Ok so I am pretty green as far as AVR programming is concerned so bear with me.

I am doing a project with an ATmega32 MCU on a dev board which has 4 VCC pins. The project includes a GPS, SD card and a TFT display. Currently, the GPS and SD card take up 2 out of the 4 VCC pins for powering themselves up. I have two free left.

The display controller I have is the SSD1289:


I've connected it to my MCU, the VCC pin on the display took up one more VCC pin so I have one VCC left. I've been reading up and I've found that the LED_A and RD pins need to be connected to 3.3V, that is VCC. The LED_A pin should also have a resistor on the line connecting it to VCC.

First question - can someone confirm this is the way to connect it?

Second question - Is it possible to, for example, connect the VCC and RD pins of the display to the same VCC using a breadboard? So then I have one VCC left to connect to LED_A pin through a resistor? I don't know if multiple pins can share the same VCC. Will the voltage drop if I connect two pins to it? Is this safe to do?


1 Answer 1


You have basic misconceptions there:

  1. Vcc is the power input (!) for the Atmega, not an output to provide current to other parts. Your power supply (whatever you use as one) should be connected to every Vcc pin at the same time. Sure, the Vcc pins are internally connected, but the Atmega has multiple of them, so that the needed current does not have to flow through a single pin. Also you don't want to draw the current for peripherals through the Atmega chip itself (which would happen, when you connect the power source to one of them and the peripherals to another). So every device in your circuit, including the Atmega, has it's own Vcc pin, which needs to be connected directly to the power supply. That is ok and correct, as long as the power supply can provide enough current for all components. If it doesn't, you need to buy a power supply, that can.

  2. When connecting device to one pin, the number of devices is not imporant. The current draw is what is important. For example let's take simple digital output pin of the Atmega. It can provide up to 20mA (recommended value; max. 40mA for a short time). We must no exceed that value. So we can connect 1 device, that draws 20mA, or 2 devices, that draw 10mA each. Or 100 devices, which draw 0.2mA each. The number is not important, only the total power draw. The current will drop, when you draw more current, that the source can provide. In that case often the source will be damaged or at least go into thermal emergency shutdown (as for the voltage regulator of a genuine Arduino).

So the Todo for you: Connect every Vcc pin, that you have, directly with the power source.

  • Thanks for the answer! But I think you may be wrong here with 1) Here is the datasheet for my board: docdro.id/YoZ3ZNp It says the 4 VCC pins are power output.
    – Machi
    May 10, 2020 at 11:36
  • Hmm, but those VCC pins are no GPIOs and therefore the 20mA limit does not apply. I assume, the motivation behind breaking out 4 VCC pins is that they can be used to power external devices. The current rating for dupont pin headers is in the order of some amps, so they are not the limit and as the atmega doesn't source the current on the VCC pins, it should be fine to connect even power lines to the remaining VCC pins.
    – Sim Son
    May 10, 2020 at 15:40
  • @Machi, why is the link to the datasheet included as an afterthought? ... that kind of information should be included in the question when you write the question ... chrisl's answer to your question is correct and it remains correct unless you edit your question
    – jsotola
    May 10, 2020 at 17:06
  • Sorry, should've included it right away. Thought it was clear what I was asking from the get go. Anyways this answers my question now so thanks everybody!
    – Machi
    May 10, 2020 at 18:30

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