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ccp take over problem

 
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Blob



Joined: 02 Jan 2006
Posts: 75
Location: Neeroeteren, Limburg, Belgium

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ccp take over problem
PostPosted: Thu May 12, 2011 6:29 am     Reply with quote
Hello All,

On my rotating disc project, I came across a new problem.

To summarise: I have a rotating disc with 18 teeth on it.
One of the teeth is slightly different. With a magnetic pick-up and a little circuit I make a square wave of the input signal

In a polling loop I determine the position of the flywheel in relation to the pick-up.

On every position of the flywheel I will have to do different things, reading sensors, setting outputs calculate delays etc.

Now the new problem is as following.


The flywheel will turn one rotation in mode 1 and one in mode 2 than back mode 1, mode 2,...

There is an output that I have to set like a kind of pwm.
According to the rpm and other inputs, it will be activated from 0.25 to 10 ms each 2 rotations.

There are however some restrictions to the pulse.
When the pulse is short it must start at position 16 and it may prolong till position 4.

No problem here, I set my output high at position 16
and use
Code:

if(mode == 2)
  setup_ccp1(CCP_USE_TIMER3 | CCP_COMPARE_CLR_ON_MATCH);

to clear it.

But when the value of CCP exceeds 120° (from position 16 to 4 = 120°)
The output should be setted before position 16.

In fact the output pulse will first grow to later side of the rotation and when it reaches position 4 it will grow to the earlier side of the rotation.

At high rpm it can even be set in mode 2 and staying high till position 4 in the next mode 2.

To solve this I use CCP the other way around, instead of clearing the output, I will set it.

The problem in using CCP is that I can not have a timer overflow.

So at low rpm I use timer 3 DIV_BY_8, at high rpm DIV_BY_4.

I calculate the duration of 1 rotation.
@ position 4
If my output duration is less than 1 rotation, I will set CCP in mode 1 cycle
like this:
Code:

if(mode == 1)
{
CCP_1 = t_rotation - out_time;
setup_ccp1(CCP_USE_TIMER3 | CCP_COMPARE_SET_ON_MATCH);
}
else
{
setup_ccp1(CCP_OFF);
output_low(output);
}


If my output duration is higher that 1 rotation, I set CCP like this:
Code:
if(mode == 2)
{
output_low(output);
CCP_1 = 2*t_rotation - out_time;
setup_ccp1(CCP_USE_TIMER3 | CCP_COMPARE_SET_ON_MATCH);
}


The take over from ccp - set from mode 1 to mode 2 works fine.

But the take over from ccp-clear at position 16 to ccp_set at position 4 gives an error output at changing point, before and after this point it is ok.

I tried to set a boolean high at position 4 when t_out > 120°.
This disables the ccp at postition 16.

Is there anything to overcome this interference?

Thanks,
BLOB
Blob



Joined: 02 Jan 2006
Posts: 75
Location: Neeroeteren, Limburg, Belgium

View user's profile Send private message

PostPosted: Fri May 13, 2011 2:05 am     Reply with quote
Got it,

I made a fail safe solid state like structure around the modes to prevent ccps getting mixed up.

Best regards,
Blob
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