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Author Topic: Is it neccesary to pull-up the reset pin on home made Arduino board?  (Read 1546 times)
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The datasheet also implies that the external-reset signal is directly connected to pin PC6
No it don't imply anything. the same physical pin can be used as either an external reset OR a GPIO pin 6 on port C. What it actually is can be determined by the fuse setting.

Page 87, the external-reset signal is connected to AIO on PC6.
Page 80, AIO is connected directly to the pin.

If it were otherwise, then why would a pull-up resistor be necessary?

« Last Edit: July 21, 2011, 02:25:43 am by tim7 » Logged

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If it were otherwise, then why would a pull-up resistor be necessary?
There is a fixed internal weak pull-up on the reset line, but the question is if this is sufficient or not. Atmel design recommendations (application note AVR042) suggest not and so we (being conservative engineers) add the external 10k resistor (as did the Arduino team for Uno and its predecessors).  We should respect that such recommendations are based on a collective experience for designs that span the full operating range of the microcontroller (supply voltage, clock frequency, temperature, …) and no individual (or company for that matter) is likely to offer better advice in this respect.

AVR042 also recommends (term used is “highly recommended”) a voltage/ESD clamp on the reset line. The post from retrolefty suggesting that the Uno may lock-up in some cases due to the lack of such protection may serve as an example that real problems can be avoided if we follow manufacturer recommendations.
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 Bigger Bigger  Smaller Smaller  Reset Reset

To answer the question  from AgeingHippy  :  YES. If you guys talk about the ATmega328 pin 1. And I agree with you Grumpy_Mike. In Every ( most ) of CPU's ( that including the computer in front of you ), a reset circuit is needed. Even a simple counter chip ( 7490 ) need a reset circuit. <-- Correct me if am wrong.  In the book title "The 8088 Project Book", page 64,  by Robert Grossblatt, a reset circuit is needed. It basicaly : 10 K pull-up config , push-on button in serie with 150 , 4.7 uF elec and 0.1 uF ( in parallel with the push-on button and the resistor) and 1N914 diode in parallel with the 10 K resistor.  Work like this : At power-up, the cap is short for the time of RC ( 10 K * ( 4.7 uF + 0.1 uF ) <--put a LOW signal ( possibility of clearing the internal Flip-Flop - counters ) , and slowly going into HIGH for the CPU to run. When you press reset, I will do the same procedure when it start-up.

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Adding a reversed biased diode across the reset pull-up resistor 'cures' the symptom.
 

Thank you retrolefty to remind me of the diode. I will place it in my breadboard Arduino/ ATmega358 setup.   
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