GPS REPEATER/SYSTEMS MONITOR using PIC16F876

Introduction

This circuit is a repeater for GPS data. It is intended to connect to Garmin-38 (and similar) hand held GPS receivers that don't have external antennas. The purpose for the repeater is to allow the GPS to be outside, exposed to the satellite view, and allow the user to see the data below decks at the navigation station.

Versions

Within the code there is opportunity to easily configure for a 16x4 display or a 20x4 display, as well as configure it for crystal speeds from 4 to 20MHz, and for nautical, metric, or statute speed display. Although the description below is for the 16x4 version, the 20x4 is quite similar.

Herb Young in Australia built a very nice boxed version of the repeater.

Features

The repeater displays several pages of data pertaining to both the GPS and the boat electrical system. A DB-9 jack is on the front panel for the purposes of connection of a laptop to upload and download GPS waypoint data, etc. This code is a good example of how to process serial data using the PIC16F87x hardware UART and the serial interrupt.

Specifications

• Power: 10-16VDC at ~150mA including GPS
• Analog: three inputs, 14.99VDC maximum displayed value
• Data: NMEA-0183 2.0 format (4800 baud, TTL level, RMB & RMC data sentences)

Operation

Apply power to the repeater. The following screen is displayed

• The three buttons cycle through the three pages, which are:
  1. Position (Latitude - Longitude - Speed - Heading)
  2. Waypoint (Waypoint name - Steering - Distance - Bearing)
  3. Battery voltages
• Pressing and holding the Position button for between one and three seconds toggles the background illumination on and off.
• Pressing and holding the Position button for four or more seconds resets the microprocessor (like power-on-reset.)
• Pressing and holding the Position button during power-up will display the Setup Page, described below.

Code:

// Change the following to change the clock frequency
#define CRYSTAL_FREQ    16000000
// Change the following to change between 16 or 20 column display
#define DISPLAY_COLS    20
// Speed units are "1" (nautical knots), "2" (metric kph), or "3" (statute mph)
#define SPEED_UNITS     1

/****************************************************************************
GPS18.c

This program receives NMEA-0183 data from a GPS and displays it.
Meant for large display version still in 16F876.
Three buttons
Automicaly resets if main loop stops (not the best solution, still don't know why it's stopping)

Next: don't display GPS screens unless GPS is active
      detect display needing reset
      preset data eeprom for first-time operation
      don't display init stuff if reseting from main loop

                 +5                       +5+5
                  |                        | |
                  20                      15 2
                ----------             ----------
~SerIn -----18-|          |-24-----11-|DB4 A Vdd |
               |          |-25-----12-|DB5       |
  ADC0 ------2-|          |-26-----13-|DB6       |
  ADC1 ------3-|  16F876  |-27-----14-|DB7     Vo| 3--
  ADC2 ------5-|          |           |    LCD   |    |
               |          |-14------6-|EN        |    |
       XTAL--9-|          |-15------4-|R/S       |    |
       XTAL-10-|          |-28-FET-16-|K         |    |
               |          |           |  RW  Vss |    |
 BUTTON 1---21-|          |            ----------     |
 BUTTON 2---22-|          |              1   5        |
 BUTTON 3---23-|          |              |   |        |
               |          |             Gnd Gnd       |
               |          |                           |
               |          |-11----------R/C-----------
               |          |
               |          |
                ----------
                  8    19
                  |     |
                 Gnd   Gnd

***************************************************************************/
#case
#include < 16F876.h >
#include < jonsinc.h >
#device = *=16 ADC=10                /* allow RAM addresses over 255 */

#if ( ( CRYSTAL_FREQ < 4000000) || ( CRYSTAL_FREQ > 20000000 ) )
#error "CRYSTAL FREQ" not defined to between 8000000 and 20000000
#endif
#if ( ( DISPLAY_COLS != 16 ) && ( DISPLAY_COLS != 20 ) )
#error "DISPLAY COLS" not defined to 16 or 20
#endif

// RMC_TIME = 1 per clock megahertz, rounded
#define RMC_TIME        CRYSTAL_FREQ/1000000

#define LCD_D0          PIN_B3
#define LCD_D1          PIN_B4
#define LCD_D2          PIN_B5
#define LCD_D3          PIN_B6
#define LCD_EN          PIN_C3
#define LCD_RS          PIN_C4
#define RX_IN           PIN_C7
#define BUTTON_1        PIN_B0
#define BUTTON_2        PIN_B1
#define BUTTON_3        PIN_B2
#define LCD_BACKLITE    PIN_B7
#define LINE_1          0x00
#define LINE_2          0x40
#if DISPLAY_COLS == 20
#define LINE_3          0x14
#define LINE_4          0x54
#endif
#if DISPLAY_COLS == 16
#define LINE_3          0x10
#define LINE_4          0x50
#endif
#define CLEAR_DISP      0x01
#define EOF             0x00
#define COMMA           ','
#define CR              13
#define SPACE           ' '
#define PERIOD          '.'
#define DEGREE          0xdf
#define DOLLAR          '$'
#define NULL            0
#define GPRMC_CODE      75
#define GPRMB_CODE      74
#define RX_BUFFER_SIZE  70
#define POSITION_SCREEN 1
#define WAYPOINT_SCREEN 2
#define BATTERY_SCREEN  3
#define HIDDEN_RMC      5
#define WARNING_MSG     0
#define NODATA_MSG      1
#define ACTIVITY_SYMBOL 0xFF
#define MAX_VOLTS       15
#define EEPROM_CONTRAST 0
#define EEPROM_INITIAL  1

/* Set the following define to "YES" to display XOR'ed GPS sentence code */
/* such as GPRMC and the display will read out the value of 74. */
#define GET_GPS_CODE    NO

#separate void Display ( void );
#separate void LCD_Init ( void );
#separate void LCD_SetPosition ( unsigned int cX );
#separate void LCD_PutChar ( unsigned int cX );
#separate void LCD_PutCmd ( unsigned int cX );
#separate void LCD_PulseEnable ( void );
#separate void LCD_SetData ( unsigned int cX );
#separate void SkipField ( char cCnt );
#separate char GetField ( void );
#separate void InitRxBuffer ( char cCode );
#separate char GetRxChar ( void );
#separate void DisplayLatLon ( void );
#separate void DisplayWaypoint ( void );
#separate void DisplayLatitude ( char cLine );
#separate void DisplayLongitude ( char cLine );
#separate void DisplayHeading ( char cLine );
#separate void DisplaySpeed ( char cLine );
#separate void DisplaySteer ( char cLine, char cX );
#separate void DisplayWaypointName ( char cLine, char cX );
#separate void DisplayDistance ( char cLine, char cX );
#separate void DisplayBearing ( char cLine, char cX );
#separate void GetUtcAndMagVar ( void );
#separate long TrueToMag ( long iH );
#separate long FieldFiveToLong ( void );
#separate void DisplayAnalog ( void );
#separate void DisplayScaledVoltage ( long iV, char cScale );
#separate void DisplayArrival ( char cLine );
#separate void DisplayMessage ( char cMsgNum );
#separate void DisplayTemplateLatLon ( void );
#separate void DisplayTemplateWaypoint ( void );
#separate void DisplayTemplateAnalog ( void );
#separate void Delay5mS ( char cCnt );

#fuses HS, NOPROTECT, PUT, NOWDT, BROWNOUT, NOLVP, NOCPD
#use standard_io ( A )
#use standard_io ( B )
#use standard_io ( C )
#use delay ( clock = CRYSTAL_FREQ )
#use rs232 ( baud=4800, xmit=PIN_C6, rcv=PIN_C7, ERRORS )    // XMIT must be assigned to enable hardward USART
#priority RDA, RTCC, EXT

static char cC [ 10 ];      // local buffer
static char cTimeOut;
static char cRxBuffer [ RX_BUFFER_SIZE ];    // Fifo
static char cRxByteCnt;         // Number of bytes in the recv fifo
static char *cRxBufferWritePtr;    // Pointers for the Rx buffer
static char *cRxBufferReadPtr;
static char cRxIsrState, cRxMsgTypeReceived, cRxMsgTypeDesired;
static char cRxMsgReady, cReceiveFlag;
static long iVar, iLastRange, iTimeOut;
static char cVarDir, cScreenChanged, cAdcDone;
static char cButtonPressed, cSkip, cButtonCount;
static char cScreen, cSavedScreen, cRmcTimer1, cRmcTimer2;
static char cToFrom [ 5 ], cIndicator, cIllumination, cRxErrorFlag;
static char cDone, cContrast;

/*******************************************************************/
#int_ad
void AdcInterrupt ( void )
    {
    /* Gets here when ADC is done conversion, sets flag */
    cAdcDone = YES;
    }

#int_timer1
void Timer1Interrupt ( void )
    {
    /* Periodic RMC data timer, gets here every 204mS */
    /* This routine forces RMC to run every 10 minutes to update */
    /* magnetic variation */
    if ( cRmcTimer1-- == 0 )
        {
        cRmcTimer1 = 255;               // 52 seconds @ 10.240MHz
        if ( cRmcTimer2-- == 0 )
            {
            cRmcTimer2 = RMC_TIME;      // triggers every 10 minutes
            cSavedScreen = cScreen;     // save current screen type
            cScreen = HIDDEN_RMC;       // force RMC to run
            }
        }
    }

#int_rtcc
void Timer0Interrupt ( void )
    {
    // Gets here every 16.4mS at 8MHz, 8.2mS at 16MHz
    // Handles data timeout and switch debounce.

    // DATA TIMEOUT TIMER
    if ( cTimeOut != 0 )
        {
        cTimeOut--;
        }

    // This timer is preset by the normal operating loop, unless the operating
    // loop stops looping, at which point iTimeOut finally decrements to zero
    // and resets CPU.
    if ( iTimeOut != 0 )
        {
        iTimeOut--;
        }
    else
        {
        reset_cpu();                // force reset
        }

    if ( input ( BUTTON_2 ) == LOW )  // if button still pressed
        {
        cScreen = WAYPOINT_SCREEN;
        cSkip = YES;                // skip out of anything in process
        cScreenChanged = YES;       // repaint complete screen
        }

    if ( input ( BUTTON_3 ) == LOW )  // if button still pressed
        {
        cScreen = BATTERY_SCREEN;
        cSkip = YES;                // skip out of anything in process
        cScreenChanged = YES;       // repaint complete screen
        }

    // SWITCH DEBOUNCE
    if ( input ( BUTTON_1 ) == LOW )  // if button still pressed
        {
        if ( cButtonCount < 255 )   // hold at 255
            {
            cButtonCount++;         // otherwise increment
            }
        }
    else            // if button is unpressed
        {
        if ( cButtonCount > 2 )     // filter out glitches
            {
            //If button press is greater than 3.3 seconds, cold reset
            if ( cButtonCount == 255 )
                {
                reset_cpu();
                }
            if ( ( cButtonCount > 57 ) && ( cButtonCount < 255 ) )
                {
                if ( cScreen != HIDDEN_RMC )       // if not in the middle of getting magnetic variation
                    {
                    // cIllumination ^= ON;
                    output_bit ( LCD_BACKLITE, cIllumination ^= ON );
                    }
                }
            // If button press is less than 0.5 second
            if ( cButtonCount <= 57 )
                {
                if ( cScreen != HIDDEN_RMC )       // if not in the middle of getting magnetic variation
                    {
                    //if ( cScreen++ >= BATTERY_SCREEN )   // increment to next screen
                        {
                        cScreen = POSITION_SCREEN;                    // wrap
                        }
                    cSkip = YES;                // skip out of anything in process
                    cScreenChanged = YES;       // repaint complete screen
                    }
                }
            }
        cButtonCount = 0;       // restart
        }
    }

#int_rda
void SerialInterrupt ( void )
    {
    /*
    Reads incoming data from the USART and puts in in a rolling buffer
    ( but in this application, it should never roll.)
    If the buffer is full, this routine just discards the received byte.
    Not checking the LRC byte at the end of the NMEA-0183 sentence.
    */
    char cChar;

    if ( rs232_errors & 0x04 )  // get framing error bit from Rx status reg
        {
        cRxErrorFlag = ON;
        }
    cChar = getchar();       // get char from UART, clear any errors

    if ( cRxByteCnt == RX_BUFFER_SIZE ) // is recv fifo full ???
        {
        goto done;
        }
    switch ( cRxIsrState )
        {
        case 0:
            {
            if ( cChar == DOLLAR )  // if start of NMEA0183 message
                {
                cRxByteCnt = 0;     // reset byte count
                cReceiveFlag = OFF;     // default to off
                cRxMsgTypeReceived = NULL;  // set hashed value to null
                cRxIsrState++;                 // next state
                }
            break;
            }
        case 1:                           // five type characters to obtain
        case 2:
        case 3:
        case 4:
        case 5:
            {
            cRxMsgTypeReceived ^= cChar;      // hash in msg type
            if ( cRxIsrState++ == 5 )        // if time to check message type
                {
                if ( cRxMsgTypeReceived == cRxMsgTypeDesired )  // if good
                    {
                    cReceiveFlag = YES;            // enable receiving
                    cRxBufferWritePtr = cRxBuffer;    // reset to beginning of buffer
                    }
                else                    // don't want this message
                    {
                    cRxIsrState = 0;    // reset to look for next msg
                    }
                }
            break;
            }
        case 6:
            {
            /* Case 6 skips the comma character following msg type */
            cRxIsrState++;
            break;
            }
        default:                          // remainder of characters
            {
            if ( cReceiveFlag == YES )        // if this message is wanted
                {
                *cRxBufferWritePtr = cChar;     // put char in fifo
                cRxBufferWritePtr++;            // increment pointer
                if ( cRxBufferWritePtr == ( cRxBuffer + RX_BUFFER_SIZE ) ) // pointer past end ?
                    {
                    cRxBufferWritePtr = cRxBuffer;      // set pointer to start of fifo
                    }
                cRxByteCnt++;              // Increment byte count
                if ( cChar == CR )
                    {
                    cRxMsgReady = YES;         // signal that message is ready
                    cReceiveFlag = NO;      // no more receive
                    }
                }
            }
        }
    done:;
    }

/*******************************************************************/

For more detail: GPS REPEATER/SYSTEMS MONITOR using PIC16F876

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Ibrar Ayyub

I am an experienced technical writer holding a Master's degree in computer science from BZU Multan, Pakistan University. With a background spanning various industries, particularly in home automation and engineering, I have honed my skills in crafting clear and concise content. Proficient in leveraging infographics and diagrams, I strive to simplify complex concepts for readers. My strength lies in thorough...