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fast io

 
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fast io
PostPosted: Sun Feb 22, 2004 9:45 am     Reply with quote
hi guys, i was programming a 16f877a to read a pin and output on another pin when it went high. I am trying to modify a pulse width modulation signal. I didnt have internet access there and only just read about the fast io just now.

my question is when i go in tomorrow and set the port for fast io will there be any noticable increase in pin reads or writes?

cant test it till tomorrow, but i gotta know Smile
rwyoung



Joined: 12 Nov 2003
Posts: 563
Location: Lawrence, KS USA

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PostPosted: Sun Feb 22, 2004 12:43 pm     Reply with quote
A quick test with the compiler and viewing the LST file:

Code:

0000                00306 .................... #use fast_io(b)
0000                00307 ....................    set_tris_b(0x00); // all output
009E 3000       00308 MOVLW  00
009F 0066       00309 TRIS   6
0000                00310 ....................    output_high(PIN_B0);
00A0 1406       00311 BSF    06.0
0000                00312 ....................    output_low(PIN_B0);
00A1 1006       00313 BCF    06.0
0000                00314 ....................
0000                00315 .................... #use standard_io(b)
0000                00316 ....................    output_high(PIN_B0);
00A2 1683       00317 BSF    03.5
00A3 1006       00318 BCF    06.0
00A4 1283       00319 BCF    03.5
00A5 1406       00320 BSF    06.0
0000                00321 ....................    output_low(PIN_B0);
00A6 1683       00322 BSF    03.5
00A7 1006       00323 BCF    06.0
00A8 1283       00324 BCF    03.5
00A9 1006       00325 BCF    06.0


At the top you see I have #use fast_io(B). Then I set all the bits to output. After that every time I set or clear an individual bit it is one instruction. This would also go for writing all bits in the port with "output_b(bytevalue)".

Then for a test I switch back to #use standard_io(B). Now you see the compiler is inserting a bunch of port direction control instructions. In fact it looks like it is flipping the port back and forth between read and write each time I change the value.

So fast_io means one instruction per port write/read (plus one to configure the port initially) and standard_io is using 4 each time I read/write to the port.

I only tested code generation with one version of the compiler (V3.182) and for one chip (PIC16F88).

If I remember correctly this information is given in the help file but I can't say if it is linked to the #use or the output/input instructions. Sometimes the help file doesn't have everything cross referenced as well as it could.
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!
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