how to make RESET after powerup ?

artohautala · Joined: 17 Nov 2011 Posts: 187

Hello ,

my software requires RESET after power up...
I'm using PIC18F4525
How to make it ?

brdgs

-arto-

Ttelmah · Joined: 11 Mar 2010 Posts: 19499

The compiler can get close to it with the 'reset_cpu' instruction, but this is not exactly the same as a power on reset. There are small differences in things like the actual clearing of the individual RAM locations in the chip. The differences are small on your chip (since it has a processor instruction designed to do this), but is larger on chips without this.

artohautala · Joined: 17 Nov 2011 Posts: 187

Ttelmah · Joined: 11 Mar 2010 Posts: 19499

Er. Of course it won't return.
The CPU _resets_. It goes back to the start of the program.

Are you sure the processor is actually running?.

One thing that can happen, is that the CPU clock is not actually starting. The hardware reset then starts this. Software can't (since the chip is not running)....

You need to fix your hardware to get the chip to start.

Things would be:
1) Ensure the rise time of the power supply is fast enough.
2) Delay releasing MCLR till after the supply is good.

artohautala · Joined: 17 Nov 2011 Posts: 187

Ttelmah · Joined: 11 Mar 2010 Posts: 19499

As you see I was still typing.

If the processor does not run, then it'll never get to the reset_cpu instruction. I suspect your chip is not actually _starting_.

artohautala · Joined: 17 Nov 2011 Posts: 187

Ttelmah · Joined: 11 Mar 2010 Posts: 19499

The classic reason for a chip that will start with a hardware reset after power is applied, would be the crystal not starting. Slow power rise can give this.
There is a maximum rise time for the supply for the internal reset to work. You can slightly improve things by putting a bias resistor across the crystal (perhaps 1M5R across the chip terminals). Adding your own circuit to hold MCLR low till a short period after the supply is stable.
Generally neither is necessary if the power to the chip does rise at a reasonable rate.

temtronic · Posted: Sat Mar 07, 2015 2:15 pm

The other classic reasons are
1) program was compiled in 'debug' mode instead of 'release'

2) ICD or programmer still connected to PIC

3) faulty xtal/caps/PCB layout whisker/etc.

Jay

artohautala · Joined: 17 Nov 2011 Posts: 187

Ttelmah · Joined: 11 Mar 2010 Posts: 19499

What you are doing now, does not affect the code at all.
The instruction at this point will not actually generate any code at all...

You have changed something else (fuses or something similar), which is now making your code work.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Ttelmah · Joined: 11 Mar 2010 Posts: 19499

Absolutely, which can obviously never work.
However the 'new' location does nothing. If his code runs with this, it'd run without the instruction just the same. The compiler generates no code at all for a reset_cpu instruction placed at the top of the file like this.

He has fiddled since his original problem (which we still don't know what it was), and now the chip is running, and he thinks it is this added instruction that has made it work (which it isn't).....

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Ttelmah · Joined: 11 Mar 2010 Posts: 19499

He started saying his processor 'required reset after power-up'.
He said the chip worked if he pressed the reset button.

We looked at the classic 'reasons' for this (slow power rise (me), so the clock was not starting, and also things like ICD connections (Temtronic)).

He now says the chip is running with reset_cpu at the top as shown, which as you, I, and a lot of other posters know, does absolutely nothing. The compiler does not generate any code at all for the instruction placed there, so the chip would have worked just as well without the instruction at all.