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Writing to lower 8 bits of a 16 bit port

 
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pebbert9



Joined: 31 Dec 2010
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Writing to lower 8 bits of a 16 bit port
PostPosted: Sat Feb 12, 2011 1:39 am     Reply with quote
I am trying to use bits 0-7 of port D as an 8-bit data buss. and the upper bits for address. ( My part is a PIC24H without a PMP port.)

Is there an easy way to write a byte to port D without affecting the upper 8 bits?

output_d(0x00) will set the entire port ( bits 0-15) to zero, not just bits 0-7.

I can set the bits individually, but that isn't too efficient.

Thank you,
FvM



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PostPosted: Sat Feb 12, 2011 2:50 am     Reply with quote
By accessing the SFRs directly:
Code:
#word LATD  = 0x2DC
#byte LATDL = 0x2DC
#byte LATDH = 0x2DD
//e.g.
LATDL = 0x34;

TRISx can defined accordingly, if required
pebbert9



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PostPosted: Sun Feb 13, 2011 12:59 am     Reply with quote
Thanks, that worked great.

BTW, I did have to manually set TRISD for it to work ( and LATD is 0x2D6 for my chip )
FvM



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PostPosted: Sun Feb 13, 2011 2:37 am     Reply with quote
Quote:
BTW, I did have to manually set TRISD for it to work

It's one of several ways. You can also use set_tris_d() to initialize the port as a whole.
bkamen



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PostPosted: Sun Feb 13, 2011 8:33 am     Reply with quote
FvM wrote:
By accessing the SFRs directly:
Code:
#word LATD  = 0x2DC
#byte LATDL = 0x2DC
#byte LATDH = 0x2DD
//e.g.
LATDL = 0x34;

TRISx can defined accordingly, if required


You can also build a structure/union and overlay that on top of the port.

I also like "getenv" now instead of hard address assignments...
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mbradley



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PostPosted: Sun Feb 13, 2011 7:37 pm     Reply with quote
Just my two cents:
Code:

int8 addr;
int8 data;

void write(void)
{
  int16 wAddr;
  wAddr = addr<<8;
  output_d(wAddr | data);
}

this still writes the address in the upper 8 bits
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