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3.3V RS232

 
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colin_turner99



Joined: 24 Nov 2005
Posts: 7

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3.3V RS232
PostPosted: Tue Nov 21, 2006 10:22 am     Reply with quote
Hi

I have a pic running a 5V and have a GPS module that has 3.3V RS232

The GPS is putting the data on B0 and my code is as follows


Code:
 if (kbhit(GPS)) {
            fprintf(HOST, "%c", fgetc(GPS));
         }


Code:

#use rs232(baud=9600,parity=N,rcv=PIN_B0,bits=8,stream=GPS)


Code:

port_b_pullups(false);
   setup_adc_ports(NO_ANALOGS|VSS_VDD);
   setup_adc(ADC_OFF);
   setup_spi(FALSE);
   setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
   setup_timer_1(T1_DISABLED);
   setup_timer_2(T2_DIV_BY_1,4,10);
   setup_comparator(NC_NC_NC_NC);
   setup_vref(FALSE);
   enable_interrupts(INT_RDA);
   //enable_interrupts(INT_TIMER1);
   //enable_interrupts(INT_TIMER2);
   enable_interrupts(GLOBAL);


All i get from the PIC is grabage

If i send Test text from the PIC all work ok

has anyone got experanice with 3.3v RS232?

Colin
Ttelmah
Guest
PostPosted: Tue Nov 21, 2006 11:14 am     Reply with quote
You don't give us much to work on.
What PIC?.
What GPS module?.
What is the RS232 statement for the 'HOST' comms?.
What compiler version?.

Best Wishes
ckielstra



Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands

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PostPosted: Tue Nov 21, 2006 6:40 pm     Reply with quote
.....and more missing info:
- Is there a 3.3V to 5V convertor between the GPS serial output and the PIC input?
- Why is INT_RDA enabled? Is it possible you are simultaneously receiving data?

Most of all these questions would have been answered had you given a small but _complete_ example program (including the fuse settings, etc.)
kender



Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley

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PostPosted: Tue Nov 21, 2006 7:50 pm     Reply with quote
ckielstra wrote:
- Is there a 3.3V to 5V convertor between the GPS serial output and the PIC input?

...And if you need a 3.3V to 5V level translator, you can, probably pick one here.
http://focus.ti.com/logic/docs/translationselection.tsp?templateId=5985&navigationId=11413
colin_turner99



Joined: 24 Nov 2005
Posts: 7

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PostPosted: Wed Nov 22, 2006 3:09 am     Reply with quote
Hi

The Pic i am using is a 16F88

Basically the pic sits inbetween the GPS module and another device and does some caluations on the GPS data before sending it throught the HOST uart

I dont use a 3.3V to 5V converter

Here is my full code



Code:


#include <16F88.h>
#device adc=8
#use delay(clock=20000000)
#fuses NOWDT, HS, NOPUT, MCLR, BROWNOUT, NOLVP, NOCPD, NOWRT, NODEBUG, NOPROTECT, FCMEN, IESO
#use rs232(baud=9600,parity=N,xmit=PIN_B5,rcv=PIN_B2,bits=8,stream=HOST)
#use rs232(baud=9600,parity=N,rcv=PIN_B0,bits=8,stream=GPS)

#define BufferLength 25
#define length 39

char GPS_code[]  = "$GPRMC,";
int8 GPS_buffer[length];
int8 HostBuffer[BufferLength];
int8 HostRead=0;
int8 HostWrite=0;
int8 c=0;
int8 GPS_write=0;
int8 GPS_state   = 0;
int1 GPS_done    = FALSE;

#int_RDA
void RDA_isr()
{
      
   HostBuffer[HostWrite] = fgetc(HOST);
   
   if (HostWrite==BufferLength) {
      HostWrite++;
   } else {
      HostWrite = 0;
   }
   
}

#int_TIMER1
void TIMER1_isr()
{

}

#int_TIMER2
void TIMER2_isr()
{
}



void main()
{

   port_b_pullups(false);
   setup_adc_ports(NO_ANALOGS|VSS_VDD);
   setup_adc(ADC_OFF);
   setup_spi(FALSE);
   setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
   setup_timer_1(T1_DISABLED);
   setup_timer_2(T2_DIV_BY_1,4,10);
   setup_comparator(NC_NC_NC_NC);
   setup_vref(FALSE);
   enable_interrupts(INT_RDA);
   //enable_interrupts(INT_TIMER1);
   //enable_interrupts(INT_TIMER2);
   enable_interrupts(GLOBAL);
   
   while(1){
         //fprintf(HOST, "Test Test Test");
         
         
         if (kbhit(GPS)) {
            fprintf(HOST, "%c", fgetc(GPS));
         }
            
   }

}

Ttelmah
Guest
PostPosted: Wed Nov 22, 2006 3:57 am     Reply with quote
You still don't say which GPS module. This is critical, because whether or not you can use a direct connection without a buffer, depends on three things. First, the Voh of the 3.3v device. Second the Vih of the PIC. Third the signal polarity of the module.
Now the input your are using on the 16F88, has a Vih, just over 2v (2v, or 2.05v, according to which line of the data sheet you believe). Does your GPS module guarantee to give this as it's Voh into this load?. If not, then extra circuitry will be needed to handle this.
The code you have, assumes that the module is generating the normal 'inverted' RS232 (signal idles high), which is normally used when driving a RS232 buffer. Some modules designed for internal use, instead develop the opposite polarity. Is the data output line of the module sitting 'high', when no data is being sent?. If not, then you need to add 'INVERT' to the GPS RS232 declaration.
The logic in the 'host' interrupt handler is hosed, and will stop the receive from pulling the data here. However this should not affect your current test. Basically, 'HostWrite', is going to permanently stay as '0'. The code should be:
Code:


#int_RDA
void RDA_isr() {
   HostBuffer[HostWrite] = fgetc(HOST);
   //HostWrite must be incremented _before_ the test, or the buffer
   //will overflow by one character, and the counter only wants to reset
   //when the limit is reached.
   if (++HostWrite==BufferLength) {
      HostWrite=0;
      }
   }
}

Add 'errors' to the HOST RS232 declaration.
You should be aware, that if a receive interrupt occurs during the software reception of a character, this will garbage the data. Hence (given the input hardware buffer on HOST), use:
Code:

   while (TRUE) {
        if (kbhit(GPS)) {
            disable_interrupts(GLOBAL)
            //You must not output more than one character in the recieve
            //time, or overruns _will_ occur. Hence use putc
            fputc(fgetc(GPS),HOST);
            enable_interrupts(GLOBAL);
        }
   }


Best Wishes
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