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writing only LSB to 16Bit port

 
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on7nh



Joined: 05 Jun 2006
Posts: 41
Location: Belgium

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writing only LSB to 16Bit port
PostPosted: Thu May 17, 2012 8:41 am     Reply with quote
hello,
Is it possible to write only the lower 8 bits to a 16 bit port (CPU is DSPIC30f6014a) ?

I want to write data to the lower 8 bits, and leave the state of the higher 8 bits at its actual state.
Ttelmah



Joined: 11 Mar 2010
Posts: 19497

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PostPosted: Fri May 18, 2012 1:49 am     Reply with quote
Should be easy. Since the chip supports a byte wide write. On a lot of '16bit' chips, this is impossible (they only do a 16bit I/O operation to the ports), and on these you have to read the whole word, mask in the new bits you want, and write the word back. On the PIC30 though a byte wide write is supported. So:
Code:


#byte PORT_B_L=getenv("SFR:PORTB")
#byte PORT_B_H=PORT_B_L+1

//Then write to the low byte with:
   PORT_B_L=val_to_output;

If you look at the assembler this generates, you will see it uses the mov.b instruction correctly accessing the single byte.

Best Wishes
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