Is it ok to leave atmega328p reset pin floating?

[abdullahseba]

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.

[sleemanj]

Pull up recommended.

[Psi]

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.

[ohdsp]

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.

[abdullahseba]

So the pullup and caps wont interfere with the icsp?

[sleemanj]

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.

[obiwanjacobi]

Caps on the RESET pin *may* interfere with ICSP.

[2c]

[matkar]

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.

[Nusa]

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.

[sleemanj]

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.

[Nusa]

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.

[sleemanj]

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.

[abdullahseba]

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.