RESET circuit tweaks

Post by **PhilC** » 10 Nov 2018 20:45

I noticed that the 7705 is also designed to perform a reset on voltage loss scenarios (probably obvious to you all) is it worth checking the VCC on pin 8 and 7 to see if its dropping in time with the pulses?

only other thing I can see is the cap on REF is too small, should be 0.1uf not 0.01uf. so that might be worth a try I guess?

Post by **Icky** » 10 Nov 2018 20:50

I got it working. :?

Have been testing using bench PSU at 5v. Cranked it up to 5.5v and the circuit is steady.

Screen Shot 2018-11-10 at 20.46.50.png

Post by **PhilC** » 10 Nov 2018 20:53

:D well we got there in the end then

Now you've fixed it I have to get distracted by X Factor...... *sigh*

Post by **Icky** » 10 Nov 2018 20:57

Forgottenmyname wrote: 10 Nov 2018 20:53 :D well we got there in the end then

Now you've fixed it I have to get distracted by X Factor...... *sigh*

Yes - Thats the prototype board sorted. Now need to translate it to the reset board and STF as I think there are a few value changes to be done.

Enjoy Xfactor :D

Post by **PhilC** » 10 Nov 2018 21:01

Icky wrote: 10 Nov 2018 20:57
Enjoy Xfactor :D

Do I have to? :geek: :roll:

Post by **exxos** » 10 Nov 2018 23:35

Just catching up here... Doesn't seem right that it needs higher voltage to work.. Maybe some resistor is to high in value and its not charging the cap up ?

Post by **Icky** » 10 Nov 2018 23:41

exxos wrote: 10 Nov 2018 23:35 Just catching up here... Doesn't seem right that it needs higher voltage to work.. Maybe some resistor is to high in value and its not charging the cap up ?

On the prototype board I am using 1K as per the Mega ST schematic as the pull up resistor connected to the reset switch. Apart from the pull-up on the ~Reset (Mega ST schematic and my test board) there are no other resistors.

Post by **exxos** » 10 Nov 2018 23:59

What about 2 1k in series on the sense on pin 7 to bias it to 2.5v ?

Post by **exxos** » 11 Nov 2018 00:02

Also pin 5 needs 10k pull up and pin 6 pull down according to the data sheet...

Post by **Icky** » 11 Nov 2018 00:14

exxos wrote: 10 Nov 2018 23:59 What about 2 1k in series on the sense on pin 7 to bias it to 2.5v ?

Gave that ago on 5V with no response. output just sat at 0V

Section 10 of the data sheet found by Forgottenmyname is interesting around having to terminate both outputs and also putting a resistor in series with the Charge Threshold capacitor http://www.ti.com/lit/ds/symlink/tl7705b.pdf

exxos's Atari, Amiga & retro forum

RESET circuit tweaks

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