fast_io on a 12F635 not working- suggestions appreciated....

[alm1006]

Hello -

I'm not seeing the 1usec pulses when fast_io(A) is invoked, in fact - the output stays low??
The list file claims the bit is being set and cleared in two cycles -
is there something I'm missing?

Thanks for the help,
Albert



//CODE:

delay_ms(200); // delay schedule 2 by 200ms (+ 10ms) //
#use fast_io(A)
output_high(PIN_A5);
#asm
nop
#endasm
output_low(PIN_A5);
#use fast_io(A)

}

//LIST FILE:

.................... delay_ms(200); // delay schedule 2 by 200ms (+ 10ms) //
0085: MOVLW C8
0086: MOVWF 44
0087: CALL 004
.................... #use fast_io(A)
.................... output_high(PIN_A5);
0088: BSF 05.5
.................... #asm
.................... nop
0089: NOP
.................... #endasm
.................... output_low(PIN_A5);
008A: BCF 05.5
.................... #use fast_io(A)
....................
.................... }

[Ttelmah]

As soon as you enable #use fast_io, it becomes _your_ responsibility to handle TRIS.
After the #use fast_io statement, you need to add the line to tell the pin to be an output. Now you can toggle the pin up/down to your heart's content.
You don't need to use assembler to get a one clock delay. use delay_cycles(1);
You don't say what your clock rate is?. Remeber the actual instruction all take time. As it stands, assuming your clock is running at 4MHz, so 1uSec/instruction, you will see the pin go high for 2uSec. Basically it is set high in the third clock cycle of the output instruction, then there is a four clock delay, then it is set low in the third cycle of the next instruction. Total of eight clock cycles.

Best Wishes

[alm1006]

The C Compiler reference manual shows the syntax as #use fast_io(port),
the example shown is written as #use fast_io(a), no mention of the tris registers??
It also shows #use fixed_io,#use standard_io,set_tris_X.

Also, why should I have to reset the tris register, it was set initially and was never changed. If I remove the #fast_io comment the output pin works as it should but takes twice the amount of time.

The clock rate is 4mhz. I'm rather new at this so please forgive the newbie mistakes, I have Embedded C Programming and the Microchip Pic
by Richard H. Barnett arriving today.

Thank you for your time and patients,

Regards,
Albert

[PCM programmer]

[Ttelmah]

The help file also gives a longer description of this. Specifically:
-------
Affects how the compiler will generate code for input and output instructions that follow. This directive takes effect until another #use xxxx_IO directive is encountered. The fast method of doing I/O will cause the compiler to perform I/O without programming of the direction register. The user must ensure the direction register is set correctly via set_tris_X().
-------

Notice the last line.
Now look at the entry for 'standard_io' (the default), and notice it's middle line:
-------
The standard method of doing I/O will cause the compiler to generate code to make an I/O pin either input or output every time it is used.
-------

The I/O pins on the processor all normally 'wake up' as inputs (avoids things being driven that shouldn't be...). The extra time involved in the standard_io form, is because it sets the direction for every operation.

You might want to consider using 'fixed IO' instead. With this, you would declare: