M1 is a stepper taken from an old disk drive. There are five pins, i.e., common, coil 1, 2, 3 and 4. Resistance measured between common pin and each coil is about 75 Ohms. Driving current for each coil is then needed about 60mA at +5V supply. A darlington transistor array, ULN2003 is used to increase driving capacity of the 2051 chip. Each output provides 500mA max at 50V. P1.4 to P1.7, four output pins are connected to the input of the ULN2003 as shown in the circuit. Four 4.7k resistors help the 2051 to provide more sourcing current from the +5V supply. The serial port is optional for your exercises. Many have provided useful technical, and application of using stepper, see the links below.

stepper.c
stepper.hex

I have changed a 10ms time base with a simple TF0 polling instead of using interrupt. The program is just to send the stepper's energizing pattern to the P1 every 10ms. Flag1 is used for intertask communication.

/*
 * STEPPER.C
 * sweeping stepper's rotor cw and cww 400 steps
 * Copyright (c) 1999 by W.Sirichote
 */

#include c:\mc51\8051io.h  /* include i/o header file */
#include c:\mc51\8051reg.h

register unsigned char j,flag1,temp;
register unsigned int cw_n,ccw_n;

unsigned char step[8]={0x80,0xc0,0x40,0x60,0x20,0x30,0x10,0x90}
#define n 400

/* flag1 mask byte
   0x01  run cw()
   0x02  run ccw()
*/

main()

{
  flag1=0;
  serinit(9600);
  disable();  /* no need timer interrupt */
  cw_n = n;    /* initial step number for cw */
  flag1 |=0x01; /* initial enable cw() */

 while(1){
  {
    tick_wait();  /* wait for 10ms elapsed */

    energize();   /* round-robin execution the following tasks every 10ms */
    cw();
    ccw();
  }
        }

}

cw(){
       if((flag1&0x01)!=0)
       {
        cw_n--;       /* decrement cw step number */
        if (cw_n !=0)
         j++;         /* if not zero increment index j */
         else
         {flag1&=~0x01; /* disable cw() execution */
          ccw_n = n;    /* reload step number to ccw counter */
          flag1 |=0x02; /* enable cww() execution */
          }
       }

}

ccw(){
       if((flag1&0x02)!=0)
       {
        ccw_n--;       /* decremnent ccw step number */
        if (ccw_n !=0)
         j--;          /* if not zero decrement index j */
         else
         {flag1&=~0x02; /* disable ccw() execution */
          cw_n = n;     /* reload step number to cw counter */
          flag1 |=0x01; /* enable cw() execution */
          }
       }

}

tick_wait(){   /* cputick was replaced by simpler ASM code 10ms wait */

    asm" JNB TCON.5,*";   /* wait for TF0 set */
    asm" CLR TCON.5";     /* clear TF0 for further set */
    asm" ORL TH0,#$DC";   /* reload TH0 with $DC, TL0 = 0 */
}

energize(){

    P1 = step[(j&0x07)];  /* only step 0-7 needed */
}

• change the speed showing the rotation becomes faster or slower than 10ms time delay.
• with an additional serial port, write an initializing function that receives ascii command from terminal to set the number of step for cw and ccw, say.
• while energizing the stepper coils, write a function that read ascii character from terminal on the fly to decrease or increase rotating speed, say.

• Jones on Stepping Motors Jones provides a tutorial covering the basic of stepping motors and control systems, including physics of the motor, circuits and software for motor control.
• Basic Stepper Motor Concepts Understanding electromagnetic principle underlying the stepper.
• Motorize Your Telescope Mel Bartels describes how to motorize telescope, many links for amateur astronomy.