Fastest way to set outputs to 0 on power up

[dazlogan]

Hi everyone,

I need to set several outputs to 0 (low) on power up as an absolute priority. I am wondering - what is the fastest way of doing this in code please anyone?

Thanks

Regards,
Darren

[Mark]

Well, that depends. If it is an entire port, then just setting that port to 0 is fast. If setting only 1 output then setting that output to 0 is probably the fastest. If setting more than 1 output but less than 8, setting the port to a known state is fast or you can even 'AND' the port with a mask of 1's and 0's.

[dazlogan]

Hi Mark,

Thanks.

Let's say port_a, port_b and port_c all need to go low.

Being more specific - is it best to put something like:

main () {

output_a(0x00);
output_b(0x00);
output_c(0x00);


or is there a quicker way of doing it?

[Mark]

That's about the fastest way that I know. You can probably save a couple of instructions by using "fast_io" and setting the TRIS registers all at one time.

[Neutone]

Have a look at the datasheet and see what the powerup timer does. During powerup all port pins are inputs and a pull down resistor will keep you safe but thats a hardware fix.

[SherpaDoug]

By default the ports power up in tri-state. So if you put a pull down resistor on them they will power up in a low state before the processor even comes out of reset. They will stay low untill your code makes them otherwise.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[Haplo]

And as Neutone said all the ports are set to input on power-up, so before setting them to zero you have to configure them as outputs (if you are using #use fast_IO).

[Guest]

Hardware is the best choice, however to make sure each port is LOW (or high as you please) Set the ouput before setting the TRIS .

This is about the fastest way I know, if Port A if more cirtical than Port C then output that first.


#use fast_io(a)
#use fast_io(b)
#use fast_io(c)

#byte portA = 0XF80 // for PIC18 series
#byte portB = 0XF81
#byte portC = 0XF82

#byte TRIS_A = 0XF92 // for PIC18 series
#byte TRIS_B = 0XF93
#byte TRIS_C = 0XF94

void main()
{
portA = portB = portC = 0; // set low before changing TRIS
TRIS_A = TRIS_B = TRIS_C = 0; // all outputs are low NO glitches

other code..
}

That's abut 600nS at 40MHz...
Here's the LST file for it.
.................... portA = 0;
0012: CLRF F80
.................... portB = 0;
0014: CLRF F81
.................... portC = 0; // set low before changing TRIS
0016: CLRF F82
.................... TRIS_A = 0;
0018: CLRF F92
.................... TRIS_B = 0;
001A: CLRF F93
.................... TRIS_C = 0; // all outputs are low NO glitches
001C: CLRF F94


hansw

[dazlogan]

Thanks Hans and everyone else.

My problem is cured !. Most appreciated.

Regards,
Darren