New to electronics and trying to understand how to read and interpret data sheets for MCUs, particularly w.r.t. voltage and current supply.

I'm trying to understand the voltage/current specifications for this Atmel evaluation board, as it has seemingly contradictory verbiage within its own user guide.

Certain verbiage suggests the board requires a 5V power supply:

From §3.3 Power Sources:

"External Power: 5V ±2% (±100mV) for USB host operation. 4.3V to 5.5V if USB host operation is not required."

From §3.4.2 Xplained Pro Power Header:

"The power header can be used to connect external power to the SAM R21 Xplained Pro kit. The kit will automatically detect and switch to any external power if supplied. The power header can also be used as supply for external peripherals or extension boards. Care must be taken not to exceed the total current limitation of the on-board regulator when using the 3.3V pin."

To me, these two sections contradict each other. One one hand its saying to operate the board around 5V, but then elsewhere it says never to bring the board higher than 33V. Also, in that same table in section 3.3, there's a column about current requirements that also seems to contradict itself. On one hand it says:

"Recommended minimum is 1A to be able to provide enough current for connected USB devices and the board itself. Recommended maximum is 2A due to the input protection maximum current specification."

Which to me tells me the board wants 1A - 2A of current at all times. But then it goes on to say that the USB devices require 0.5A (which I understand is the USB spec).

I am trying to understand:

  1. What voltage/current the board itself recommends/wants to power itself without issue; and
  2. What voltage/current is supplied to and expected from the GPIO pins (so I know what voltage/current compatibilities need to exist with any peripheral I hook up to them)

Thanks for any and all clarifications here!

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    \$\begingroup\$ No conflict; these points address distinct situations. As with your previous question electronics.stackexchange.com/questions/533987/… you have failed to answer specific queries as to why you think this particular board is useful to you. Do you actually intend to host USB peripherals? Have you even begun to think through the software complexity of doing so? As before, for someone just starting a journey of learning this is almost certainly not where you want to begin. \$\endgroup\$ Commented Nov 25, 2020 at 18:57
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    \$\begingroup\$ Sure these are all valid questions. I've played around with RPi and Arduino boards in the past, and gotten LEDs blinking and servos activating, and flashed simple apps to them for driving all those things. I honestly found it a little boring because they all have modules/hardware that does all this design work for you. For instance, want a camera for you RPi project? Just buy the camera module off Adafruit. Boom. Camera. Now I want to take it a step further with a more "production-like" MCU where there's not as much hand-holding as you get with those other systems. \$\endgroup\$ Commented Nov 25, 2020 at 19:08
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    \$\begingroup\$ As for this board, I like the microprocessor for it, it has the right specs for what I plan to run on it, and this board allows me to connect that processor to just about any peripheral (camera, servos, LEDs, etc.) that I will want, at least for the foreseeable future in my electronics trajectory. \$\endgroup\$ Commented Nov 25, 2020 at 19:09
  • \$\begingroup\$ Stepping outside the walled garden is about reading specs and interpreting their application to your unspecified intented usage yourself. \$\endgroup\$ Commented Nov 25, 2020 at 19:20
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    \$\begingroup\$ *intended, I think you mean intended \$\endgroup\$ Commented Nov 25, 2020 at 19:23

2 Answers 2


The first statement is about the power you supply to the board. You could do this at the USB input or at pin 1 of the power header.

The second statement is about pin 4 of the power header, which is an output of the board, not an input. You must not connect loads to this pin that draw more current than the on-board regulator can supply.

enter image description here

To determine the power limit, you will presumably have to dig into the schematics or BOM to find out what the regulator actually is and what other loads are connected to it.

Also, in that same table in section 3.3, there's a column about current requirements that also seems to contradict itself. ...

Which to me tells me the board wants 1A - 2A of current at all times. But then it goes on to say that the USB devices require 0.5A

It's saying that with all peripherals (USB or daughter cards) it might require up to 1 A. If you don't connect peripherals, it will require less, but they don't know what peripherals you will connect, so they recommend using a supply able to provide current to the maximum possible peripheral load.

If you are powering the board from USB, you should design it not to draw more than 500 mA on start-up. Before drawing more current, you must "negotiate" with the USB host for permission to do so (the details of this negotiation process are beyond my knowledge).

They also recommend using a supply that doesn't supply enough current to overload the input protection device.

  • \$\begingroup\$ Your claim that the power header is an output and not an input is literally contradicted by the quoted text, eg "The power header can be used to connect external power to the SAM R21 Xplained Pro" \$\endgroup\$ Commented Nov 25, 2020 at 19:22
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    \$\begingroup\$ @ChrisStratton, read the document. Pin 1 of the header is a power input. Pin 4 of the header is a 3.3 V regulated output. \$\endgroup\$
    – The Photon
    Commented Nov 25, 2020 at 19:23
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    \$\begingroup\$ @ChrisStratton, if OP already knew which part of the document was important, they wouldn't have any question to ask. Sometimes the actual question is "how do I find this information in the datasheet", and posting the complete datasheet in the question is not practical. \$\endgroup\$
    – The Photon
    Commented Nov 25, 2020 at 19:29
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    \$\begingroup\$ Realistically the OP is in over their head and prematurely worrying about hardware issues without having determined if they have sufficient software and support resources to achieve their unexplained actual goal when using this platform from a starting point of inexperience. \$\endgroup\$ Commented Nov 25, 2020 at 19:31
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    \$\begingroup\$ @ChrisStratton, nonetheless showing them how to interpret the document will get them closer to understanding that. \$\endgroup\$
    – The Photon
    Commented Nov 25, 2020 at 19:35

As for the specific ratings of the GPIO pins, refer to the manufacturer's datasheet for the device. There is usually a per pin rating and a total device rating - it might suggest some like 20mA per pin but 200mA total or suchlike. There are voltage ratings as well, so you need to ensure you stay within the required voltage range. Exceed the manufacturer's specs and expect smoke.

With the spec on the 3.3V regulator on the board - this is thermally limited. If you try drawing too much current from it, the device will get hot and eventually fail. It could be the regulator is rated for 1A, but due to thermal limitations, it will not achieve that in practice. Datasheet specifications are for what the manufacturer can guarantee - it is up to the designer to understand these and their implications.


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