Hello

Thanks for much of your comments regarding the PWM.  I have finally
found the problem with changing of the PWM frequency.  I believe it to
be a device flaw.  However please look at both code examples below and
you will see what I am talking about.  Both are logically correct, but
only one works.  Any ideas????

The problem did lay in the changing of the timer2 prescale value.

The code that does not work, but is logically correct for setting the
prescale for timer2:

SetTrickel:
1          ;1, set the period register.
2          MOVFF TRICKEL_FREQ_PERIOD,PR2
3          ;2. Write the duty cycle
4          MOVFF TRICKEL_DUTY_H,CCPR1L
5          MOVLW            B'11001111'
6          ANDWF            CCP1CON,W,A
7          IORWF TRICKEL_DUTY_L,W,BANKED
8          MOVWF           CCP1CON,A
9          ;3. Set prescale
10         MOVLW            B'00000011'
11         ANDLW            0X03                 ;Prescale bit for 16
12         IORLW  0xfc
13         ANDWF            T2CON,F,A
14         ;           BCF                  T2CON,T2CKPS0,A
15         ;           BSF                  T2CON,T2CKPS1,A
16         ;4. Turn timer 2 on and configure for PWM mode.
17         BSF                  T2CON,TMR2ON,A
18         RETURN

The above logic (Lines 9-13) set the prescale using a bit masking method
as follows
10.        Sets bits 0 and 1 high into working register.
11.        Masks out bits 2-7.  Normally this would be done to a
register variable, but I have to test with a constant to ensure that
there was in fact a flaw.
12.        Adds masking bits 2-7 to the working register while keeping
the previous lower bits 0 and 1 content.
13.        Takes the masking bits in working register and removes all
unwanted 1's from T2CON.

This logic is a correct method, but does not work.  Now below is another
method which does work.

SetTrickel:
1          ;1, set the period register.
2          MOVFF TRICKEL_FREQ_PERIOD,PR2
3          ;2. Write the duty cycle
4          MOVFF TRICKEL_DUTY_H,CCPR1L
5          MOVLW            B'11001111'
6          ANDWF            CCP1CON,W,A
7          IORWF TRICKEL_DUTY_L,W,BANKED
8          MOVWF           CCP1CON,A
9          ;3. Set prescale
10         ;MOVLW           B'00000011'
11         ;ANDLW           0X03                 ;Prescale bit for 16
12         ;IORLW 0xfc
13         ;ANDWF           T2CON,F,A
14         BCF                  T2CON,T2CKPS0,A
15         BSF                  T2CON,T2CKPS1,A
16         ;4. Turn timer 2 on and configure for PWM mode.
17         BSF                  T2CON,TMR2ON,A
18         RETURN

Now this above code comments out lines 10-13 and uncomments lines 14 and
15.  Line 14 clears bit 0 of T2CON.  Line 15 Sets bit 1 of T2CON.  These
two bits are hex equivalent of line 11's result of 0x03, correct!  Also
bits 2-7 of T2CON remain unchanged, which is the desired effect of the
bit masking logic in the first example.

Now,  why is it that using bit instructions makes the entire code work,
but on the other hand using bit masking logic and ANDing the contents of
T2CON with the mask does not work.

This is either a flaw or I have missed something all together.

Any clues.  You have to agree with my sanity here.  I was going crazy
over this logic.  I know that the masking logic is correct.  So why does
only bit twiddling work???????

One further comment, in example 1, which did not work, I substituted
line13 with the following two lines of code in the hopes that this had
something to due with it:

13.        ANDWF            T2CON,W,A      ;Make the result go back into
the working register.
13a.      MOVWF           T2CON,A          ;Move the working into
register T2CON.

Regards,


James


--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body