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A bit in a byte of an array

 
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rovtech



Joined: 24 Sep 2006
Posts: 262

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A bit in a byte of an array
PostPosted: Thu Apr 25, 2013 1:03 pm     Reply with quote

I am trying to access a bit in a byte of a char array.
The following works:

Code:

// incoming packets
   char packet [5];                          // packets to receive
  #define ackn          (packet [0])
  #define status        (packet [1])    // compass
  #define bat_volts    (packet [2])
  #define bat_amps   (packet [3])
  #define spare         (packet [4])
  status = 0x3C;                            // initialize to NO COMPASS
// compass signals
  char hdg_att;
  hdg_att     = status;
  #bit sp1    = hdg_att.0
  #bit sp2    = hdg_att.1
  #bit south = hdg_att.2
  #bit east   = hdg_att.3
  #bit north = hdg_att.4
  #bit west  = hdg_att.5
  #bit sp3   = hdg_att.6
  #bit sp4   = hdg_att.7


I would like to use the following but the compiler gives an error.
Is there a better way to do this?

Code:

// incoming packets
   char packet [5];                          // packets to receive
  #define ackn          (packet [0])    // acknowledge
  #define status        (packet [1])    // SROV Status, compass for now
  #define bat_volts    (packet [2])    // SROV Raw Battery voltage
  #define bat_amps   (packet [3])    // SROV Raw Battery current
  #define spare         (packet [4])    // Spare
  status = 0x3C;                            // initialize to NO COMPASS
// compass signals
  #bit sp1    = status.0
  #bit sp2    = status.1
  #bit south = status.2
  #bit east   = status.3
  #bit north = status.4
  #bit west  = status.5
  #bit sp3   = status.6
  #bit sp4   = status.7


The use is:
Code:

  if (south)
// do the following
Ttelmah



Joined: 11 Mar 2010
Posts: 19540

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PostPosted: Thu Apr 25, 2013 1:30 pm     Reply with quote

Just use bit_test.
Code:

#define SP1 0
#define SP2 1
#define SOUTH 2
#define EAST 3
#define NORTH 4
#define WEST 5
#define SP3 6
#define SP4 7

if (bit_test(status,SOUTH))


#bit can _only_ map to a fixed location.

Best Wishes
rovtech



Joined: 24 Sep 2006
Posts: 262

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a bit in a byte of an array
PostPosted: Sat Apr 27, 2013 5:30 pm     Reply with quote

Thanks Ttelmah, I was hoping for something that would read easier.

if (NORTH) is much easier than if (bit_test(status, NORTH))

My method that works (first example) is nice except for having to equate the array byte to another variable. Why is the array variable "status" different from a "simple" variable "hdg_att", apart from being an element in an array.

The manual says that #bit id = x.y where id is a valid C identifier, x is a constant or a C variable, and y is a constant 0-7. However the error when I try the second example is that x must be a "simple variable".

I will try your suggestion and see if it works. bit_test may also require a simple variable.
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Sat Apr 27, 2013 8:40 pm     Reply with quote

You can always use #define statements to make the code look like
whatever you want. Example:
Code:

#include <16F877.H>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock=4M)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)

char packet[5];

#define status (packet[1])

#define North (bit_test(status, 4))

//==========================================
void main()
{
 
if(North)
   printf("N");
else
   printf("?");

while(1);
}
andrewg



Joined: 17 Aug 2005
Posts: 316
Location: Perth, Western Australia

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PostPosted: Sat Apr 27, 2013 8:46 pm     Reply with quote

Why not bit fields?:
Code:
struct {
  int8 ackn;
  int8 sp1:1;
  int8 sp2:1;
  int8 south:1;
  int8 east:1;
  int8 north:1;
  int8 west:1;
  int8 sp3:1;
  int8 sp4:1;
  int8 bat_volts;
  int8 bat_amps;
  int8 spare;
} packet;

// ...

if (packet.north) // ...

_________________
Andrew
Ttelmah



Joined: 11 Mar 2010
Posts: 19540

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PostPosted: Sun Apr 28, 2013 2:05 am     Reply with quote

Yes. The key point is that #bit, has to be locatable at compile time.
bit fields should work, but I'd probably just use a #define. It has the feeling of being 'safer', knowing the compiler....

Best Wishes
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