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Topic: MEGA 2560 reset voltage (Read 5262 times) previous topic - next topic

floresta

#15
Sep 10, 2014, 07:48 pm Last Edit: Sep 10, 2014, 07:53 pm by floresta Reason: 1
Quote
I don't quite understand what it means. Does it mean I have to supply 0.9Vcc to keep it from resetting? Thanks.

The answer to the specific question that you asked is 'Yes'.  Going back to the first line of the data sheet excerpt in the original post - here is my interpretation.

If you have a whole boxfull of these devices and if you randomly test them under all sorts of conditions you will find that:

- all of them will reset if the voltage is less than 0.2VCC
- none of them will reset if the voltage is greater than 0.9VCC

This means that if you want to assure that your processor does not reset you must keep the voltage above 0.9VCC and if you want to assure that your processor does reset then you must make sure the voltage is less than 0.2VCC

Also, if you only specify that your controlling circuit outputs a logic low that is less than 0.9VCC then some devices will reset and some will not.

Of course you also have to make sure that you do not inadvertently go into the HV programming mode mentioned by the other Doc.


Don

liuzengqiang

Notes for the DC characteristic:

I think the description Don gave about testing a whole bunch of processors is consistent with my understanding. I am not trying to discredit raschemmel's test. It showed that one such processor behaves in such a way when supplied with 5V and around room temperature. ATMEL must have done a number of processors and guarantees a wide temperature and supply voltage so they gave a very strict range (with what probability of resetting?), which is what I will follow. If I were to do some simple internet of things stuff to have fun, I'll follow raschemmel's suggestion and expect it to work years after years without a single problem. But the reason I asked was I wanted to reset Arduino under more extreme conditions including hot and cold temperature and possible less-than-stable supply voltage. I'll use a MOSFET. I think that will give 0.1Vcc and 0.9Vcc easily.
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Budvar10

Quote
Quote
LOW from XBee (max 0.5V isn't it ?) will be LOW for mega also. But if HIGH output will be applied directly to RESET (something between 2,8-3,3V) it is not enough to keep mega surely out of reset even if it has reset pin pulled up to Vcc.

Try the test I described in my last post and I believe you will find the above to not be correct.

@raschemmel
I have no doubts about your claims. I am not trying to contradict to you but what is the guarantee of proper operation under various conditions.

It is just my personal opinion  to follow the datasheet.
Arduino clone with ATmega1284P   http://forum.arduino.cc/index.php?topic=277260.0

Budvar10

@liudr
Quote
So for the transistor diagram in adafruit's website, is the capacitor there to stabilize the logic level?

No, it is high-pass filter. I think you need just transistor with ~1k serial resistor in base.
Function of this is opposite: HIGH level opens the transitor which shorts the reset pin to GND and will be LOW.  LOW level closes the transistor then the reset pin is tied to Vcc via pullup resistor and will be HIGH.
Arduino clone with ATmega1284P   http://forum.arduino.cc/index.php?topic=277260.0

raschemmel

Quote
1k serial resistor in base.


I think you mean series resistor
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Budvar10

Arduino clone with ATmega1284P   http://forum.arduino.cc/index.php?topic=277260.0

raschemmel

Not that it matters any more but the Mwga Reset pin would need a logic level converter from the xbee to have 5V on it when the xbee ouputs a high of 3.3V.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Docedison

@ Raschemmel... The operative point is the the threshold is not an absolute value, It depends on the actual applied Vcc..
Thus Vhigh is .9 X Vcc and Vlow or reset is .2 X Vcc not .2V or .9V

...
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raschemmel

Right, hence my comment, since the logic HIGH from the xbee is only 3.3V which is < 0.9 x Vcc (when Vcc=5V). Thus the xbee , after
resetting the Mega, will not be able to keep it out of reset (also mentioned by someone else). The CD4050 is a level converter capable of solving that issue but it is a moot point since the OP opted for a single transistor solution (admittedly cheaper and easier).
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

CrossRoads

Xbee just needs to drive an open collector/open drain transistor to pull reset low, and let 10K pullup to 5V take it back high.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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