RESET circuit tweaks

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Icky
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Re: RESET circuit tweaks

Post by Icky » Sat Nov 10, 2018 4:58 pm

It's a bit odd, not consistent.

Sometimes when turning on the ~Reset line out of the TL7705 goes straight to high and stays there and it won't boot

Other times on switch on it will go high then low and the board will reset and boot. the reset switch has no effect pressing it the line does not go high and low again.

And on other times on switch on the line goes high and on press of reset button will go low and back to high. Screen also displays reset flicker.

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Re: RESET circuit tweaks

Post by exxos » Sat Nov 10, 2018 5:02 pm

Odd... I think the circuit is from the mega st... Might be worth a look?
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Re: RESET circuit tweaks

Post by Icky » Sat Nov 10, 2018 5:05 pm

Yes - Have been comparing. I noticed the pull up on the reset switch is 1K on MEGA ST and 10K on STF. Have tried both still the same.

Also the caps are different 10uF for the electrolytic where as STF is 47uF.

I will test this circuit out on breadboard for behaviour as I have a spare TL7705. That way I can play around with it easily.

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Re: RESET circuit tweaks

Post by Icky » Sat Nov 10, 2018 7:08 pm

OK - prototype version

This is the exact match of the Mega ST reset circuit from the schematic

IMG_3258.png
IMG_3258.png (1.64 MiB) Viewed 1032 times

Power it up and this is what I am getting on LA. You can see that the inverted reset out is going high and low sporadically.

Screen Shot 2018-11-10 at 19.02.01.png
Screen Shot 2018-11-10 at 19.02.01.png (20.81 KiB) Viewed 1032 times

Second is with selecting reset several times which is the yellow wire to GND.

Screen Shot 2018-11-10 at 19.05.39.png
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Re: RESET circuit tweaks

Post by Icky » Sat Nov 10, 2018 7:32 pm

Interestingly was also looking at other schematics around the reset circuit and ventured into the MegaSTE as well. It looks like Atari went towards a 556 away from the TL7705 on the Mega.

Screenshot 2018-11-10 at 19.29.37.png
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Re: RESET circuit tweaks

Post by PhilC » Sat Nov 10, 2018 7:51 pm

If it helps, here's a photo of my Mega reset circuit, in case something is a different value to the schematic.
20181110_194921.jpg
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Re: RESET circuit tweaks

Post by PhilC » Sat Nov 10, 2018 7:56 pm

Actually, noticed that a second cap is connected across C3 on my Mega, so maybe this is the problem?

Second cap is a 470uf. The other Mega doesn't have one but is also more unreliable, so might try that on the other one too.
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Re: RESET circuit tweaks

Post by Icky » Sat Nov 10, 2018 7:58 pm

Forgottenmyname wrote:
Sat Nov 10, 2018 7:51 pm
If it helps, here's a photo of my Mega reset circuit, in case something is a different value to the schematic.

20181110_194921.jpg
Thanks Forgottenmyname - I notice your C3 is two caps in parallel. What value are they?

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Re: RESET circuit tweaks

Post by Icky » Sat Nov 10, 2018 7:59 pm

Icky wrote:
Sat Nov 10, 2018 7:58 pm
Forgottenmyname wrote:
Sat Nov 10, 2018 7:51 pm
If it helps, here's a photo of my Mega reset circuit, in case something is a different value to the schematic.

20181110_194921.jpg
Thanks Forgottenmyname - I notice your C3 is two caps in parallel. What value are they?
Must of seen the same thing at the same time :D

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Re: RESET circuit tweaks

Post by PhilC » Sat Nov 10, 2018 8:01 pm

Icky wrote:
Sat Nov 10, 2018 7:59 pm
Icky wrote:
Sat Nov 10, 2018 7:58 pm


Thanks Forgottenmyname - I notice your C3 is two caps in parallel. What value are they?
Must of seen the same thing at the same time :D
470 and original is the 10
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