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#FAST_IO for certain pins

 
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rikotech8



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#FAST_IO for certain pins
PostPosted: Mon Sep 22, 2014 5:48 am     Reply with quote

Hello
I would like to ask you this:
How can I tell the compiler to produce a TRIS direction code only for some pins? Let say we have PORT A with 8 pins form A0 to A7. I want a fast reaction from pin A0 when I set it's state. For the rest 6 pins (A1-A7) I want the compiler to generate a TRIS direction code.
When I read the manual, its written:
Quote:
Syntax:
#USE FAST_IO (port)
Elements:
port is A, B, C, D, E, F, G, H, J or ALL

In other words I can adjust the compiler settings only to generate a TRIS direction code for the entire port, not pin by pin. BUT the next directive
#USE FIXED_IO (port_outputs=pin, pin?) is able to handle pin by pin.
So if I set the code for PORTA this way:

Code:
#USE FAST_IO (A)
#USE FIXED_IO (a_outputs=PIN_A1,PIN_A2, PIN_A3,PIN_A4,PIN_A5,PIN_A6,PIN_A7)

Does it mean that I will have PIN_A0 as fast I/O?
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PCM programmer



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PostPosted: Mon Sep 22, 2014 3:23 pm     Reply with quote

No, because only the last directive takes effect. The fixed_io cancels
the fast_io. And fixed_io writes to the whole TRIS register every time
anyway.
rikotech8



Joined: 10 Dec 2011
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PostPosted: Tue Sep 23, 2014 6:30 am     Reply with quote

10x
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asmboy



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PostPosted: Tue Sep 23, 2014 10:04 am     Reply with quote

in my work , deciding to declare FAST_IO depends
on how a PORT will be used.

in specific - if speed MATTERS - such as accessing a byte wide SRAM chip's
data pins- ONLY FAST_IO makes sense, since a single unmasked instruction
"does it all". the rest of the time , default CCS "smart" / on the fly TRISIO
is much simpler. pick one or the other and you will be happy.

The other case where fast_io is helpful is where you never, ever anywhere in your code change TRIS, though this might be a CCS version dependent optimization.
Then set TRIS at the top of main() and forget it.
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