Accessing bits of variable, XC8

Ian Rogers

User Extraordinaire

Forum Supporter

Most Helpful Member

• Jul 25, 2017

• #21

All that volatile does, is to let the optimiser skip over the declaration.. This way the compiler lets it stay where you put it... If, for example, you are testing but not actually using the variable, the optimiser will just leave it out!! Very annoying whilst trying to debug!!

If you are using the free version, then forget the volatile bit as there is no / little optimisation done..

 

Ian Rogers

User Extraordinaire

Forum Supporter

Most Helpful Member

• Jul 25, 2017

• #22

Also... I never use true and false... Just use 0 and 1..

 

TucsonDon

Member

• Jul 25, 2017

• #23

I tried your suggestion to assign it to an address and the compiler did that but in the main code it still doesn't call the function when the bit is changed. I looked @ the program memory for the "if" statement and it shows MOVF tstatus, W, ACCESS. If I understand this it is looking @ the whole byte and not the bit. I have tried to mask i.e. if (tstatus.a &0x01 ==true)call function ,but it still will not execute that line.

 

TucsonDon

Member

• Jul 25, 2017

• #24

1

 

Last edited: Jul 25, 2017

Ian Rogers

User Extraordinaire

Forum Supporter

Most Helpful Member

• Jul 25, 2017

• #25

Can I see your code? I can run it through my sim...

 

TucsonDon

Member

• Jul 25, 2017

• #26

I figured it out. When I declared the bits I was doing

bool a:0;
bool b:0;
ect.....

so apparently the 0 is not initializing the bit as I thought, as I have change them all to 1's and now it works.

 

Last edited: Jul 25, 2017

Ian Rogers

User Extraordinaire

Forum Supporter

Most Helpful Member

• Jul 26, 2017

• #27

The whole point of creating a union is to tell the compiler what is where, you can have groups of bits 1 ~ 7

C:

union{
   struct{
   unsigned flags:3;
   unsigned over:1;
   unsigned startup:1;
   unsigned mode:2;
   };
} mystuf;

So any thing that has 7 states can fit into the flags.... mystuf.flags = 6;

 

TucsonDon

Member

• Jul 26, 2017

• #28

Ian Rogers

Thank you for your assistance and patience with this I have learned a lot.