !

My application on c6678 reads data from UART using this code:

int getMsgFromUART(uint8_t *buff)
{
   uint8_t ch = 0x00;
   if (UartReadData(&ch, 1000000) == 0)
   {
       *buff = ch;
       return 0;
   }

   return 1;
}

where UartReadData function is:

UART_STATUS UartReadData(uint8_t *buf, uint32_t delay)
{
   uint32_t delayCount = delay;

   while( (UartIsDataReady()) != 1)
   {
        if (--delayCount > 0) asm (" nop ");
        else
        {
           *buf = CSL_FEXT(hUartRegs->RBR, UART_RBR_DATA);
           return ( (UART_STATUS) UART_EFAIL);
        }
   }

   *buf = CSL_FEXT(hUartRegs->RBR, UART_RBR_DATA);

   return ( (UART_STATUS) UART_EOK);
}

Here UartIsDataReady is:

int UartIsDataReady(void)
{
   int DR_val = 0;

   if (CSL_UART_LSR_DR_READY == (CSL_FEXT(hUartRegs->LSR, UART_LSR_DR))) {
        DR_val = 1;
    }

   return (DR_val);
}
But I found that I cannot read sequencies longer than 20 symbols. Probably the UART buffer size is limited by 20 symbols. How to increase the UART buffer ?

over 13 years ago

0 Fawad Nackvi62335 over 13 years ago

Expert 1080 points

Andrey,

Most UARTs have 16 byte hardware FIFOs. They can only accept 16 bytes before they have to be read or they will overflow. So you have to set up a software buffer and read the UART before it overflows.

Fawad

0 Tom Kopriva over 13 years ago

TI__Mastermind 20480 points

Hi Andrey,

Assuming your using your read function in a task, you might also be starving it with a higher priority task and/or Hwi/Swi. This could disrupt your --delayCount

0 Renjith Thomas over 13 years ago

Guru 31670 points

Andrey,

Your whole logic is wrong.

Andrey Savinkov said:
while( (UartIsDataReady()) != 1)

Will never be true in the beginning and once it becomes empty it will go inside the while loop and try to read garbage. And since outside the while loop, you are again reading the data, you are getting the first 20bytes properly.

This whole logic can be implemented in much easier way.

0 Andrey Savinkov over 13 years ago in reply to Renjith Thomas

Intellectual 375 points

Thomas, thank you for answer !

I have tried to correct the logic, but was not successful. Could you show the much easier way which you mentioned in your last post ?

0 Renjith Thomas over 13 years ago in reply to Andrey Savinkov

Guru 31670 points

Andrey,

Refer this UART driver implementation from x-loader for omap-3. This is a very simple implementation.

http://gitorious.org/x-load-omap3/mainline/blobs/master/drivers/ns16550.c

0 Andrey Savinkov over 13 years ago in reply to Renjith Thomas

Intellectual 375 points

Thanks a lot !

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Size of UART buffer