writing a byte to 16 bit port

Hi,
What is the best (simplest) way to write a byte to a 16 bit port pin with 
out disrupting the other MSB pins. In my case I'm trying to write a byte to 
the LSByte programmed in C on a dsPIC.

Something like:

unsigned char temp;
temp = 19;
PORTB = temp;

Even though the variable 'temp' is only a byte long, I'm pretty sure it will 
toggle the MSByte in the 16 bit PORTB register.

Any ideas?

Thomas Magma

On Thu, 14 Feb 2008 19:04:59 GMT, "Thomas Magma"
<somewhere@overtherainbow.com> wrote:

>Hi,
>What is the best (simplest) way to write a byte to a 16 bit port pin with 
>out disrupting the other MSB pins. In my case I'm trying to write a byte to 
>the LSByte programmed in C on a dsPIC.
>
>Something like:
>
>unsigned char temp;
>temp = 19;
>PORTB = temp;
>
>Even though the variable 'temp' is only a byte long, I'm pretty sure it will 
>toggle the MSByte in the 16 bit PORTB register.

If you can't do a read-modify-write on the port, something like:
temp = 19;
foo = ((PORTB & 0xFF00) | temp);
PORTB = foo;

then try keeping a shadow register, i.e., *all* changes to PORTB are
first done to ShadowB and then ShadowB is written to the port.

-- 
Rich Webb     Norfolk, VA

Thomas Magma wrote:
> Hi,
> What is the best (simplest) way to write a byte to a 16 bit port pin with 
> out disrupting the other MSB pins. In my case I'm trying to write a byte to 
> the LSByte programmed in C on a dsPIC.
> 
> Something like:
> 
> unsigned char temp;
> temp = 19;
> PORTB = temp;
> 
> Even though the variable 'temp' is only a byte long, I'm pretty sure it will 
> toggle the MSByte in the 16 bit PORTB register.
> 
> Any ideas?
> 
> Thomas Magma
> 
> 
> 
It kind of depends on how you C compiler encodes the assembly code, but 
I am sure they do it right.

Take a look at the dsPIC30F/33F Programmer�s Reference Manual

http://ww1.microchip.com/downloads/en/DeviceDoc/70157B.pdf

Section 4.4, all will be reveled.

donald

> If you can't do a read-modify-write on the port, something like:
> temp = 19;
> foo = ((PORTB & 0xFF00) | temp);
> PORTB = foo;
>

Thanks Rich,

What is the 'foo' for? Couldn't I just:
PORTB = ((PORTB & 0xFF00) | temp);

to simplify things?

Thomas

On Thu, 14 Feb 2008 19:31:40 GMT, "Thomas Magma"
<somewhere@overtherainbow.com> wrote:

>
>
>> If you can't do a read-modify-write on the port, something like:
>> temp = 19;
>> foo = ((PORTB & 0xFF00) | temp);
>> PORTB = foo;
>>
>
>Thanks Rich,
>
>What is the 'foo' for? Couldn't I just:
>PORTB = ((PORTB & 0xFF00) | temp);
>
>to simplify things?

Probably, although [disclaimer: never worked on dsPICs or with that
specific compiler] it's likely that PORTB is not a simple object and
there could be side effects that I'm not aware of that come into play
when PORTB is referenced. Using a throw-away temp should guarantee
that bad things won't happen.

-- 
Rich Webb     Norfolk, VA

> It kind of depends on how you C compiler encodes the assembly code, but
> I am sure they do it right.
>
> Take a look at the dsPIC30F/33F Programmer�s Reference Manual
>
> http://ww1.microchip.com/downloads/en/DeviceDoc/70157B.pdf
>
> Section 4.4, all will be reveled.
>
> donald

Thanks Donald,

I hadn't seen that programming reference manual before, but it only talks 
about byte addressing in assembly language and not in C. I'm kind of looking 
for a clean way to do it in C.

Thomas 

On Thu, 14 Feb 2008 19:04:59 GMT, "Thomas Magma"
<somewhere@overtherainbow.com> wrote:

>Hi,
>What is the best (simplest) way to write a byte to a 16 bit port pin with 
>out disrupting the other MSB pins. In my case I'm trying to write a byte to 
>the LSByte programmed in C on a dsPIC.
>
>Something like:
>
>unsigned char temp;
>temp = 19;
>PORTB = temp;
>
>Even though the variable 'temp' is only a byte long, I'm pretty sure it will 
>toggle the MSByte in the 16 bit PORTB register.
>
>Any ideas?
>
>Thomas Magma

Not an answer to your question-- I think that is partly in donald's
suggestion and partly in the C30 compiler User's Guide if you want to
use the byte mode capability-- or use the and/or-- but 99.9% of the
time you should NOT be writing to PORTB in the first place-- rather
use LATB (or B sorry). 
Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

>but 99.9% of the
> time you should NOT be writing to PORTB in the first place-- rather
> use LATB (or B sorry).

I did mean LATB. I've already been through that painful LAT vs PORT learning 
curve, but thanks anyways.

On Feb 15, 12:18=A0am, Rich Webb <bbew...@mapson.nozirev.ten> wrote:
> On Thu, 14 Feb 2008 19:04:59 GMT, "Thomas Magma"
>
> <somewh...@overtherainbow.com> wrote:
> >Hi,
> >What is the best (simplest) way to write a byte to a 16 bit port pin with=

> >out disrupting the other MSB pins. In my case I'm trying to write a byte =
to
> >the LSByte programmed in C on a dsPIC.
>
> >Something like:
>
> >unsigned char temp;
> >temp =3D 19;
> >PORTB =3D temp;
>
> >Even though the variable 'temp' is only a byte long, I'm pretty sure it w=
ill
> >toggle the MSByte in the 16 bit PORTB register.
>
> If you can't do a read-modify-write on the port, something like:
> temp =3D 19;
> foo =3D ((PORTB & 0xFF00) | temp);
> PORTB =3D foo;
>
Nice one.
This is the famous trick to be adapted in this scenario :):)

Karthik Balaguru

"Thomas Magma" <somewhere@overtherainbow.com> wrote:

>I hadn't seen that programming reference manual before, but it only talks 
>about byte addressing in assembly language and not in C. I'm kind of looking 
>for a clean way to do it in C.

Assuming you are using the Microchip C30 compiler include generic.h and use
an ugly cast like. 

	((WORD_VAL*)&LATB)->byte.LB = 0x23;

	((WORD_VAL*)&LATB)->byte.HB = 0x45;

or define your own structure to map over LATB like LATBbits is done in the
processor header file and add a line to the linker script file to define
its address. 

--