2

I want to build my first barebones AVR circuit.

I plan on NOT having a RESET button. For this reason, is it valid if I connect the ATMEGA's RESET pin to Vcc directly?

What I mean by this, is that in this scenario, I will not be using a pull-down resistor as well. Just straight to Vcc.

EDIT: This is a part of the schematic:

enter image description here

Improve this question
9
  • what does the datasheet say about allowable voltages on the reset pin?
    – jsotola
    Jun 15 at 17:25
  • what you are proposing makes no sense ... what advantage do you gain by removing the external reset function? ... you are not using the pin for some other function ... you could bridge two solder pads with a screwdriver if you don't want a physical switch
    – jsotola
    Jun 15 at 17:32
  • What Atmega are you using exactly? If you really don't want to connect anything for resetting and also not using it otherwisen you can probably leave this pin unconnected, if there is an internal pullup on it. You can check in the controllers datasheet. This is true for the Attiny controllersn so probably also for the Atmegas
    – chrisl
    Jun 15 at 18:42
  • How do you intend to program the ATmega328P if the reset pin is tied to Vcc ?
    – 6v6gt
    Jun 15 at 21:02
  • 3
    If you are intend to build your own MCU board, you would need to read datasheet. This kind of question can be answered by reading the datasheet, see page 40 of the Reset Logic diagram.
    – hcheung
    Jun 16 at 0:49

2 Answers 2

Reset to default
2

I updated the question with a pic. I am using ATMega328. I intend to program the chip using Sparkfun's AVR Programmer.

Then don't tie /RESET to VCC. The programming won't work. What are you doing? Trying to save one resistor? If you have J2 as a programming interface, then /RESET needs to be able to be pulled low to start the programming sequence. Either remove J2 altogether and program the chip in advance of going on the board, or use the pull-up resistor. Don't tie it to VCC.

Improve this answer
6
  • 1
    Is it OK if I use a pull-down resistor and omit the button? Or should I include the button in the design, but never solder it?
    – user1584421
    Jun 16 at 12:37
  • @user1584421 What pull-down resistor? The ATmega328P reset pin already has a weak pullup resistor. However, it is usual to supplement this with a 10k pullup to Vcc. It is very unusual to tie the reset pin to Vcc because you cannot then program it, as already mentioned several times. You can simply omit the reset button if you don't need to do manual resets.
    – 6v6gt
    Jun 17 at 4:07
  • @user1584421 If you use a pull-down resistor, and it is stronger than the internal pull-up then the processor will be constantly held in Reset, which hardly seems useful.
    – Nick Gammon ♦
    Jun 17 at 4:18
  • Or should I include the button in the design, but never solder it? - an imaginary button? Include a pull-up resistor (not a pull-down) to VCC and perhaps have a couple of adjacent pads connected to Reset and Ground, so you can short them with a screwdriver if you need to reset the processor.
    – Nick Gammon ♦
    Jun 17 at 4:21
  • Also, since the design appears to have a (6 pin) ICSP header which includes the ground pin and the (adjacent) reset pin, a manual reset can be forced, if required, by momentarily shorting those two pins together.
    – 6v6gt
    Jun 17 at 4:53
2

According to the datasheet linked to by hcheung in their comment, datasheet, on page 41 it states "Figure 10-2. MCU Start-up, RESET Tied to VCC".

I believe it would be OK to tie the pin directly to Vcc.

I also believe the graphical data presented in "Figure 10-2" could be miss-leading if the reader didn't know what Vcc meant, and also because there is no horizontal axis to specify that they are talking about elapsed time, not voltage change.

EDIT

Regarding your comment "Thanks, but now I am not sure if this setup will work with Sparkfun's Pocket AVR Programmer.".

Looking at page 40 of the datasheet, a internal pull-up resistor is built into the MCU.

enter image description here

You could leave the reset pin not connected and see if it still works as expected. Perhaps a high impedance DVOM would show Vcc on that pin. Some people say leave it disconnected if there are no issues from EMI. EEV Blog - Is it ok to leave atmega328p reset pin floating?

The data sheet also mentions the power on reset timeout which may allow you to program the MCU with your programmer once it first starts, (assuming you have tied the reset pin to Vcc and you leave the reset pin disconnected on your programmer).

Another option would be to program the MCU, then connect the reset pin to Vcc through a couple of solder pads you can bridge with a blob of solder.

Once the reset pin is tied to Vcc, using a programmer connected to that pin will likely result in the magic smoke being released from the MCU, your programmer, or both.

Improve this answer
2
  • 2
    FWIW, I have an ATtiny at home with RST tied to Vcc. It has been running flawlessly 24×7 for the past six years.
    – Edgar Bonet
    Jun 16 at 7:17
  • Thanks, but now I am not sure if this setup will work with Sparkfun's Pocket AVR Programmer.
    – user1584421
    Jun 16 at 12:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.