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gfoot
PostPosted: Thu Dec 14, 2023 10:44 pm 
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If you have wired the reset switch between the MCP130's output and ground, as you would for a DS1813, is it possible that your reset button is shorting the MCP130's output to ground, leading to it at least getting confused about the supply voltage quality and performing a reset as a result? i.e. it might be doing the right thing but for the wrong reasons.

Putting the switch in the power line sounds fine though if you don't need software-driven resets, so you can keep using these parts in that case.
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BigDumbDinosaur
PostPosted: Thu Dec 14, 2023 11:10 pm 
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drogon wrote:
30 million Atari 2600s had a resistor, small capacitor and a push button.. so that's what I have in my latest minimal system. Working fine so-far.

Apples and oranges, with no pun intended.  :D  WDC’s official reset requirements are more stringent than that of the NMOS hardware in the Ataris.

That said, I have a suspicion the WDC 65C02 has Schmitt action on its reset input, despite what WDC says about achieving a proper reset.¹  Since reset only has to be low for a few clock cycles to get a clean start, almost any kind of R-C delay, even if only lasting a few milliseconds, should work.

The main advantage to using a device such as the DS1813 or MPC130 is it delays release of reset until after VCC has stabilized.  This can be an important feature in cases where something like a wall wart is the power source for your unit.  Although the output of a PC power supply (what I use with my POC units) is guaranteed to monotonically rise at power-on, there is no such assurance with a wall wart.  Hence the DS1813/MPC130 method of controlling reset becomes useful if there is any uncertainty about how VCC rises when you flip the switch.

——————————————————————————————————
¹I do know NMIB has Schmitt action, something I was told by Bill Mensch in an E-mail message some time ago.  It wouldn’t surprise me if other control inputs, such as RESB, are the same.

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Last edited by BigDumbDinosaur on Thu Dec 14, 2023 11:36 pm, edited 1 time in total.

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BigDumbDinosaur
PostPosted: Thu Dec 14, 2023 11:36 pm 
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gfoot wrote:
Putting the switch in the power line sounds fine though if you don’t need software-driven resets, so you can keep using these parts in that case.

If you are going to open the VCC connection to the MPC130 to initiate a reset, I suggest you either use a form-C push button or add a pull-down resistor to assure the MPC130’s VCC input does go to zero volts when VCC is interrupted.  The form-C method would be better, with the push button arranged to ground the MPC130’s VCC input when pressed.

The theory behind grounding with a form-C push button, or having a pull-down resistor, is the reset line is continuously pulled up to VCC.  Some internal leakage in the MPC130 could keep the device “powered” if its VCC input is floating.  I suspect that effect would mess with reset timing, possibly causing the MPC130 to not respond to the push button.

Yet another method would be to wire a form-A push button across VCC and ground at the MPC130, with series resistance to limit current flow when the push button is pressed.  That resistance has to be low enough to not overly-influence the MPC130’s behavior, but high enough to avoid overloading the push button.  YMMV, as usual.

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drogon
PostPosted: Fri Dec 15, 2023 8:54 am 
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BigDumbDinosaur wrote:
That said, I have a suspicion the WDC 65C02 has Schmitt action on its reset input, despite what WDC says about achieving a proper reset.¹  Since reset only has to be low for a few clock cycles to get a clean start, almost any kind of R-C delay, even if only lasting a few milliseconds, should work.


The 65C134 must have the internal schmitt as the schematic for the 134-SXB board has a 2MΩ resistor to Vcc, 1µF capacitor (to Gnd) and a button to short out the capacitor. Also a diode over the resistor to Vcc. Nothing more.

-Gordon

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plasmo
PostPosted: Fri Dec 15, 2023 2:18 pm 
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sburrow wrote:
Whelp, I suppose I have now seen a near-fatal flaw in my previous designs. Oof! I'm glad it is still working, but I won't do this in the future. Thank you BDD, good call, this helped the future me a lot!

I wouldn't call that near-fatal flaw. My earliest design needs two kinds of resets, a power-on 250mS-ish reset and a manual reset that negates as soon as the button is released. So I've designed a push button across the reset output of MCP130 just like your design. That worked so well, I applied it to all subsequent designs. 300+ MCP130 later, it is still the same design. The cheap tactile pushbutton switch is surprisingly bounce-free and even if it did have bounces that caused a bad boot once in a long while, press the button again. MCP130 is 1/5 the price of DS1813 and you can also add a 100ohm resistor if you really want it to work like DS1813. I'm sticking with it.
Bill
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fachat
PostPosted: Mon Dec 18, 2023 8:18 am 
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Just a little note, the DS1813 is for 5V VCC. For 3.3V there is DS1818 IIRC

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