A PWM Algorithm Realized With Only One Add Instruction

 What's wrong?

MATLAB shows the duty cycle is 50% for N=2 and H=1:

>>

npulse = 8;

N = 2;

H = 1;

CNT = 0;

disp( '     k  Pout for init. CNT=0' )

for k = 1:npulse

  CNT = CNT + H;

   if CNT >= N

   CNT=mod(CNT,N) ;

   Pout = 1 ; 

 else 

     Pout = 0 ;

   end

  disp( [k Pout] )

end

     k  Pout for init. CNT=0

     1     0

     2     1

     3     0

     4     1

     5     0

     6     1

     7     0

     8     1

>> ...

     k  Pout for init. CNT=1

     1     1

     2     0

     3     1

     4     0

     5     1

     6     0

     7     1

     8     0

 Hi H L,

I don't think there is any difference between the mod() and the %-operator.

The debug result (see attached screen copy) of following programm runing in a real CPU with the %-operator

shows the same duty cycle of 50%.

void main()

{

uint16 N = 2;

uint16 H = 1;

uint16 CNT = 1;

uint8 Pout[8];

uint16 k=0;

 do {

    CNT += H;

  if (CNT >= N)

 { CNT %= N ; Pout[k] = 1 ; }

    else Pout[k] = 0 ; 

k++;

 }while(k<8);

    for(;;) {    }

}

May the defferent compiler cause the defferent result of %-operator ?

http://files.chinaaet.com/images/blog/2013/07/24/442064405399.jpg

Sorry for misunderstanding your meaning.

Is OK for following sentanses?

 CNT += H;

    if (CNT >= N)

 { Pout = 1 ;  CNT %= N ;}

   else Pout = 0 ; 

or

 CNT += H;

    if (CNT >= N) Pout = 1 ;

   else Pout = 0 ; 

 CNT %= N ;

But maybe some one says : the if-branch and the else-branch use different numbers of CPU instructions.

And even in hardware imlemention with datapath there still has the difference of a few nano-seconds 

caused by the difference between the pulse rise time and the pulse fall time. The duty cycle is not 50% in absolutely accurate meaning.

In my blog above, i just give a principle of the algorithm in style of C. In the real projects, I usualy use assembly language for software realization, and always pay attantion to minimazing the number of instructions and the balance of instruction cycles between branches.

As i declared in my chinese blog, for the convinience of reading i do not use the assembly language to describe the algorithm.

Thank you for your precision of CPU instruction number level.

Any more difference between us ?

 All discussions above are helpful for me.

I found that my poor English is not enough to express my opinion correctly.

To avoid misunderstanding I will finish this topic on Cypress Forum.

Before finishing I’ll give a summary.

 Summary

1. History

My software-PDM was first used for DC-motor speed control in MCS-51 system since 1990 year and even earlier.

Some codes for 3 PDM-outputs were simply as following:

                            Timer_ISR:

                                ...

                                ;1 Timer for 3 PWM.

                                ;Operation interval is determined by timer with higher priority.

                                ;Default N=256, no mod-operation is needed.

                                ;Each PDM-operation takes only a few CPU instruction cycles.

                                MOV  A,CNT1

                                ADD  A,H1

                                MOV  CNT1,A

                                MOV  Px.1,C

                                ;

                                MOV  A,CNT2

                                ADD  A,H2

                                MOV  CNT2,A

                                MOV  Px.2,C

                                ;

                                MOV  A,CNT3

                                ADD  A,H3

                                MOV  CNT3,A

                                MOV  Px.3,C

                                ...

                                RETI                          

Why not hardware PWM?

No any hardware PWM could be used in that years, and the MCS-51 MCU had even no Timer2 at that time.