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Ttelmah



Joined: 11 Mar 2010
Posts: 19384

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PostPosted: Tue Dec 20, 2011 2:44 am     Reply with quote

aaronik19 wrote:
Ttelmah, thanks for your reply. But acccording to my knowledge, since you are OR gate the val an Opval, the output will be always 11111111 most of the time. I do not know why you used that working. Can you please explain more if you can?

Thanks


Be careful of asmboy's suggestion. What happens here is that the port is changed to be an _input_, the value on it is read, then the low two bits of this are combined with the bits you want to send, and the port then changed to an output, and the whole result sent.
Unfortunately also, if the output loads are high a port pin that has had a '1' written to it, may not always read as a '1' back, and in which case the bit gets lost.
It _may_ work, but depends on the loads on the pins, capacitances, etc. etc., but may also cause problems....

On my code, OR does not work the way you think. You may be confusing the effect of a logical OR '||' from a bitwise OR '|'.

Imagine you have:

0b01010101 sent out on the port using 'output_latched', and therefore stored in 'opval'. Then you send 0b10101010 using 'outputb_masked'.

The incoming 10101010 gets changed to 10101000 by the '&'.
Then the opval value gets changed to 0b00000001 by it's '&'.

10101000
00000001
or them together
10101001

OR gives a '1' wherever there is a '1' in either source byte, and nowhere else. So the top six bits of your value get output, combined with the low two bits of what has previously been sent.

On the PIC 18, the value previously sent, is available in the 'LATB' register, so asmboy's suggestion, can be made to work perfectly (without any glitches), using:

Code:

#byte LATB=getenv("SFR:LATB")

// per your example -to  preserve the low order 2 bits :
// NEW bits is expected to contain the hi order 6 bits  you want
// and we mask out the low order bits in case they were not 0
void changebits ( byte newbits ){
   output_b( (LATB&3) | (newbits&0b11111100) );   
}
// any easier to understand ??


This uses the hardware latch so that the byte is output without having to change the port direction for a few cycles, and gets rid of the potential glitch, and any potential problems if the port loading is high. However doesn't work on the PIC16 chips.

Best Wishes
asmboy



Joined: 20 Nov 2007
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PostPosted: Tue Dec 20, 2011 6:42 am     Reply with quote

Quote:

port is changed to be an _input_, the value on


I think that is NOT for sure true, you see -
IF you use fast_IO() and declare portb tris as all 0
as i ALWAYS DO for output - i simply don't want the compiler involved in
port I/O direction - as i don't trust compiler generated TRIS in a program i am going to sell - i am willing and even eager to be in charge of I/o direction ;-))
Anyway - with fast_io(b) - you get this on an 18F part :
Code:

.................... void changebits ( byte newbits ){
....................    output_b( (input_b()&3) | (newbits&0b11111100) );
0004:  MOVF   F81,W
0006:  ANDLW  03
0008:  MOVWF  0E
000A:  MOVF   0C,W
000C:  ANDLW  FC
000E:  IORWF  0E,W
0010:  MOVWF  F8A
.................... }

but if you are STILL anxious - you can always ( memory permitting)
maintain a phantom portb var byte --
and operate on IT - then output the phantom variable.
aaronik19



Joined: 25 Apr 2011
Posts: 297

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PostPosted: Wed Dec 21, 2011 7:28 am     Reply with quote

thanks to all. Both methods worked fine Smile great help thanks guy and best wishes
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