Icky wrote: ↑Sun Nov 11, 2018 5:33 pm
My thoughts too that’s why I am cleaning up the board to use the proper switch etc
I guess thats the problem with switches, it would be better if it driven a "one shot" timer like the 555..
may be we can’t cut things down as much as we want. Let me get the circuit in place and we’ll see how it performs.
Re: RESET circuit tweaks
Posted: Sun Nov 11, 2018 6:15 pm
by PhilC
Is there any interest in me checking out the mega reset circuit any more? In case the circuit isn't as drawn?
Still needs the inverter though doesn't it?
Re: RESET circuit tweaks
Posted: Sun Nov 11, 2018 6:35 pm
by Icky
Forgottenmyname wrote: ↑Sun Nov 11, 2018 6:15 pm
Is there any interest in me checking out the mega reset circuit any more? In case the circuit isn't as drawn?
Still needs the inverter though doesn't it?
That TL7705 is a bit weird. It only seems to operate at around 5.4 - 5.5V. Considering the data sheet it’s operating range is 3.6 a 18V.
I looked as well at the schematic and apart from the pull up and pull down on the respective outputs there is not much difference.
May be if we know how it’s wired exactly on a real Mega it may help but I expect it is as the schematic says.
Re: RESET circuit tweaks
Posted: Sun Nov 11, 2018 6:44 pm
by PhilC
Ok, so I'll pop a multimeter on the chip and see what kind of voltages the mega is working at then.
Can't imagine it's as high as 5.4 as I'm sure when I recapped that psu it was around 4.9v
Re: RESET circuit tweaks
Posted: Sun Nov 11, 2018 8:23 pm
by PhilC
The 7705 in one of my Megas has a VCC of 5v and CT sits at around 3.16v when not in reset.
/reset is at 5v and drops to 0 on reset press, Reset sits at 0.84v till reset pressed and jumps to 4.7v for reset pulse.
R2 measures at 1K but R1 is 440ohm as fitted but marked as 1K, so probably C1 having an effect on measurement I presume.
hope that helps so far.
P.S. reset and /reset only change over as long as the button is held.
Re: RESET circuit tweaks
Posted: Sun Nov 11, 2018 8:41 pm
by Icky
Forgottenmyname wrote: ↑Sun Nov 11, 2018 8:23 pm
The 7705 in one of my Megas has a VCC of 5v and CT sits at around 3.16v when not in reset.
/reset is at 5v and drops to 0 on reset press, Reset sits at 0.84v till reset pressed and jumps to 4.7v for reset pulse.
R2 measures at 1K but R1 is 440ohm as fitted but marked as 1K, so probably C1 having an effect on measurement I presume.
hope that helps so far.
P.S. reset and /reset only change over as long as the button is held.
Thanks Forgottenmyname - Odd that I can only get the chip to operate at 5.3V+
Interesting R1 is 440R and not 1K as stated.
Re: RESET circuit tweaks
Posted: Sun Nov 11, 2018 8:47 pm
by Icky
So this is the circuit actually on the board.
The resistor is two 470K in series as I was getting better response between 900R and 1K.
Its still not stable which going back to Exxos quote reset sometimes does not reset but freezes the desktop.
exxos wrote: ↑Sun Nov 11, 2018 5:38 pm
I guess thats the problem with switches, it would be better if it driven a "one shot" timer like the 555..
IMG_3266.png (574.69 KiB) Viewed 5838 times
Re: RESET circuit tweaks
Posted: Sun Nov 11, 2018 9:14 pm
by Icky
I'm starting to think that the 555 circuit is probably the best one so far.
Here is a video of the board booting with the simple circuit and then reseting and on the second reset it goes into the strange frozen bigger desktop and takes several resets to get it to reboot.
Apologies for the shaky video but was juggling holding the phone, switching on and reseting. Must get myself a tripod for phone.
Re: RESET circuit tweaks
Posted: Sun Nov 11, 2018 9:47 pm
by exxos
odd.. that looks like RAM corruption type of thing, like those TSR programs which you can only get rid of by a cold reset. Probably its not resetting fully, RAM is getting corrupted somehow, and a warm boot isn't enough to start up anymore..
The 555 in my STF, I seem to have a iffy reset switch on mine, but the switch shorts out the cap just the same as the simple rest mod.. so not sure whats going on there.
Re: RESET circuit tweaks
Posted: Sun Nov 11, 2018 10:11 pm
by exxos
What capacitor value do you have in at the moment 22uf ? and 1K ?
may be we can’t cut things down as much as we want. Let me get the circuit in place and we’ll see how it performs.