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Problems with the 16F628 USART

 
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Posts: 23

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Problems with the 16F628 USART
PostPosted: Tue Feb 21, 2006 1:15 pm     Reply with quote
Hi all,

I've set up the USART of my program with a 4MHz internal OSC to run 9600 bps which is supported according to the PIC data sheet. However, the data sent is being corrupted. I use TeraTerm on my box to read the data. I get the first four or five characters correct, the rest is then just garbage. Can anyone tell me what I'm missing?

Much appreciated. Here is the <short> code, the same code I've posted in another query, but this is a different question so I thought it deserved another thread.

thanks,
DLC

Code:



#include <16F628.h>
#fuses INTRC_IO,NOWDT,NOPROTECT,NOLVP,NOCPD,NOBROWNOUT,MCLR
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_B2, rcv=PIN_B1)
#use fast_io(A)
#use fast_io(B)

#define MAXBIT 13         //5 address, 7 control and one junk timing
int rise[MAXBIT];
int rW=0;
int gotOne = 0;

//save_w and save_status are at 7f and 7e which are available in ALL banks
int save_w;
#locate save_w=0x7f
int save_status;
#locate save_status=0x7e
#byte status = 3
#byte TMR1L = 0x0E
#byte TMR1H = 0x0F
#byte PIR1 = 0x0C
#byte CCPR1H = 0x16

#int_global
void isr(void)
{
   #asm
      //store current state of processor
      MOVWF save_w
      SWAPF status,W
      MOVWF save_status
      BCF   status,5                              //Set to page 0 for SFR's
      BCF   status,6
   #endasm
   
      rise[rW++] = CCPR1H;                        //save the pulse time
      if (rW == MAXBIT)
      {
         rW = 0;
   }
   gotOne = 1;
      TMR1L = 0;
      TMR1H = 0;
   
      #asm
      // restore processor and return from interrupt
      SWAPF save_status,W
      MOVWF status
      SWAPF save_w,F
      SWAPF save_w,W
      BCF PIR1,2
      #endasm 
}                               

void main()
{
   int x;
   int n=0;
   
   set_tris_B(0x03);
   set_tris_A(0x00);

   //Turn off comparator
   setup_comparator(NC_NC_NC_NC);

      setup_ccp1(CCP_CAPTURE_FE);    // Configure CCP1 to capture falling edge detection
      setup_timer_1(T1_INTERNAL);    // Start timer 1

   for (x=0;x<MAXBIT;x++)
      rise[x]=0;
      
      enable_interrupts(INT_CCP1);   // Setup interrupt on falling edge
      enable_interrupts(GLOBAL);

   printf("SONY IR decoder\n\r");
   
      while(TRUE)
   {
         delay_ms(1000);
         if (n)
         {
            output_low(PIN_A1);
            n=0;
       }
       else
       {
          output_high(PIN_A1);
          n=1;
      }
         if (gotOne)
         {
            for (x=0;x<MAXBIT;x++)
               printf("\n\r%u %u counts ",x, rise[x]);
         gotOne = 0;
      }
      }
}

_________________
--
------------------------------------
Dennis Clark [email protected]
http://www.techtoystoday.com
------------------------------------
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Tue Feb 21, 2006 1:35 pm     Reply with quote
According to the 16F628 data sheet, in Table 17-4, Parameter 4,
it says the nominal 4 MHz internal RC oscillator can vary from
3.65 to 4.28 MHz. That's at least 7 or 8% off. That's beyond
the 3% maximum allowed. To be safe, you should limit the
error to 1%.
Guest
PostPosted: Tue Feb 21, 2006 3:02 pm     Reply with quote
PCM programmer wrote:
According to the 16F628 data sheet, in Table 17-4, Parameter 4,
it says the nominal 4 MHz internal RC oscillator can vary from
3.65 to 4.28 MHz. That's at least 7 or 8% off. That's beyond
the 3% maximum allowed. To be safe, you should limit the
error to 1%.


I noted that, but I've never had a PIC be THAT far off that it corrupted the serial data stream. However, I tend to write my own bit-bang code so perhaps my code is more forgiving than the USART is.

thanks,
DLC
[email protected]



Joined: 05 Nov 2003
Posts: 23

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PostPosted: Fri Feb 24, 2006 10:14 am     Reply with quote
PCM programmer wrote:
According to the 16F628 data sheet, in Table 17-4, Parameter 4,
it says the nominal 4 MHz internal RC oscillator can vary from
3.65 to 4.28 MHz. That's at least 7 or 8% off. That's beyond
the 3% maximum allowed. To be safe, you should limit the
error to 1%.


You nailed that one. I assumed that the F628 had a calibration byte like the 12F and 12C devices to tweak it in. I find no mention of a calibration byte nor a calibration register in the F628. A 10MHz ceramic resonator later and my serial ops work fine.

thanks,
DLC
_________________
--
------------------------------------
Dennis Clark [email protected]
http://www.techtoystoday.com
------------------------------------
yes
Guest
PostPosted: Fri Feb 24, 2006 2:47 pm     Reply with quote
I had same problem with 4 Mhz....i didnt tested at 10MHz...
I had changed registers value to control tha operation, but nothing good..

628A does not that problem...INT_RC works very well...

Bye
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