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LCDscope screen shot

How to build a simple and low-cost oscilloscope with a text LCD ?

Contents

LCDscope : a text LCD oscilloscope

A short video clip is sometimes better than a long explanation :


The idea of this circuit is use text LCD programmable characters to display a signal. A function generator was used to generate the signal we can see in the the videoclip.

Circuit Schematic

LCDscope-schematic.png

  • LCD1 is a 4 x 20 text LCD, but it works also with a classic 2x 16 text LCD (see source code) :

LCDscope-LCD-2x16.jpg

  • input signal is 0...5V
  • you can find the LCD at CircuitED

C Source code

/*
 *******************************************************************************
 * LCDscope : a PIC18 oscilloscope on a 20x4 text LCD
 * also works with a standard 16x4 text LCD
 *******************************************************************************
 *
 * This program shows how to use custom chars on a text LCD to build a tiny graphic screen
 * To illustrate this, here is a quick & dirty mini oscilloscope.
 *
 * Author : Bruno Gavand, February 2009
 * see more details on http://www.micro-examples.com/
 *
 * source code for mikro C compiler V8.2
 * feel free to use this code at your own risks
 *
 * target : PIC18 with 10 Mhz crystal, HS PLL
 *
 * PIC PIN Assignemnt :
 *
 * RA0 : analog input, 0-5V
 *
 * control buttons on PORTB (internal pull-ups, switchs to GND) :
 * RB0/RB1 : change horizontal frequency
 * RB2/RB3 : change input range
 * RB4 : hold/release screen
 *
 * 4-bit LCD on PORTD :
 * RD2 : RS
 * RD3 : E
 * RD4 : D4
 * RD5 : D5
 * RD6 : D6
 * RD7 : D7
 * Note : R/W pin of LCD must be tied to ground.
 *
 * Credits to CircuitED for the 20x4 black/yellow LCD
 * http://www.circuit-ed.com/
 * Thank you Warren !
 *
 *******************************************************************************
 */

/*********************
 * CONSTANTS
 *********************/

// if you have a 16x2 standard LCD, untag this line :
//#define LCD16x2
// otherwise, you are supposed to have the nice 20x4 LCD
#ifndef LCD16x2
#define LCD20x4
#endif

#define C_X             4               // number of columns in pseudo-graphic screen
#define C_Y             2               // number of rows in pseudo-graphic screen

#define miniGLCD_x      (5*C_X)         // number of X pixels in pseudo-graphic screen
#define miniGLCD_y      (8*C_Y)         // number of Y pixels in pseudo-graphic screen

#define LEVEL           16              // trigger sensitivity (number of ADC points)

// colors of pseudo-graphic screen
#define miniGLCD_COLOR_WHITE         0
#define miniGLCD_COLOR_BLACK         1
#define miniGLCD_COLOR_REVERSE       2

// pseudo-graphic function prototypes
void    miniGLCD_fill(unsigned char c) ;
void    miniGLCD_setPixel(char x, char y, unsigned char mode) ;
void    miniGLCD_line(char x0, char y0, char x1, char y1, unsigned char pcolor) ;
unsigned char miniGLCD_getPixel(char x, char y) ;

/***************
 * RAM VARIABLES
 ***************/

// horizontal frequency structure
struct TIMEBASE
        {
        unsigned char   t0con ;         // timer 0 configuration
        unsigned char   period[8] ;     // period value
        unsigned char   unit ;          // period unit
        } timeBase[] =
                {
                        {       0b10000100, "1.04857", ' ' },
                        {       0b10000011, "524.288", 'm' },
                        {       0b10000010, "262.144", 'm' },
                        {       0b10000001, "131.072", 'm' },
                        {       0b10000000, "65.536 ", 'm' },
                        {       0b11000111, "32.768 ", 'm' },
                        {       0b11000110, "16.384 ", 'm' },
                        {       0b11000101, "8.192  ", 'm' },
                        {       0b11000100, "4.096  ", 'm' },
                        {       0b11000011, "2.048  ", 'm' },
                        {       0b11000010, "1.024  ", 'm' },
                        {       0b11000001, "512    ", '\xe4' },
                        {       0b11000000, "256    ", '\xe4' },
                        {       0b11001000, "128    ", '\xe4' }
                } ;
unsigned char   tbase = 0 ;             // current timebase index

// vertical input range structure
struct INPUT
        {
        unsigned char   div ;           // power of 2 of input divider
        unsigned char   *ampl ;         // range value in volts
        } input[] =
                {
                        {       4, "2.500" },
                        {       2, "1.250" },
                        {       1, "0.625" },
                } ;
unsigned char   ipt = 0 ;               // current input range index
unsigned char   vdiv ;                  // current power of 2 of input divider

#ifdef LCD20x4
// scrolling message
unsigned char msg[][8] =
        {
        "       ",
        "       ",
        "       ",
        "       ",
        "  LCD  ",
        " SCOPE ",
        "  By   ",
        "BrunoG ",
        "*      ",
        "       ",
        "       ",
        "  see  ",
        " more  ",
        "details",
        "  and  ",
        "dwnload",
        "mikroC ",
        "source ",
        "code on",
        "       ",
        "www.   ",
        "micro- ",
        "example",
        "s.com  ",
        "*      ",
        "       ",
        "       ",
        "20x4LCD",
        "Black/ ",
        "Yellow ",
        "BkLight",
        "specs.&",
        "price :",
        "       ",
        "www.   ",
        "circuit",
        "-ed.com",
        "*      ",
        "       ",
        "       ",
        " Thank ",
        "  you  ",
        "Warren!",
        ""
        } ;
unsigned char   firstMsg = 0 ;          // scrolling message index
#endif

unsigned char   miniGLCD_screen[C_X * C_Y * 8] ;        // pseudo-screen bitmap

unsigned char   samples[miniGLCD_x] ;   // sample table
unsigned char   sIdx = 0 ;              // sample index

unsigned char   trigger = 0 ;           // trigger status
unsigned char   trigValue = 0 ;         // trigger value

unsigned char   hold = 0 ;              // hold screen flag

unsigned int    t0ctr ;                 // timer 0 overflow counter

/****************************
 * INTERRUPT ROUTINE
 ****************************/
void    interrupt()
        {
        // only timer 0 overflow is due to service
        if(INTCON.TMR0IF)
                {
                if(sIdx < miniGLCD_x)                   // is sampling in progress ?
                        {
                        if(trigger == 2)                // is sampling triggered ?
                                {
                                // read ADC sample, adjust to range and store to sample buffer
                                samples[sIdx++] = miniGLCD_y - (ADRESH >> vdiv) ;
                                }
                        else if(trigger == 1)           // maximum was detected
                                {
                                // is signal rising down ?
                                if((trigValue > LEVEL) && (ADRESH < trigValue - LEVEL))
                                        {
                                        // yes, triggers sampling
                                        trigger = 2 ;
                                        }
                                else
                                        {
                                        // no, update maximum value
                                        if(ADRESH > trigValue)
                                                {
                                                trigValue = ADRESH ;
                                                }
                                        }
                                }
                        else                            // looking for maximum
                                {
                                // is signal rising up ?
                                if((trigValue < 255 - LEVEL) && (ADRESH > trigValue + LEVEL))
                                        {
                                        // yes, next step is to wait for signal rising down
                                        trigger = 1 ;
                                        trigValue = 0 ;
                                        }
                                else
                                        {
                                        // no, update minimum value
                                        if(ADRESH < trigValue)
                                                {
                                                trigValue = ADRESH ;
                                                }
                                        }
                                }
                                
                        // start ADC, no sampling is required since ADC
                        // is always connected to the same input
                        ADCON0.GO = 1 ;
                        }

                t0ctr++ ;
                INTCON.TMR0IF = 0 ;
                }
        }

/*******************************
 * UPDATE SETTINGS & DRAW SCREEN
 *******************************/
void    mkScreen()
        {
        T0CON = timeBase[tbase].t0con ;                         // new timer 0 settings
        vdiv = input[ipt].div ;                                 // store input divider

        // ADC settings
        ADCON1 = 0b00001110 ;
        ADCON0 = 0b11000001 ;

#ifdef  LCD20x4
        LCD_out(1, 14, timeBase[tbase].period) ;                // display period value
        LCD_chr(2, 15, timeBase[tbase].unit) ;                  // display period unit

        LCD_out(3, 16, input[ipt].ampl) ;                       // display input range

        // hold screen ?
        LCD_out(1, 9, hold ? "Hold" : "\xff\xff\xff\xff") ;
#else
        LCD_out(1, 12, timeBase[tbase].period) ;                // display period value
        LCD_chr(2, 12, timeBase[tbase].unit) ;                  // display period unit

        LCD_out(1, 1, input[ipt].ampl) ;                       // display input range

        // hold screen ?
        LCD_out(1, 6, hold ? "H" : "\xff") ;
#endif
        }

/************************
 * UPDATE SCREEN AND DEBOUNCE PORTB KEYS
 ************************/
void    debounce()
        {
        mkScreen() ;
        
        Delay_ms(20) ;
        while(PORTB != 0xff) ;
        Delay_ms(20) ;
        }

/***********************
 * PSEUDO GRAPHIC FUNCTIONS
 ***********************/
 
/************************************************
 * miniGLCD_fill : fill graphic screen with pattern
 * parameters :
 *      c : filling pattern
 *              for example : 0 for black screen, 0xff for white screen
 * returns :
 *      nothing
 * requires :
 *      miniGLCD_init must have been called
 * notes :
 *      none
 */
void    miniGLCD_fill(unsigned char c)
        {
        memset(miniGLCD_screen, c, sizeof(miniGLCD_screen)) ;
        }

/********************************************
 * miniGLCD_setPixel : write pixel
 * parameters :
 *      x : pixel row
 *      y : pixel column
 *      mode : miniGLCD_COLOR_WHITE or miniGLCD_COLOR_BLACK or miniGLCD_COLOR_REVERSE
 * returns :
 *      nothing
 * requires :
 *      miniGLCD_init must have been called
 * notes :
 *      none
 */
void    miniGLCD_setPixel(char x, char y, unsigned char mode)
        {
        unsigned char *ptr ;
        unsigned char mask ;

        /*
         * do nothing if pixel is out of bounds
         */
        if(x < 0) return ;
        if(y < 0) return ;
        if(x > miniGLCD_x) return ;
        if(y > miniGLCD_y) return ;

        ptr = miniGLCD_screen + (((y * (C_X * 8)) + x) / 8) ;      // points to byte in screen map
        mask = 1 << (x & 7) ;                           // pixel bit mask

        switch(mode)
                {
                case miniGLCD_COLOR_BLACK:
                        *ptr &= ~mask ;                 // clear bit
                        break ;
                case miniGLCD_COLOR_WHITE:                   // set bit
                        *ptr |= mask ;
                        break ;
                default:
                        *ptr ^= mask ;                  // toggle bit
                        break ;
                }
        }

/********************************************
 * miniGLCD_setPixel : read pixel
 * parameters :
 *      x : pixel row
 *      y : pixel column
 * returns :
 *      color of pixel at (x, y)
 * requires :
 *      miniGLCD_init must have been called
 * notes :
 *      none
 */
unsigned char miniGLCD_getPixel(char x, char y)
        {
        unsigned char *ptr ;
        unsigned char mask ;

        /*
         * do nothing if pixel is out of bounds
         */
        if(x < 0) return(0) ;
        if(y < 0) return(0) ;
        if(x > miniGLCD_x) return(0) ;
        if(y > miniGLCD_y) return(0) ;

        ptr = miniGLCD_screen + (((y * (C_X * 8)) + x) / 8) ;      // points to byte in screen map
        mask = 1 << (x & 7) ;                           // pixel bit mask

        return(*ptr & mask) ;
        }

/******************************
 * miniGLCD_line : draw a line
 * parameters :
 *      x0, y0 : pixel start coordinates
 *      x1, y1 : pixel end coordinates
 *      pcolor : miniGLCD_COLOR_WHITE or miniGLCD_COLOR_BLACK or miniGLCD_COLOR_REVERSE
 * returns :
 *      nothing
 * requires :
 *      miniGLCD_init must have been called
 * notes :
 *      uses Bresenham's line drawing algorithm
 */
void miniGLCD_line(char x0, char y0, char x1, char y1, unsigned char pcolor)
        {
        int     dy ;
        int     dx ;
        int     stepx, stepy ;

        dy = y1 - y0 ;
        dx = x1 - x0 ;

        if(dy < 0)
                {
                dy = -dy ;
                stepy = -1 ;
                }
        else
                {
                stepy = 1 ;
                }

        if(dx < 0)
                {
                dx = -dx ;
                stepx = -1 ;
                }
        else
                {
                stepx = 1 ;
                }

        dy <<= 1 ;
        dx <<= 1 ;

        miniGLCD_setPixel(x0, y0, pcolor) ;

        if(dx > dy)
                {
                int fraction = dy - (dx >> 1) ;

                while(x0 != x1)
                        {
                        if(fraction >= 0)
                                {
                                y0 += stepy ;
                                fraction -= dx ;
                                }
                        x0 += stepx ;
                        fraction += dy ;
                        miniGLCD_setPixel(x0, y0, pcolor) ;
                        }
                }
        else
                {
                int fraction = dx - (dy >> 1) ;

                while(y0 != y1)
                        {
                        if(fraction >= 0)
                                {
                                x0 += stepx ;
                                fraction -= dy ;
                                }
                        y0 += stepy ;
                        fraction += dx ;
                        miniGLCD_setPixel(x0, y0, pcolor) ;
                        }
                }
        }

/*************************************************************
 * program custom character n at line pos_row column pos_char
 * if mode is not zero, also write custom char to LCD
 */
void CustomChar(unsigned char mode, unsigned char n, char pos_row, char pos_char)
        {
        unsigned char    i, j ;

        LCD_Cmd(64 + n * 8) ;
        for(i = 0 ; i < 8 ; i++)
                {
                unsigned char bm = 0 ;

                for(j = 0 ; j < 5 ; j++)
                        {
                        bm <<= 1 ;
                        bm |= miniGLCD_getPixel(pos_char * 5 + j, pos_row * 8 + i) ? 1 : 0 ;
                        }
                LCD_Chr_Cp(bm) ;
                }
        LCD_Cmd(LCD_RETURN_HOME) ;
#ifdef LCD20x4
        if(mode) LCD_Chr(pos_row + 2, pos_char + 9, n) ;
#else
        if(mode) LCD_Chr(pos_row + 1, pos_char + 7, n) ;
#endif
        }

/******************
 * MAIN LOOP
 ******************/
void    main()
        {
        unsigned char i, j ;
        unsigned int    wait ;

        TRISA = 0xff ;                          // set PORTA as inputs

        TRISB = 0xff ;                          // set PORTB as inputs
        INTCON2.NOT_RBPU = 0 ;                  // enables PORTB weak pull-ups
        
        TRISD = 0 ;                             // PORTD is output (LCD in 4bit mode)

        // enables timer 0 overflow interrupt
        INTCON.TMR0IF = 0 ;
        INTCON.TMR0IE = 1 ;
        INTCON.GIE = 1 ;

        // LCD configuration
        LCD_Init(&LATD) ;                      // Initialize LCD connected to PORTD
        LCD_Cmd(Lcd_CLEAR) ;                   // Clear display
        LCD_Cmd(Lcd_CURSOR_OFF) ;              // Turn cursor off
        
        // display layout
#ifdef LCD20x4
        LCD_out(1, 8, "\xff\xff\xff\xff\xff\xff") ;
        LCD_out(2, 8, "\xff    \xff  s/Div") ;
        LCD_out(3, 8, "\xff    \xff") ;
        LCD_out(4, 8, "\xff\xff\xff\xff\xff\xff  V/Div") ;
#else
        LCD_out(1, 1, "     \xff    \xff") ;
        LCD_out(2, 1, "V/Div\xff    \xff s/Div") ;
#endif

        // send custom chars
        CustomChar(1, 0, 0, 0) ;
        CustomChar(1, 1, 0, 1) ;
        CustomChar(1, 2, 0, 2) ;
        CustomChar(1, 3, 0, 3) ;
        CustomChar(1, 4, 1, 0) ;
        CustomChar(1, 5, 1, 1) ;
        CustomChar(1, 6, 1, 2) ;
        CustomChar(1, 7, 1, 3) ;

        mkScreen() ;

        for(;;)                 // forever
                {
                // if not in hold mode and samples buffer is full
                if((hold == 0) && (sIdx == miniGLCD_x))
                        {
                        // clear pseudo-screen
                        miniGLCD_fill(0) ;

                        // draw wave
                        for(i = 0 ; i < miniGLCD_x - 1 ; i++)
                                {
                                j = i + 1 ;

                                miniGLCD_line(i, samples[i], j, samples[j], miniGLCD_COLOR_WHITE) ;
                                }

                        // program custom chars
                        CustomChar(0, 0, 0, 0) ;
                        CustomChar(0, 1, 0, 1) ;
                        CustomChar(0, 2, 0, 2) ;
                        CustomChar(0, 3, 0, 3) ;
                        CustomChar(0, 4, 1, 0) ;
                        CustomChar(0, 5, 1, 1) ;
                        CustomChar(0, 6, 1, 2) ;
                        CustomChar(0, 7, 1, 3) ;

                        // restart trigger and samples index
                        trigValue = 255 ;
                        trigger = 0 ;
                        sIdx = 0 ;
                        }

                // change horizontal frequency
                if(PORTB.F0 == 0)
                        {
                        tbase++ ;
                        if(tbase == sizeof(timeBase) / sizeof(struct TIMEBASE))
                                {
                                tbase = 0 ;
                                }
                        hold = 0 ;
                        debounce() ;
                        }
                else if(PORTB.F1 == 0)
                        {
                        if(tbase == 0)
                                {
                                tbase = sizeof(timeBase) / sizeof(struct TIMEBASE) - 1 ;
                                }
                        else
                                {
                                tbase-- ;
                                }
                        hold = 0 ;
                        debounce() ;
                        }

                // change vertical range
                else if(PORTB.F2 == 0)
                        {
                        ipt++ ;
                        if(ipt == sizeof(input) / sizeof(struct INPUT))
                                {
                                ipt = 0 ;
                                }
                        hold = 0 ;
                        debounce() ;
                        }
                else if(PORTB.F3 == 0)
                        {
                        if(ipt == 0)
                                {
                                ipt = sizeof(input) / sizeof(struct INPUT) - 1 ;
                                }
                        else
                                {
                                ipt-- ;
                                }
                        hold = 0 ;
                        debounce() ;
                        }

                // hold/release screen
                else if(PORTB.F4 == 0)
                        {
                        hold ^= 1 ;
                        debounce() ;
                        }

                // scrolling message
#ifdef LCD20x4
                if(wait)
                        {
                        if(t0ctr > (1u << (tbase + 5)))
                                {
                                firstMsg++ ;
                                if(msg[firstMsg][0] == 0)
                                        {
                                        firstMsg = 0 ;
                                        }

                                t0ctr = 0 ;
                                wait = 0 ;
                                }
                        }
                else if(t0ctr > (1u << (tbase + 1)))
                        {
                        j = firstMsg ;
                        for(i = 1 ; i <= 4 ; i++)
                                {
                                if((i == 4) && (msg[j + 1][0] == '*'))
                                        {
                                        wait++ ;
                                        }
                                if(msg[j][0] == '*')
                                        {
                                        LCD_out(i, 1, "       ") ;
                                        }
                                else
                                        {
                                        LCD_out(i, 1, msg[j]) ;
                                        }

                                j++ ;
                                if(msg[j][0] == 0)
                                        {
                                        j = 0 ;
                                        }
                                }

                        firstMsg++ ;
                        if(msg[firstMsg][0] == 0)
                                {
                                firstMsg = 0 ;
                                }

                        t0ctr = 0 ;
                        }
#endif
                }
        }

Download project

Download LCDscope-project.ZIP file for mikroC : File:LCDscope-project.zip

Includes :

  • mikroC PRO project files for PIC18F4620, should work also with most of PIC
  • LCDscope C source code
  • LCDscope .HEX files

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