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Your are proud of your expensive oscilloscope, now you can turn it into a cheap digital clock !

How to display a clock on an oscilloscope

Contents

PicoOClock : Turn your oscilloscope into a digital clock

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


See also : PicOscilloMeter project !

For those are not into electronics, you must know that an oscilloscope has basicaly only one timebase to move the spot horizontaly from left to right with the same intensity. The vertical deviation is function to the input voltage. You understand immediately that you can't directly display 7 segment digits, because you can't move the spot from right to left.

By using X/Y mode, where the spot is controlled on two axes by two different voltages, it is possible to draw a picture (as in the examples mentioned below), but a fast digital to analog converter with two channels and at least 8 bits of resolution would be needed.

So we have to deal with a spot that always goes from left to right in the same period of time. If we want to have a 7segment-like display, we have to draw :

vertical segments : easy to do, just change voltage up and down quickly a few times. horizontal segments : easy to do, just set a voltage level and keep it as long as you need. By using 2 PIC outputs and a basic R2R digital to analog converter, we can have up to four different voltage levels : 3 for the vertical segment, and another one where to put the spot when it is not in use to draw a segment.

But the problem is that a 7 segment digit may have up to 3 horizontal lines at a time (like 2, 3, 8, 9..) but we can draw only one during one spot deviation. So we will have to cheat with retinal persistance and use multiple frames : since we can have only one vertical segment per period, three periods will be needed to draw a full 7 segment digit.

Suppose we want to display 12:34:56
PicOClock-frame1.jpg During the first period, we will draw all vertical segments, and horizontal upper segments only :

As a game, I let you try to find out the spot trajectory. Don't forget the rules :

  • you can't go backward
  • you can't clear the spot

But you can move so fast verticaly that the eye can't see the spot moving.

The lowest line under the digits is not significant, it his the place where the spot is parked when not used to draw a segment

PicOClock-frame2.jpg During the second period, we do exactly the same, with horizontal middle segments only
PicOClock-frame3.jpg Then third period draws horizontal lower segments
PicOClock-frame4.jpg This process is repeated again and again very quickly, and this is what the eyes can see, thanks to retinal persistance

How to use PicOClock :

  • Connect X1 (see circuit schematic) to the Y input (vertical deviation) of the oscilloscope
  • Connect X2 (see circuit schematic) to the external trigger input of the oscilloscope
  • Set on your oscilloscope :
    • timebase to 0.1 ms
    • vertical deviation 1/V division
    • external trigger
  • Power the circuit : display starts with 0:00:00
  • Press hours button to change hours
  • Press minutes button to change minutes

If you like it, please see also my Pic Pal Video Library, and also PicoOSD

Circuit Schematic

PicoClock-schematic.jpg

  • a 2-bit digital to analog converter is made by resistor ladder R3...R6 driven by RB0 and RB1 output ports
  • RA0 triggers oscilloscope vertical sync

C Source code

/*
 *******************************************************************************
 * PICOCLOCK : PIC Oscilloscope CLOCK
 *******************************************************************************
 *
 * This program shows how to display a digital clock on an oscilloscope
 * with a PIC and only 4 resistors.
 *
 * Circuit schematics :
 *
 * ------------+
 *         RA0 +----------------> to oscilloscope X trigger input
 *             |
 *         RA1 +----------------> pull-up, button to ground : minutes adjustment
 *         RA2 +----------------> pull-up, button to ground : hours adjustment
 *  PIC        |     ____
 *         RB1 +----|____|-----+---------> to oscilloscope Y input
 *             |     680       |
 *             |              +-+
 *             |              | | 330
 *             |              +-+
 *             |     ____      |
 *         RB0 +----|____|-----+
 *             |     680       |
 * ------------+              +-+
 *                            | | 680
 *                            +-+
 *                             |
 *                           -----
 *                            ---   GND
 *                             -
 *
 * Oscilloscope setup :
 * set timebase to 0.1 ms, V/div = 1 V
 * select external trigger.
 *
 * source code for mikro C compiler V7.0.0.3
 * feel free to use this code at your own risks
 * and don't bother me if you get addicted watching this clock.
 *
 * target : PIC16 or PIC18, 16 Mhz crystal
 * HS clock, no watchdog.
 *
 * tested with PIC16F84A and PIC16F877A
 *
 * Author : Bruno Gavand, October 2007
 * see more details on http://www.micro-examples.com/
 *
 *******************************************************************************
 */
#define TRIGGER         PORTA.F0        // this output is to be connected to oscilloscope trigger input
#define KEY             PORTA & 0b110   // input keys mask
#define KEY_MIN_UP      PORTA & 0b010   // minute adjust button
#define KEY_HH_UP       PORTA & 0b100   // hour adjust button

/*
 * 2 bits R2R DAC gives 4 output levels :
 */
#define HIGH            PORTB = 0b11    // uper line
#define MID             PORTB = 0b10    // middle line
#define LOW             PORTB = 0b01    // lower line
#define ZERO            PORTB = 0b00    // lowest line

#define MAX_SCALER      15625           // number of timer 0 overflow per second @ 16 Mhz = 16000000 / 4 / 256

#define MAX_DIGIT        6              // number of digits to be displayed
#define SLOTS           (MAX_DIGIT * 3 + 4)     // number of time slots : 2 for header, 3 per digits, 2 for trailer

/*
 * 10 digits 7 segment encoding + blank
 */
const unsigned char     septSeg[11] = { 0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d, 0x07, 0x7f, 0x6f, 0x00 } ;

/*
 * slot index for digit start
 */
const unsigned char     sIdx[] = {1, 4, 8, 11, 15, 18} ;

unsigned char display[MAX_DIGIT] ;      // digit to be displayed

/*
 * time slot encoded line flags :
 * bit 0 is upper line
 * bit 1 is middle line
 * bit 2 is lower line
 * (if no line flag is set, spot is redirected to lowest line)
 * bit 6 is lower vertical bar
 * bit 7 is upper vertical bar
 */
unsigned char line[SLOTS] ;

unsigned char dIdx = 0 ;        // time slot counter
unsigned char fIdx = 0 ;        // frame counter

unsigned int scaler = 0 ;       // RTC scaler
unsigned char   ss = 0, mn = 0, hh = 0 ;        // RTC

/*
 * around 10 micro-second delay
 */
void    delay10us()
        {
        Delay_us(10) ;
        }

/*
 * ISR
 */
void    interrupt(void)
        {
        if(INTCON.T0IF)                                         // if timer 0 overflow
                {
                scaler++ ;                                      // increment scaler
                if(scaler > MAX_SCALER)                         // one second has expired ?
                        {
                        scaler = 0 ;                            // clear scaler
                        ss++ ;                                  // next second
                        if(ss == 60)                            // last second in minute ?
                                {
                                ss = 0 ;                        // clear second
                                mn++ ;                          // next minute
                                if(mn == 60)                    // last minute in hour ?
                                        {
                                        mn = 0 ;                // clear minute
                                        hh++ ;                  // next hour
                                        if(hh == 24)            // last hour in day ?
                                                {
                                                hh = 0 ;        // clear hour
                                                }
                                        }
                                }
                        }

                if(line[dIdx].F6 && line[dIdx].F7)              // if full vertical bar
                        {
                        LOW, HIGH, LOW, HIGH ;
                        LOW, HIGH, LOW, HIGH ;
                        LOW, HIGH, LOW, HIGH ;
                        LOW, HIGH, LOW, HIGH ;
                        LOW, HIGH, LOW, HIGH ;
                        LOW, HIGH, LOW, HIGH ;
                        LOW, HIGH, LOW, HIGH ;
                        LOW, HIGH, LOW, HIGH ;
                        }
                else if(line[dIdx].F6)                          // if lower vertical bar
                        {
                        MID, LOW, MID, LOW ;
                        MID, LOW, MID, LOW ;
                        MID, LOW, MID, LOW ;
                        MID, LOW, MID, LOW ;
                        MID, LOW, MID, LOW ;
                        MID, LOW, MID, LOW ;
                        MID, LOW, MID, LOW ;
                        MID, LOW, MID, LOW ;
                        }
                else if(line[dIdx].F7)                          // if upper vertical bar
                        {
                        MID, HIGH, MID, HIGH ;
                        MID, HIGH, MID, HIGH ;
                        MID, HIGH, MID, HIGH ;
                        MID, HIGH, MID, HIGH ;
                        MID, HIGH, MID, HIGH ;
                        MID, HIGH, MID, HIGH ;
                        MID, HIGH, MID, HIGH ;
                        MID, HIGH, MID, HIGH ;
                        }

                if(dIdx == 7)                                   // hour : minute separator
                        {
                        LOW, Delay10us() ;
                        MID, Delay10us() ;
                        }
                else if(dIdx == 14)                             // minute : second separator
                        {
                        if(scaler < MAX_SCALER / 2)             // blink 0.5 Hz
                                {
                                LOW, Delay10us() ;
                                MID, Delay10us() ;
                                }
                        }
                        
                switch(fIdx)                                    // depending on frame index
                        {
                        case 0:                                 // upper line
                                if(line[dIdx] & 1)
                                        {
                                        HIGH ;
                                        }
                                else
                                        {
                                        ZERO ;
                                        }
                                break ;
                        case 1:                                 // middle line
                                if(line[dIdx] & 2)
                                        {
                                        MID ;
                                        }
                                else
                                        {
                                        ZERO ;
                                        }
                                break ;
                        case 2:                                 // lower line
                                if(line[dIdx] & 4)
                                        {
                                        LOW ;
                                        }
                                else
                                        {
                                        ZERO ;
                                        }
                                break ;
                        }

                dIdx++ ;                                        // next slot
                if(dIdx == SLOTS)                               // last slot ?
                        {
                        dIdx = 0 ;                              // clear slot

                        TRIGGER = 1 ;                           // triggers the scope
                        fIdx++ ;                                // next frame
                        if(fIdx == 3)                           // last frame ?
                                {
                                fIdx = 0 ;                      // clear frame
                                }
                        TRIGGER = 0 ;                           // end trigger
                        }
                INTCON.T0IF = 0 ;                               // clear timer 0 overflow
                }
        }
        
/*
 * main entry
 */
void    main()
        {
#ifdef P16F877A
        /*
         * set PORTA as digital I/O
         */
        ADCON1 = 7 ;
        CMCON = 7 ;
#endif

        TRISA = 0b110 ;                                        // PORTA direction register
        PORTA = 0 ;
        
        TRISB = 0 ;                                             // PORTB is output
        PORTB = 0 ;

        /*
         * clear buffers
         */
        memset(&line, 0, sizeof(line)) ;
        memset(display, 0, sizeof(display)) ;

        OPTION_REG = 0b11011000 ;                               // timer 0 prescaler is 1:1
        INTCON = 0b10100000 ;                                   // start interrupts

        for(;;)                                                 // main loop
                {
                unsigned char i ;

                if(KEY)                                         // is a button pressed ?
                        {
                        if(KEY_MIN_UP)                          // adjust minutes
                                {
                                ss = 0 ;
                                mn++ ;
                                }
                        else if(KEY_HH_UP)                      // adjust hours
                                {
                                ss = 0 ;
                                hh++ ;
                                }
                        mn %= 60 ;                              // prevent minute overflow
                        hh %= 24 ;                              // prevent hours overflow
                        Delay_ms(100) ;                         // debounce
                        }

                /*
                 * prepare display buffer
                 */
                display[5] = ss % 10 ;                          // seconds
                display[4] = ss / 10 ;

                display[3] = mn % 10 ;                          // minutes
                display[2] = mn / 10 ;

                display[1] = hh % 10 ;                          // hours
                display[0] = (hh > 9) ? hh / 10 : 10 ;          // blank first digit if zero

                /*
                 * prepare time slot flags
                 */
                for(i = 0 ; i < MAX_DIGIT ; i++)                // for each digit
                        {
                        unsigned char s ;
                        unsigned char *p ;
                        
                        s = septSeg[display[i]] ;               // get 7 segment encoding

                        p = &line[sIdx[i]] ;                    // get pointer to time slot, left part of the digit

                        (*p).F0 = s.F0 ;                        // a segment
                        (*p).F1 = s.F6 ;                        // g segment
                        (*p).F2 = s.F3 ;                        // d segment

                        (*p).F6 = s.F4 ;                        // e segment
                        (*p).F7 = s.F5 ;                        // f segment

                        p++ ;                                   // next slot, center part of the digit

                        (*p).F0 = s.F0 ;                        // a segment (continuation)
                        (*p).F1 = s.F6 ;                        // g segment (continuation)
                        (*p).F2 = s.F3 ;                        // d segment (continuation)

                        p++ ;                                   // next slot, right part of the digit

                        (*p).F6 = s.F2 ;                        // b segment
                        (*p).F7 = s.F1 ;                        // c segment
                        }
                }
        }


Download project

Please copy/paste C source code above in your favorite PIC C compiler !

External links to other projects of the same kind

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