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RESET circuit tweaks

Posted: Thu Sep 27, 2018 8:41 pm
by exxos
Some little tweaks to try..

Replace C901 with a 1uF ceramic... This is basically what the Falcon circuit does. It affects the delay time after the reset button has been pressed. I don't think this will affect anything..

Mostly reset needs long delay on power up to make sure things settle down before the CPU starts doing stuff. This capacitor is 22uF with 10K resistor...And takes about 0.1second to reach 2V. So likely a 1uF ceramic capacitor with 200K should pretty much do the same job...Though I think a slightly higher value capacitor of 2.2uF to increase the delay little longer probably be a good thing anyway..

Really I am just looking for ways to get rid of all those nasty electrolytic's...

Re: RESET circuit tweaks

Posted: Thu Sep 27, 2018 8:43 pm
by PhilC
If it's of any help, my board quite often needs a reset on the first power up.

Re: RESET circuit tweaks

Posted: Thu Sep 27, 2018 8:48 pm
by exxos
Forgottenmyname wrote:
Thu Sep 27, 2018 8:43 pm
If it's of any help, my board quite often needs a reset on the first power up.
Ah right.. you could try swapping R919 with a higher value like 22K or 47K initially ... I think much higher than that, the cap will struggle to charge.

Might be worth checking the voltage across C903 after power down, make sure the voltage is crashing down to 0V quickly.

Re: RESET circuit tweaks

Posted: Sun Oct 07, 2018 5:50 pm
by PhilC
exxos wrote:
Thu Sep 27, 2018 8:48 pm
Forgottenmyname wrote:
Thu Sep 27, 2018 8:43 pm
If it's of any help, my board quite often needs a reset on the first power up.
Ah right.. you could try swapping R919 with a higher value like 22K or 47K initially ... I think much higher than that, the cap will struggle to charge.

Might be worth checking the voltage across C903 after power down, make sure the voltage is crashing down to 0V quickly.
Ok, apologies, I had forgotten I'd said I'd try this. I replaced R919 with a 47K resistor and the board no longer needs a reset after powering up :D

Re: RESET circuit tweaks

Posted: Sun Oct 07, 2018 5:58 pm
by PhilC
actually, ignore my last comment, on the 10th power up it failed, so I've put a 22K into R919 instead and so far so good.

Re: RESET circuit tweaks

Posted: Sun Oct 07, 2018 9:29 pm
by exxos
Forgottenmyname wrote:
Sun Oct 07, 2018 5:58 pm
actually, ignore my last comment, on the 10th power up it failed, so I've put a 22K into R919 instead and so far so good.
OK thanks. So probably the cap needs a larger value in there and anything over 22K is to high to charge it.

Re: RESET circuit tweaks

Posted: Sun Oct 07, 2018 9:37 pm
by PhilC
I can change the cap too if you'd like. Which ones and what value would you like me to try?

Re: RESET circuit tweaks

Posted: Sun Oct 07, 2018 9:45 pm
by exxos
Forgottenmyname wrote:
Sun Oct 07, 2018 9:37 pm
I can change the cap too if you'd like. Which ones and what value would you like me to try?
Sure... C903, higher value such as 47uF.... You may need to put back the 10K in there for it to work...

Re: RESET circuit tweaks

Posted: Mon Oct 22, 2018 2:03 pm
by exxos
While I was looking the MEGA ST circuit, I noticed the reset circuit used a different chip than the normal 555..

What really caught my attention was that it wasn't using a inverter on the output.. As I want to reduce the amount of parts and do away with most of the inverters if possible.. This would mean I can do it with this new chip as it is uses buffers...

rst.JPG
rst.JPG (70.36 KiB) Viewed 1986 times
The chip is still available..

https://www.mouser.co.uk/ProductDetail/ ... gIhdvD_BwE

Re: RESET circuit tweaks

Posted: Mon Oct 22, 2018 2:11 pm
by PhilC
And it's a reasonable price too. Works fine on my Megas