AT89S52 Mini Development Board – Servo Motor Interfacing

Servos have simple electrical interface – they have 3 wires, one for power, one for ground and the other for the pulse train. Once the servo is powered, the signal wire is ready to receive signal in the form of pulse width modulation (PWM) from an external source.
The signal expects to be updated every 20 ms with a pulse between 0.5ms and 2.5ms. With a 1.5 ms pulse, the servo motor will be at the natural 90° position. With a 0.5 ms pulse, the servo will be at the 0° position, and with a 2.5 ms pulse, the servo will be at 180°. You can obtain the full range of motion by updating the servo with a value in between.

AT89S52 mini development board has a dedicated connector K13 to interface Servo motor. Regulated 5V is available on power supply pin and the signal pin is connected at the port pin P2.3. Sample code to check servo motor is given below, Upon code execution, servo turns each angle.

Note: Use adapter for powering since USB doesn’t provide required current

Schematic

Sample Code

/*************************************************************************
                           HEADER FILE
**************************************************************************/
#include <REGX52.H>
#define servo P2_3 

/*************************************************************************
                       VARIABLE DECLARATIONS
**************************************************************************/
int value=0,i=0;
char rec_data;
int count=0;

/*************************************************************************
                           MAIN FUNCTION
**************************************************************************/
void main()
{
 TMOD = 0x21; 	
 SCON = 0X50;           /* 8-BIT UART, Serial reception enable           */
 TF1  = 0;              /* Clear Timer 1 Overflow Flag                   */
 TR1  = 0;              /* Clear TR1 to turn off timer 1                 */
 TF0  = 0;              /* Clear Timer 0 Overflow Flag                   */
 TR0  = 0;              /* Clear TR0 to turn off timer 0                 */
 TH1  = 0XFD;           /* Load value to Timer 1 High Byte Register      */
 TL1  = 0XFD;           /* Load value to Timer 1 Low Byte Register       */
 TH0  = 0xFE;           /* Load value to Timer 0 High Byte Register      */
 TL0  = 0x32;           /* Load value to Timer 0 Low Byte Register       */
 TR1  = 1;              /* Set TR1 to turn on timer 1                    */
 TR0  = 1;              /* Set TR0 to turn on timer 0                    */		
 TI   = 0;

 value = 0;

 while(1)
 {
  if(TF0==1)            /* Set when timer0 register overflows            */
  {
   TF0 = 0;
   TH0 = 0xFE;          /* Load value to Timer 0 High Byte Register      */
   TL0 = 0x3d;          /* Load value to Timer 0 Low Byte Register       */

   count++;             /* Increment count each time the timer overflows */
   if(count==value)
   {
    servo = 0;  
   } 
   if(count==40)        /* When count is 40, time duration = 20ms        */
   {
    servo = 1;
    count = 0;
   }  
  }

  if(RI==1)
  {
   RI = 0;
   rec_data = SBUF;

   if(rec_data=='a')  
         value = 3;     /* Load count for 90° when 'a' is received       */

   if(rec_data=='b')  
         value = 5;     /* Load count for 180° when 'b' is received      */
   if(rec_data=='c')   
         value = 1;     /* Load count for 0° when 'c' is received        */
  }
 }
}

/***************************  END OF PROGRAM  ****************************/

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

HEADER FILE

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

#include <REGX52.H>

#define servo P2_3

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

VARIABLE DECLARATIONS

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

int value=0,i=0;

char rec_data;

int count=0;

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

MAIN FUNCTION

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

void main()

{

TMOD = 0x21;

SCON = 0X50; /* 8-BIT UART, Serial reception enable */

TF1 = 0; /* Clear Timer 1 Overflow Flag */

TR1 = 0; /* Clear TR1 to turn off timer 1 */

TF0 = 0; /* Clear Timer 0 Overflow Flag */

TR0 = 0; /* Clear TR0 to turn off timer 0 */

TH1 = 0XFD; /* Load value to Timer 1 High Byte Register */

TL1 = 0XFD; /* Load value to Timer 1 Low Byte Register */

TH0 = 0xFE; /* Load value to Timer 0 High Byte Register */

TL0 = 0x32; /* Load value to Timer 0 Low Byte Register */

TR1 = 1; /* Set TR1 to turn on timer 1 */

TR0 = 1; /* Set TR0 to turn on timer 0 */

TI = 0;

value = 0;

while(1)

{

if(TF0==1) /* Set when timer0 register overflows */

{

TF0 = 0;

TH0 = 0xFE; /* Load value to Timer 0 High Byte Register */

TL0 = 0x3d; /* Load value to Timer 0 Low Byte Register */

count++; /* Increment count each time the timer overflows */

if(count==value)

{

servo = 0;

}

if(count==40) /* When count is 40, time duration = 20ms */

{

servo = 1;

count = 0;

}

if(RI==1)

{

RI = 0;

rec_data = SBUF;

if(rec_data=='a')

value = 3; /* Load count for 90° when 'a' is received */

if(rec_data=='b')

value = 5; /* Load count for 180° when 'b' is received */

if(rec_data=='c')

value = 1; /* Load count for 0° when 'c' is received */

}

/*************************** END OF PROGRAM ****************************/

OutputThe following screenshots explain how to test the sample code.

Step 1: Open Realterm

Step 2: RealTerm opens as shown below

Step 3: Go to ‘Port’ option, set correct baudrate (which is set as 9600 in the sample code) and correct port

Step 4: Click ‘Change’ (encircled in red) to apply the changes. Now check the status of Port. If it is closed, click ‘Open’ button (encircled in green) to open it.

Step 5: Type the command to transmit in the corresponding data box and then click over Send ASCII as shown below .The transmitted data available in black window , as per this servo turns.