Setting Internal Oscillator for PIC16F627A

I love to use PIC16F627A and PIC16F628 because they come with internal oscillators. That means I can make a project with lower component count (without 1 crystal and 2 load capacitors). The project setting of MikroC for using internal oscillator of the PIC16F627A shows below:

6 Digits 7-Segment LED Multiplexing using a Shift Register

Multiplexing technique can reduce number of needed I/O pins of the MCU as I have explained in 'LED 7-Segment Multiplexing' and '6 Digits LED 7-Segment Multiplexing'. In those posts, I used 13 I/O pins for driving 6 digits LED 7-Segment. However, the PIC16F627A and PIC16F628 have only 15 usable I/O pins that include 2 pins for external 32.768KHz oscillator. So, there is no pin left for time setting buttons. I can change to the PIC that has more I/O pins, but I don't think it's a good solution. From my searches, I can overcome this I/O pins shortage problem by using shift register to expand the MCU I/O pins.

The concept is very similar to led dot matrix driving technique. Each digit is multiplexed via a shift register 74HC595 which is required 3 pins of the MCU. Each segment of the 7-segment display is driven by PORTA of the PIC16F628. As a result, the required pins for driving 6-Digit 7-Segment display are just 3+7 = 10 pins!. With this configuration, there are 3 I/O pins that are free for time setting buttons and driving blinking second LEDs.

I use TMR2 module for scanning digits. TMR2 is an 8-bit timer which overflows every 256 (0xFF) counts. It's known that the refresh rate above 50Hz would be enough for human's eyes to see the display without recognizing the flickering. If I set TMR2 with 1:8 Prescaler (T2CON = 0x3C), the multiplexing frequency will be 81.3Hz (4MHz/4/256/8/6 = 81.3Hz) which is enough for flicker free display.

PORTA is used to drive each segment of the 7-segment displays. However, I have to skip the RA5 as it's a MCLR pin and it can be only input pin. So, my 7-segment digit mask is different then the normal 7-segment digit mask.

my PORTA 7-segment digit mask : {0x5F, 0x06, 0x9b, 0x8f, 0xC6, 0xCd,0xDD, 0x07,
0xDf, 0xCf}
Normal 7-segment digit mask : {0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d, 0x07, 0x7f, 0x6f} for number 0-9 respectively.

Below is the example schematic of 999999-second counter using the PIC16F627A or PIC16F628 and a shift register. I will not implement a clock with this configuration as I need more free MCU pins for driving Alarm buzzer and other things.



The source code for 999999-second counter in MikroC is listed below

//PIC16F627A or PIC16F628
//4MHz Internal OSC
//MUX by using Shift Register 595
// Critical parameters:
// Delay, Postscaler, Prescaler
// Low delay + High Prescaler
// 03/11/2008
// punkky@gmail.com
#define SH_CP PORTB.F0
#define DS PORTB.F1
#define ST_CP PORTB.F2
// 7-Segment code is skipping RA5, so the code is not as normal 7-segment coding
unsigned short number [10] = {
    0x5F, 0x06, 0x9b, 0x8f, 0xC6, 0xCd, 0xDD, 0x07,
    0xDf, 0xCf
};
unsigned short digit [6] = {
    0, 0, 0, 0, 0, 0
};
unsigned short i;
unsigned short n;
unsigned short counter;
unsigned short tick;
unsigned short shift_register;
unsigned short x1;
unsigned short x2;
unsigned short x3;
unsigned short x4;
unsigned short x5;
unsigned short x6;
void interrupt ()
{
    if (PIR1.TMR2IF)
    {
        PIR1.TMR2IF = 0;
        if (counter == 5)
        {
            //Shift data
            DS = 0;
            //Store data
            SH_CP = 1;
            SH_CP = 0;
            Delay_us (250);
            counter = 0;
        }else
        {
            //Shift data
            DS = 1;
            //Store
            SH_CP = 1;
            SH_CP = 0;
            Delay_us (250);
            counter ++;
        }
        ST_CP = 1;
        ST_CP = 0;
        PORTA = 0x00;
        PORTA = number [digit [counter]];
    }
    if (PIR1.TMR1IF)
    {
        TMR1H = 0x80;
        PIR1.TMR1IF = 0;
        tick = 1;
        x6 ++;
        if (x6 > 9)
        {
            x6 = 0;
            x5 ++;
            if (x5 > 9)
            {
                x5 = 0;
                x4 ++;
                if (x4 > 9)
                {
                    x4 = 0;
                    x3 ++;
                    if (x3 > 9)
                    {
                        x3 = 0;
                        x2 ++;
                        if (x2 > 9)
                        {
                            x2 = 0;
                            x1 ++;
                            if (x1 > 9)
                            {
                                x1 = 0;
                            }
                        }
                    }
                }
            }
        }
    }
}
void main ()
{
    //Digital I/O for PORTA
    CMCON = 0x07;
    TRISA = 0x00;
    PORTA = 0x00;
    TRISB = 0x00;
    PORTB = 0x00;
    //Internal Clock 4MHz
    PCON.OSCF = 1;
    counter = 0;
    // Set GIE, PEIE
    INTCON = 0xC0;
    //1:8 post scaler
    T2CON = 0x3C;
    // enable interupt
    PIE1.TMR2IE = 1;
    T1CON = 0x0F;
    //Initial value TMR1: 0x8000
    TMR1H = 0x80;
    TMR1L = 0x00;
    // enable interupt
    PIE1.TMR1IE = 1;
    //Data
    DS = 0;
    //Store
    SH_CP = 0;
    ST_CP = 0;
    x1 = 0;
    x2 = 0;
    x3 = 0 ;
    x4 = 0;
    x5 = 0;
    x6 = 0;
    while (1)
    {
        if (tick)
        {
            tick = 0;
            digit [0] = x1;
            digit [1] = x2;
            digit [2] = x3;
            digit [3] = x4;
            digit [4] = x5;
            digit [5] = x6;
        }
    }
}

6 Digits LED 7-Segment Multiplexing

In the post 'LED 7-Segment Multiplexing', I have explained about the concept and benefits of multiplexing. Multiplexing implementation is very similar to driving Led Dot Matrix. I use Timer0 interrupt for switching through each digit. Timer0 or TMR0 is an 8-bit timer which overflows every 256 (0xFF) counts. It's known that the refresh rate above 50Hz would be enough for human's eyes to see the display without recognizing the flickering. If I set TMR0 with 1:8 Prescaler, the multiplexing frequency will be

4MHz(internal OSC.)/4(working OSC)/8(prescaler)/256(max counts of TMR0)/6(number of digits) = 81.3 Hz which is good for a display.

Just an example, I have implemented (in Proteus) a 999999-second counter by using 6 Digits LED 7-Segment Multiplexing technique. There are 2 main components in the project, PIC16F627A or PIC16F628 and 6 x LED7-segment display. The schematic shows below. The crystal is 32.768KHz as usual. There is a 10KOhm pull up resistor at RA4 pin as this pin is an open-drain pin as I described in "Open-Drain RA4 pin on PIC Microcontroller".


The source code in MikroC is listed below: (.hex is also available, please feel free to contact me)

//PIC16F627A
//4MHz Internal OSC
//MUX by the MUC itself with Interrupt
//TMR0 .. check the prescelar+delay in scan routine as they are related
//punkky@gmail.com
unsigned short number [10] = {
    0x5F, 0x06, 0x9b, 0x8f, 0xC6, 0xCd, 0xDD, 0x07,
    0xDf, 0xCf
};
unsigned short digit [6];
unsigned short counter;
unsigned short shift_register;
unsigned short x1;
unsigned short x2;
unsigned short x3;
unsigned short x4;
unsigned short x5;
unsigned short x6;
unsigned short tick;
void interrupt ()
{
    if (INTCON.T0IF)
    {
        //Scan digits with TMR0
        INTCON.T0IF = 0;
        if (counter == 5)
        {
            PORTA = number [digit [counter]];
            Delay_us (500);
            shift_register = 0x01;
            PORTB = ~shift_register;
            PORTA = 0x00;
            counter = 0;
        } else
        {
            PORTA = number [digit [counter]];
            Delay_us (500);
            shift_register = shift_register << 1;
            PORTB = ~shift_register;
            PORTA = 0x00;
            counter ++;
        }
    }
    if (PIR1.TMR1IF)
    {
        TMR1H = 0x80;
        PIR1.TMR1IF = 0;
        tick = 1;
        //update current time
        x6 ++;
        if (x6 > 9)
        {
            x6 = 0;
            x5 ++;
            if (x5 > 9)
            {
                x5 = 0;
                x4 ++;
                if (x4 > 9)
                {
                    x4 = 0;
                    x3 ++;
                    if (x3 > 9)
                    {
                        x3 = 0;
                        x2 ++;
                        if (x2 > 9)
                        {
                            x2 = 0;
                            x1 ++;
                            if (x1 > 9)
                            {
                                x1 = 0;
                            }
                        }
                    }
                }
            }
        }
    }
}
void main ()
{
    //Digital I/O for PORTA
    CMCON = 0x07;
    TRISA = 0x00;
    PORTA = 0x00;
    TRISB = 0x00;
    PORTB = 0x00;
    //Internal Clock 4MHz
    PCON.OSCF = 1;
    counter = 0;
    // Enable TMR0
    OPTION_REG.T0CS = 0;
    // Enable Prescaler
    OPTION_REG.PSA = 0;
    // PS0,1,2 = 010 = 3
    // 3 means 1:8 prescaler
    // 1:2, 1:4, 1:8, 1:16, 1:32, 1:64, 1:128, 1:256
    OPTION_REG.PS2 = 0;
    OPTION_REG.PS1 = 1;
    OPTION_REG.PS0 = 0;
    INTCON.T0IF = 0;
    INTCON.T0IE = 1;
    INTCON.GIE = 1;
    INTCON.PEIE = 1;
    T1CON = 0x0F;
    TMR1H = 0x80;
    TMR1L = 0x00;
    // Enable TMR1 interrupt
    PIE1.TMR1IE = 1;
    shift_register = 0x01;
    x1 = 0;
    x2 = 0;
    x3 = 0;
    x4 = 0;
    x5 = 0;
    x6 = 0;
    while (1)
    {
        if (tick)
        {
            tick = 0;
            //update digits
            digit [0] = x1;
            digit [1] = x2;
            digit [2] = x3;
            digit [3] = x4;
            digit [4] = x5;
            digit [5] = x6;
        }
    }
}

LED 7-Segment Multiplexing

In my first Digital Clock, I use 6 pcs. of CD4543,BCD to 7-Segment decoder, to drive 6 digit LED 7-Segment display for the sake of simplicity of the software. However, the hardware needs many components. As you can see in the post, the PCB of the clock is quite big and containing a lot of solder points. To reduce the number of components, I will integrate the function of CD4543 into the firmware. One digit requires 7 connections (wires) for all segments and 1 connection for common cathode (or anode). If I connect 6 digits to the MCU without any modification, I will need 7-segment x 6 digit = 42 connections . That means I need to use MCU with atleast 42 I/O pins. As you know, it is a waste for using a lot of MCU pins just for display. The required pins can be reduced dramatically by using a technique called Multiplexing.

Multiplexing

Multiplexing technique is based on the idea of Persistence of vision of the human eyes. The sample schematic of 3 digits multiplexing is shown below. Segment a-g of each digit are connected together. Each digit is switched on-off by controlling signal at Digit 1, Digit 2 and Digit 3. For example, if Digit 1 is '1' , Digit 1 will be on. If Digit 1 is '0', Digit 1 will be off. People will see all 3 digits display in the same time if each digit switch on and off fast enough.

By using multiplexing technique the number of required connections for 6 digits display is reduced from 42 pins to 7-Segment+6 digits = 13 pins Wow!! I can drive 6 digits 7-segment display by using just a PIC16F627A.
Next time, I will post the program for LED 7-Segment Multiplexing

--- Updated 21 Dec 2008 --
The program for LED 7-Segment Multiplexing is now posted at 6 Digits LED 7-Segment Multiplexing

PCB for 7-Segment PIC Digital Clock

I have designed a single-sided PCB for the PIC Digital Clock. Because the autorouter is not good for routing single-sided PCB so I have to route by hand. It was my first hand routed PCB design and it was a time consuming task. I haven't tested the PCB yet. Use it by your own risk. I will produce this PCB and test it later.

As I don't have server to upload the Eagle file, please contact me if you want the file or PDF version of the image below.



Components side of the PCB. The red lines are jumpers.

7-Segment PIC Digital Clock : The photographs

The prototype of the 7-Segment PIC Digital Clock. Check out 7-Segment Digital Clock for the schematic and source code.

Making a Digital Clock (Updated)

My fist Microcontroller project, PIC Digital Clock, is shown below.


I have added time setting feature to make it to be a usable clock. The updated version is shown below:


More features will be added in the future.
There is no fast or slow time setting as in normal/simple digital clocks. In this clock, each digit of the clock display can be set one by one via 2 setting buttons.
Features
1. Bright Led 7-Segment display without Multiplexing
2. Display: Hour, Minute, Second
3. Set time via 2 buttons
4. Set time Digit by Digit
5. Use internal oscillator of PIC16F627a or PIC16F628
6. Use 32.768KHz crystal for better clock accuracy


Setting Time:
1. The clock shows 12:34:56 when power the clock on. The first digit of hour (it's number 1 in this case) will be blinking to notify that the time is not correct and need to be set.
2. Press SET button to count up the digit.
3. Press MODE button when the digit is the correct time. The next digit will be blinking.
4. Repeat step 2 and 3 for setting minute and second.
5. Make sure that pressing MODE button for setting the second digit of second at the correct time.


Shecmatic of the Digital Clock


Example of a PCB design of the Digital Clock


The firmware ( source code in C ) written in MikroC

//6 digit  clock
//Using timer1 16bit counter interrupt
// PIC16F627A or PIC16F628
// Internal Clock 4MHz
// PUNKKY@gmail.com
#define MODE PORTB.F4
#define SET PORTB.F5
#define Sec_port_l PORTA.F6
#define Sec_port_h PORTA.F4
#define Min_port_l PORTA.F3
#define Min_port_h PORTA.F2
#define Hr_port_l PORTA.F1
#define Hr_port_h PORTA.F0
#define Blink PORTA.F7
#define HTMR1 0x80
#define LTMR1 0x00
typedef unsigned short uns8;
uns8 i;
uns8 hr_h;
uns8 hr_l;
uns8 min_h;
uns8 min_l;
uns8 sec_h;
uns8 sec_l;
uns8 tick;
uns8 myTimer;
uns8 setting_time;
void setup ();
void set_time ();
void show_time ();
void display (uns8 digit);
void blink_digit (uns8 digit);
void check_bt ();

//void check_bt(); //chech button
void interrupt ()
{
        PIR1.TMR1IF = 0;
        // clears TMR1IF
        TMR1H = HTMR1;
        tick = 1;
        Blink = 1;
        sec_l ++;
        if(sec_l>9){
            sec_l = 0;
            sec_h++;
        }
        if(sec_h>5){
            sec_h=0;
            min_l++;
        }
        if(min_l>9){
            min_l = 0;
            min_h++;
        }
        if(min_h>5){
            min_h = 0;
            hr_l++;
        }
        if(hr_l>9){
            hr_l = 0;
            hr_h++;
        }
        if(hr_h >2){
            hr_h = 0;
        }
        if(hr_h >=2 && hr_l>3){
           hr_h = 0;
           hr_l = 0;
        }
}
void main ()
{
        setup ();
        
        //Set time
        hr_h = 1;
        hr_l = 2;
        min_h = 3;
        min_l = 4;
        sec_h = 5;
        sec_l = 6;
        show_time ();
        setting_time = 1;
        set_time();
        while (1)
        {
                //blink_digit();
                if (tick)
                {
                        tick = 0;
                        show_time ();
                        Delay_ms (300);
                        Blink = 0;
                }
                check_bt ();
        }
}
void setup ()
{
        tick = 0;
//Digital output on PORTA
        CMCON = 0x07;
        //Input buttons + external clock
        TRISB = 0xB0;

        PORTB = 0x00;
        TRISA = 0x00;
        PORTA = 0x00;
        //Internal Clock 4MHz
        PCON.OSCF = 1;
        // Prescaler 1:1   external clock
        T1CON = 0x0F;

        PIE1.TMR1IE = 0;  // disable interupt to stop the clock

        INTCON = 0xC0;
        // Set GIE, PEIE
        TMR1L = LTMR1;
        TMR1H = HTMR1;
        // TMR1 starts at 0x0BDC = 3036 to make TMR1 counts to 62500 and
        // overclows in every 0.1 sec
        // Math: 1/500000*8*62500 = 0.1
        // 1/5000000 : time for 20MHz crystal (internal clock will be 20/4 = 5MHz)
        // 8: prescaler
        // 62500: TMR1 counts to 62500
        // Counting number of overflows to 10 will get 1 sec.

}

void show_time ()
{
        display (1);
        display (2);
        display (3);
        display (4);
        display (5);
        display (6);
}
void display (uns8 digit)
{
        switch (digit)
        {
                case 1 :
                PORTB = hr_h;
                Hr_port_h = 1;
                Hr_port_h = 0;
                break;
                case 2 :
                PORTB = hr_l;
                Hr_port_l = 1;
                Hr_port_l = 0;
                break;
                case 3 :
                PORTB = min_h;
                Min_port_h = 1;
                Min_port_h = 0;
                break;
                case 4 :
                PORTB = min_l;
                Min_port_l = 1;
                Min_port_l = 0;
                break;
                case 5 :
                PORTB = sec_h;
                Sec_port_h = 1;
                Sec_port_h = 0;
                break;
                case 6 :
                PORTB = sec_l;
                Sec_port_l = 1;
                Sec_port_l = 0;
                break;
        }
}
void blink_digit (uns8 digit)
{
        switch (digit)
        {
                case 1 :
                PORTB = 0xFF;
                Hr_port_h = 1;
                Hr_port_h = 0;
                Delay_ms (100);
                display (1);
                Delay_ms (100);
                break;
                case 2 :
                PORTB = 0xFF;
                Hr_port_l = 1;
                Hr_port_l = 0;
                Delay_ms (100);
                display (2);
                Delay_ms (100);
                break;
                case 3 :
                PORTB = 0xFF;
                Min_port_h = 1;
                Min_port_h = 0;
                Delay_ms (100);
                display (3);
                Delay_ms (100);
                break;
                case 4 :
                PORTB = 0xFF;
                Min_port_l = 1;
                Min_port_l = 0;
                Delay_ms (100);
                display (4);
                Delay_ms (100);
                break;
                case 5 :
                PORTB = 0xFF;
                Sec_port_h = 1;
                Sec_port_h = 0;
                Delay_ms (100);
                display (5);
                Delay_ms (100);
                break;
                case 6 :
                PORTB = 0xFF;
                Sec_port_l = 1;
                Sec_port_l = 0;
                Delay_ms (100);
                display (6);
                Delay_ms (100);
                break;
        }
}
void set_time ()
{

        i = 1;
        while (setting_time)
        {
                blink_digit (i);
                while (SET == 0)
                {
                        Delay_ms (5);
                        switch (i)
                        {
                                case 1 :
                                hr_h ++;
                                if (hr_h > 2)
                                {
                                        hr_h = 0;
                                }
                                break;
                                case 2 :
                                hr_l ++;
                                if (hr_l > 9)
                                {
                                        hr_l = 0;
                                }
                                if (hr_h >= 2 && hr_l > 3)
                                {
                                        hr_l = 0;
                                }
                                break;
                                case 3 :
                                min_h ++;
                                if (min_h > 5)
                                {
                                        min_h = 0;
                                }
                                break;
                                case 4 :
                                min_l ++;
                                if (min_l > 9)
                                {
                                        min_l = 0;
                                }
                                break;
                                case 5 :
                                sec_h ++;
                                if (sec_h > 5)
                                {
                                        sec_h = 0;
                                }
                                break;
                                case 6 :
                                sec_l ++;
                                if (sec_l > 9)
                                {
                                        sec_l = 0;
                                }
                                break;
                        }
                        while (SET == 0)
                        {
                                Delay_ms (5);
                        }
                }
                while (MODE == 0)
                {
                        Delay_ms (5);
                        i ++;
                        if (i > 6)
                        {
        sec_l--;
        TMR1H = 0x80;
        TMR1L = 0x00;
        PIE1.TMR1IE = 1;
        setting_time = 0;
                                break;
                        }
                        while (MODE == 0)
                        {
                                Delay_ms (5);
                        }
                }
        }
}
void check_bt ()
{
        myTimer = 0;
        if (setting_time == 0)
        {
                while (MODE == 0)
                {
                        Delay_ms (5);
                        myTimer ++;
                        if (myTimer > 200)
                        {
                                setting_time = 1;
                                myTimer = 0;
                                break;
                        }
                }
        }
        while (MODE == 0)
        {
                PIE1.TMR1IE = 0;
                //Stop clock
                Delay_ms (5);
                blink_digit (1);
        }
        set_time ();
}

I don't have the picture of the prototype as I haven't made it yet. But I have tested the circuit and firmware with proteus already.

--- Update ---
- The prototype of this updated version is done. Please see its photographs at 7-Segment PIC Digital Clock : The photographs
- The PCB is ready: please check out PCB for PIC Digital Clock