Question

Port in Embedded C

Asked by: viralss

Hi
Can someone please help me out greatly and explain the following piece of code to me?  Of most interest I suppose is the line if ((Value >>i)&0x01).  What does that mean exactly?  

I assume it has something to do with a port that I am setting up.  I need to set up another line of communication over the same RS232 port and was thinking this would be a good place to start.

Thanks

void SetInBitsInterruptMode(MIL_ID MilSystem, int Value)
{
   int NbBits=8,Offset=0;
 
   for (int i=0; i<NbBits; i++)
   {
       if ((Value>>i)&0x01)
           MsysControl(MilSystem, M_USER_BIT_INTERRUPT_STATE + i +Offset, M_ENABLE);
       else
           MsysControl(MilSystem, M_USER_BIT_INTERRUPT_STATE + i +Offset, M_DISABLE);
   }
}

                                  
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Asked On
2009-08-21 at 08:47:56ID24671761
Tags

Embedded C

,

Ports

,

MIL

,

Matrox

Topics

Embedded Hardware

,

C Programming Language

Participating Experts
2
Points
500
Comments
6

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Answers

 

by: Anthony2000Posted on 2009-08-21 at 09:17:24ID: 25153350

See my comments below, I hope this helps.


void SetInBitsInterruptMode(MIL_ID MilSystem, int Value)
{
   int NbBits=8,Offset=0;
 
   for (int i=0; i<NbBits; i++)
   {
       // Value is shifted each time through this loop, i times
       // basically the Value passed in is looked at one bit at 
       // time. If the bit is set, then MsysControl is call with 
       // the value of i (bit position) and notice the M_ENABLE
       // otherwise if the bit is 0, then M_DISABLE
 
       if ((Value>>i)&0x01)
           MsysControl(MilSystem, M_USER_BIT_INTERRUPT_STATE + i +Offset, M_ENABLE);
       else
           MsysControl(MilSystem, M_USER_BIT_INTERRUPT_STATE + i +Offset, M_DISABLE);
   }
}
                                              
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by: evilrixPosted on 2009-08-21 at 09:32:03ID: 25153464

Observation. This....

       if ((Value>>i)&0x01)
           MsysControl(MilSystem, M_USER_BIT_INTERRUPT_STATE + i +Offset, M_ENABLE);
       else
           MsysControl(MilSystem, M_USER_BIT_INTERRUPT_STATE + i +Offset, M_DISABLE);


...can be simplified to this...

       MsysControl(MilSystem, M_USER_BIT_INTERRUPT_STATE + i +Offset, ((Value>>i)&0x01) ? M_ENABLE : M_DISABLE);

 

by: Anthony2000Posted on 2009-08-21 at 09:39:53ID: 25153536

In case it is important, the bits in Value are shifted to the least significant bit position (usually known as bit 0).

if an int is 16 bits then
and  Value = 0110000000000001

when i is equal to 0, then the bit on the right will be looked at first.

Are you clear now?

 

by: viralssPosted on 2009-08-23 at 11:34:18ID: 25163666

Mmmm no not really.  I understand the bit shifting idea but the line if ((Value >>i)&0x01) means what? why the ampersand and why the 0x01?  Can you explain what the function is up to in general?  It makes no real sense to me at all.  Thanks

And thanks for your suggestion evilrix

 

by: evilrixPosted on 2009-08-23 at 13:06:40ID: 25164039

The & in C/C++ is a binary/bitwise 'and' operator. It means if bit A 'and' bit B are the 1 then the result is 1 else the result is 0. The 0x01 means bit 0; bits are number from 0, with bit 0 being the least significant bit. If it's set it represents a 1 and if it's not it represents a 0. So to test if bit 0 (the first bit) is set you just and a number with 0x01 and if the result is 1 it's set and if the result is 0 it's not.

For example, say Value contains 01010101 when you and it with one you get this...

01010101
00000001 &
-------------
00000001 < --- the result is 1 so the first bit is set

Now what about 10101010...

10101010
00000001 &
-------------
00000000 < --- the result is 0 so the first bit is NOT set

So all the function is doing is waling each bit to see if it's set  and if it is the function is called with M_ENABLED otherwise it's called with M_DISABLED

It might be simpler to think of the byte as an array of bools

This is the "truth table" for binary AND
A    B
0 & 0==0
0 & 1==0
1 & 0==0
1 & 1==1

So A and B must be 1 for the result to be 1.

http://en.wikipedia.org/wiki/Binary_numeral_system
http://en.wikipedia.org/wiki/Bitwise_operation#AND
http://msdn.microsoft.com/en-us/library/z0zec0b2.aspx

 

by: viralssPosted on 2009-08-23 at 14:46:08ID: 25164418

Awesome thats exactly what I was looking for, thanks!!

20120131-EE-VQP-002

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