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ROBOTICAR
Joined: 28 Aug 2007
Posts: 45
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| problem with #define !!!!!! |
 Posted: Sat Mar 26, 2011 11:52 pm |
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Hi
I read the "Embedded C programming and the microchip PIC" book, - the bits and bitfields part - and I have problem with some part.
It use bitfields for describing I/O port, for example instead of output_high(PIN_D0) it use this code :
| Code: |
////////////////////////PIC 18F452/////////////////////////////////
typedef struct P{
unsigned int bit_0 : 1;
unsigned int bit_1 : 1;
unsigned int bit_2 : 1;
unsigned int bit_3 : 1;
unsigned int bit_4 : 1;
unsigned int bit_5 : 1;
unsigned int bit_6 : 1;
unsigned int bit_7 : 1;
}bits;
#define PORTD (*(bits *)0xf83)
void main()
{
set_tris_d(0);
PORTD.bit_0=1;
}
////////////////////////////////////////////////////////////////////
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This code works very well. But I could not understand what
"#define PORTD (*(bits *)0xf83)" means??? !!!!!! specially *(bits *) !!!
(I know 0xf83 is the address of PORTD register in pic18f452.)
Could someone explain this code for me ? (+please  )
THANKS. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sun Mar 27, 2011 12:40 am |
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It casts a pointer to a structure of type 'bits' for the byte at address
0xF83, and then takes the contents of it. |
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n-squared
Joined: 03 Oct 2006
Posts: 99
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| Posted: Sun Mar 27, 2011 1:53 am |
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Hi
I use a different method for addressing bits in SFR's. This method stems from CCS's way of producing register definition files.
You define the register using:
| Code: |
#byte PORTD = 0xf83
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and define the bits as:
| Code: |
#bit PORTD0 = PORTD.0
.
.
#bit PORTD7 = PORTD.7
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You then use the ports bits like this:
| Code: |
if (!PORTD3 && PORTD0)
do_something();
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Please note that in PIC18, PIC24 and PIC3x you have to use LATx registers for output operations, and not PORTx as you did in your example.
See section "9.0 I/O PORTS" in PIC18FXX2 data sheet.
Another safe method is to use the compiler's output_xxx(PIN_Dx); functions.
BR
Noam
_________________
Every solution has a problem. |
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demonspells
Joined: 06 Jan 2011
Posts: 26
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| Posted: Sun Mar 27, 2011 10:07 am |
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I think structs are easier to handle
there is an easier way to handle a certain port or register
it goes like this
| Code: | struct {
int1 F0;
int1 F1;
int1 F2;
int1 F3;
int1 F4;
int1 F5;
int1 F6;
int1 F7;
} LATB;
#byte LATB = 0xF8A |
then you can set/reset each pin like this
| Code: | LATB.F0 = 0; // Reset pin B0
LATB.F6=1; // Set pin B6
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_________________
Badr Ghatasheh |
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bkamen
Joined: 07 Jan 2004
Posts: 1615
Location: Central Illinois, USA
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| Posted: Sun Mar 27, 2011 10:28 am |
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| n-squared wrote: |
Please note that in PIC18, PIC24 and PIC3x you have to use LATx registers for output operations, and not PORTx as you did in your example.
See section "9.0 I/O PORTS" in PIC18FXX2 data sheet.
Another safe method is to use the compiler's output_xxx(PIN_Dx); functions.
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Well, no -- he CAN use PORTx, but then he can run into read-modify-write issues when reading back the PORT to see what value it is.
LATx is the best way though.
Also, I would avoid stuff like:
| Code: |
#define PORTD (*(bits *)0xf83)
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Instead, use
| Code: |
#byte PORTD = getenv("sfr:LATD")
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Now when you change the device, PORTD follows the address of LATD according to the compiler's database for the device.
-Ben
_________________
Dazed and confused? I don't think so. Just "plain lost" will do. :D |
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