Author Topic: Board review?  (Read 4577 times)

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Offline donotdespisethesnake

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Re: Board review?
« Reply #25 on: February 25, 2018, 08:46:28 pm »
Also if you put a blocking cap, you should also add the diode to prevent overvoltage on RESET.

Why?  A cap will only ever output the voltage it was charged to.  The ATMega can handle 12V on it's reset pin.

Whatever, you're the expert  :-DD
Bob
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Offline Rerouter

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Re: Board review?
« Reply #26 on: February 25, 2018, 09:18:12 pm »
To give clarification, More so as a "what not to do"

By switching the resetswitch to ground, it would couple via the capacitor and pull the micro reset pin towards ground (actual voltage doesn't matter in this example), however if you kept that switch held, the micro pin would charge back up to VCC, meaning when you released your resetswitch, it becoming positive would couple through the capacitor and bring the reset pin to VCC + something,

Now in reality the majority of micro controllers have protection diodes that shunt out of range signals to VCC or ground, but its more a case of avoid if possible

In any case, I would not recommend this arrangement for a reset pin, and with FTDI chips being shunned by a lot of the arduino crowd these days, I suspect its relevance has faded.
 

Offline rs20

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Re: Board review?
« Reply #27 on: February 25, 2018, 10:19:38 pm »
paulca is correct, the max rating on just the reset pin for the AtMega328P is -0.5V to +13.0V. Given that the highest voltage you'd expect from a worst-case scenario as outline by Rerouter is 5V + 5V = 10V, it seems that the diode would be totally unnecessary, as correctly pointed out by paulca and bizarrely, rudely, incorrectly and unconstructively mocked by donotdespisethesnake.

BTW, the reason for the high voltage tolerance of the reset pin is to support parallel programming, in which one routinely places +12V on the reset pin.
 

Offline paulca

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Re: Board review?
« Reply #28 on: February 25, 2018, 11:09:45 pm »
Also if you put a blocking cap, you should also add the diode to prevent overvoltage on RESET.

Why?  A cap will only ever output the voltage it was charged to.  The ATMega can handle 12V on it's reset pin.

Whatever, you're the expert  :-DD

Apologies it was not meant as a challenge, but as a statement of my (miss?) understanding.  I should have prefixed it with "My understanding was:"
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline paulca

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Re: Board review?
« Reply #29 on: February 25, 2018, 11:20:15 pm »
Let me try and follow here.  In my case I have:

Code: [Select]
5V --> 10K --> RESET --> 100nF --> GND
                 |
          ISP RESET PIN

In normal operation the 5V will charge the capacitor so there is 5V across it, 0 across the resistor, 0 current?

When the ISP programmer pulls the RESET to ground, assuming it has no resistor, then 5V will drop across the resistor wasting about 5uA.    The capacitor will discharge into the ISPs ground.  If it doesn't have a resistor on that pull down then it will take a flash pulse of current from the 100nF but finish up with the RESET pin at ground potential.  The 100nF will have 0V on both sides an stay discharged.

When the ISP releases the RESET pull down then 5V will pass through the 10K resistor, charging the 100nF cap back up until again we have 5V at RESET, 0V across the resistor and 5V across the capacitor.

... or I'm missing something here.
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline Deridex

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Re: Board review?
« Reply #30 on: February 26, 2018, 05:23:47 am »
All good. Yeah, it will help reduce noise on the reset line, but it also adds a capacitative load that the ISP programmer has to contend with. Without checking the specifications of the ISP programmer, I'd be hesitant to guess what value to use; and given that you're unlikely to need it at all, leaving it unpopulated is a good way to go.

Just for context, you can see an example of a coupling capacitor on a reset line on the Arduino schematic (C5 on this schematic):



As I mentioned before, this is basically a horrible hack specific to pressing an FTDI chip/ATMEGA16U2 into service as a programmer, so of no relevance to your design.

I think i made a bit a mistake here, by not telling him, that the cap can be left unpopulated aslong there are no problems.  :palm:
Sadly i enountered already a board with another ucontroller that needed this cap to not reset when the loads were switched. It was a seriosly funny problem ... that took a while to figure out. |O
So i tend to add the pads for a Cap on the reset line on the most boards that switch bigger loads.
 

Offline rs20

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Re: Board review?
« Reply #31 on: February 26, 2018, 10:33:09 am »
Let me try and follow here.  In my case I have:

Code: [Select]
5V --> 10K --> RESET --> 100nF --> GND
                 |
          ISP RESET PIN

In normal operation the 5V will charge the capacitor so there is 5V across it, 0 across the resistor, 0 current?

When the ISP programmer pulls the RESET to ground, assuming it has no resistor, then 5V will drop across the resistor wasting about 5uA.    The capacitor will discharge into the ISPs ground.  If it doesn't have a resistor on that pull down then it will take a flash pulse of current from the 100nF but finish up with the RESET pin at ground potential.  The 100nF will have 0V on both sides an stay discharged.

Your analysis is perfect, except you're kinda dismissing the importance of that "flash pulse of current" that you mention. Yes, the reset line will eventually finish at the ground potential, but how long does that take? If the ISP programmer thinks that it's pulled the RESET line low for x microseconds, but the RC filter formed by the output impedance of the ISP programmer and the capacitor values has a time period of 2x microseconds, then the ATMega328P will completely fail to be reset successfully. Different ISP programmers and various clones might have different output impedances and reset line pulse widths too, which means that a poor choice of capacitor value could result in inexplicable, intermittent operation and a lot of tears.

Put another way; if you placed a 1 Farad capacitor on the reset line, I'd wager that most ISP programmers would fail. The difficult question is, how small do you have to make the capacitance before it becomes reliable? Is it easier/better to just use a lower resistance resistor?
 

Offline paulca

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Re: Board review?
« Reply #32 on: February 27, 2018, 03:41:21 am »
If it works without it, I'll leave it unpopulated :)

The board is now 100% built, just waiting on them shipping it.
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline paulca

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Re: Board review?
« Reply #33 on: March 04, 2018, 08:33:29 am »
So, I thought this was worth doing:

"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline paulca

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Re: Board review?
« Reply #34 on: March 17, 2018, 10:40:39 pm »
So I wanted to thank everyone for their help with this.  Got the board on Thursday.  Soldered it together and... wait for it... not one bug was found with only the push buttons untested still.  Spent Friday night programming the RTC and LEDs and this morning doing the oLED.  Works a treat.



C7 - The 100nF on RESET has indeed been left unpopulated.
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 
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