Author Topic: Is it ok to leave atmega328p reset pin floating?  (Read 8323 times)

0 Members and 1 Guest are viewing this topic.

Offline abdullahseba

  • Frequent Contributor
  • **
  • Posts: 335
  • Country: gb
Is it ok to leave atmega328p reset pin floating?
« on: April 10, 2017, 09:28:06 pm »
Hi
Is it ok to leave the atmega328p reset pin floating on the ICSP header?
Or should I add a pull-up resistor?
Thanks in advance.
This is my right hand this is my wrong hand
 

Offline sleemanj

  • Super Contributor
  • ***
  • Posts: 2854
  • Country: nz
  • Professional tightwad.
    • The electronics hobby components I sell.
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #1 on: April 10, 2017, 09:37:43 pm »
Pull up recommended.
~~~
EEVBlog Members - get yourself 10% discount off all my electronic components for sale just use the Buy Direct links and use Coupon Code "eevblog" during checkout.  Shipping from New Zealand, international orders welcome :-)
 
The following users thanked this post: abdullahseba

Offline Psi

  • Super Contributor
  • ***
  • Posts: 8367
  • Country: nz
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #2 on: April 10, 2017, 09:46:53 pm »
They do have a pullup on reset internally, so it's not really possible to have reset "floating".
If you don't connect the pin to anything then reset simply has a week pullup.

I've left them totally unconnected without problems.
however it's recommended to have a stronger external pull up just in case there is excessive EMI around.
A small cap to ground is also recommended to prevents glitches.
« Last Edit: April 10, 2017, 09:48:48 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 
The following users thanked this post: abdullahseba

Offline ohdsp

  • Contributor
  • Posts: 32
  • Country: gb
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #3 on: April 10, 2017, 09:50:56 pm »
As PSI/sleemanj said use an external pull. Even if an IC has internal pull-ups these can often be very high impedance (~100kohm).

When adding an external pull up resistor (say 1 to 10kohm) a small capacitor (1nF or 10nF ceramic) between reset and ground can help improve EMI/EMC performance; meaning less "unexplained" resets. If the IC needs a delay on the reset after power up it can also serve this purpose as well by using a larger capacitor.
Check out the Open Hardware DSP Platform:
http://www.ohdsp.org
http://github.com/ohdsp
 
The following users thanked this post: abdullahseba

Offline abdullahseba

  • Frequent Contributor
  • **
  • Posts: 335
  • Country: gb
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #4 on: April 10, 2017, 11:22:34 pm »
So the pullup and caps wont interfere with the icsp?
This is my right hand this is my wrong hand
 

Offline sleemanj

  • Super Contributor
  • ***
  • Posts: 2854
  • Country: nz
  • Professional tightwad.
    • The electronics hobby components I sell.
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #5 on: April 10, 2017, 11:38:18 pm »
So the pullup and caps wont interfere with the icsp?

Typically use a 10k for R1.  It won't affect the ICSP, which connects direct to the reset pin.
~~~
EEVBlog Members - get yourself 10% discount off all my electronic components for sale just use the Buy Direct links and use Coupon Code "eevblog" during checkout.  Shipping from New Zealand, international orders welcome :-)
 
The following users thanked this post: abdullahseba

Offline obiwanjacobi

  • Frequent Contributor
  • **
  • Posts: 987
  • Country: nl
  • What's this yippee-yayoh pin you talk about!?
    • Marctronix Blog
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #6 on: April 11, 2017, 07:11:31 am »
Caps on the RESET pin *may* interfere with ICSP.

[2c]
Arduino Template Library | Zalt Z80 Computer
Wrong code should not compile!
 

Offline matkar

  • Regular Contributor
  • *
  • Posts: 153
  • Country: si
  • Sixty percent of the time it works EVERY time.
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #7 on: April 11, 2017, 03:52:02 pm »
So the pullup and caps wont interfere with the icsp?

Keep the capacitor small enough and it won't interfere. I usually place a 100nF between reset and GND with a pullup of 1k.
 

Offline Nusa

  • Super Contributor
  • ***
  • Posts: 2223
  • Country: us
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #8 on: April 11, 2017, 04:22:13 pm »
If you don't need the reset pin function, you could program the fuses so that it's not a reset pin anymore. Then you won't need an external pullup and you'll also have another gpio pin that you can use. You can still reset via a power cycle.

This trick is more commonly used on smaller packages like the ATTinys where an extra gpio pin is far more valuable, since you have far fewer gpio pins to start with.
 

Offline sleemanj

  • Super Contributor
  • ***
  • Posts: 2854
  • Country: nz
  • Professional tightwad.
    • The electronics hobby components I sell.
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #9 on: April 11, 2017, 10:31:06 pm »
If you don't need the reset pin function, you could program the fuses so that it's not a reset pin anymore. Then you won't need an external pullup and you'll also have another gpio pin that you can use. You can still reset via a power cycle.

This trick is more commonly used on smaller packages like the ATTinys where an extra gpio pin is far more valuable, since you have far fewer gpio pins to start with.

Trouble is you can not program it anymore without a high voltqge programmer if you disable reset.
~~~
EEVBlog Members - get yourself 10% discount off all my electronic components for sale just use the Buy Direct links and use Coupon Code "eevblog" during checkout.  Shipping from New Zealand, international orders welcome :-)
 

Offline Nusa

  • Super Contributor
  • ***
  • Posts: 2223
  • Country: us
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #10 on: April 11, 2017, 11:10:18 pm »
If you don't need the reset pin function, you could program the fuses so that it's not a reset pin anymore. Then you won't need an external pullup and you'll also have another gpio pin that you can use. You can still reset via a power cycle.

This trick is more commonly used on smaller packages like the ATTinys where an extra gpio pin is far more valuable, since you have far fewer gpio pins to start with.

Trouble is you can not program it anymore without a high voltqge programmer if you disable reset.

Sure you can, it just takes a timely power cycle instead of a button/host control of reset line. It still gets you into bootloader mode where it looks for an incoming upload for a couple seconds, and the timing isn't that difficult to master. You're right that it's a pretty big inconvenience just to save a resistor. I wouldn't do it unless you really need the io pin.

Now if you screw up programming the fuses, that's another matter entirely! The HV programmer may be the only way out.
 

Offline sleemanj

  • Super Contributor
  • ***
  • Posts: 2854
  • Country: nz
  • Professional tightwad.
    • The electronics hobby components I sell.
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #11 on: April 11, 2017, 11:15:39 pm »
Sure you can, it just takes a timely power cycle instead of a button/host control of reset line. It still gets you into bootloader mode

It's a good bet that you don't have a bootloader installed on a tiny.  But yes, if you do, then you could do a power cycle reset.
~~~
EEVBlog Members - get yourself 10% discount off all my electronic components for sale just use the Buy Direct links and use Coupon Code "eevblog" during checkout.  Shipping from New Zealand, international orders welcome :-)
 

Offline abdullahseba

  • Frequent Contributor
  • **
  • Posts: 335
  • Country: gb
Re: Is it ok to leave atmega328p reset pin floating?
« Reply #12 on: April 11, 2017, 11:17:21 pm »
It's a good bet that you don't have a bootloader installed on a tiny.  But yes, if you do, then you could do a power cycle reset.
I think I will install the arduino bootloader on it.
This is my right hand this is my wrong hand
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf