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Author Topic: Does the USB Interaface chip waste energy on battery power?  (Read 812 times)
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Hello!
I've heard that these components on the Arduino board is most power consuming:
- USB Interface Chip
- 5V voltage regulator
When the Arduino runs on battery power (e.g. 4 AA batteries directly into 5V and GND pins, or 9V battery into DC power jack), is the USB Interface Chip wasting energy? If it is, how can I disable it? I looked at Atmel datasheet, and don't understand all the clk_io abbrevations, and how to manipulate them. Also, what does the USB chip do, anyway?
Thanks!
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I'm not really sure.  On the schematic, I see the part that(I think) auto switches between USB/VIN power, but it seems that the Vcc pin on the AtMega8U2 it connected regardless, as it's not powered directly from the USB 5V pin.  I may be missing something though.
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Hello!
I've heard that these components on the Arduino board is most power consuming:
- USB Interface Chip
- 5V voltage regulator
When the Arduino runs on battery power (e.g. 4 AA batteries directly into 5V and GND pins, or 9V battery into DC power jack), is the USB Interface Chip wasting energy? If it is, how can I disable it? I looked at Atmel datasheet, and don't understand all the clk_io abbrevations, and how to manipulate them. Also, what does the USB chip do, anyway?
Thanks!

I measured an Uno at about 42mA. The ATmega328P accounts for 11-12mA of that. Not sure how the rest breaks down.

4xAA cells is a bit much, absolute maximum voltage for the ATmega328P is 5.5V.

The USB chip (which is just another Atmel microcontroller, specifically an ATmega8U2 or ATmega16U2 on the Uno R3) provides the serial interface between the ATmega328P and the USB connection. This includes uploading sketches via the bootloader.

I'm not aware of any straightforward (or otherwise) way to turn off the ATmega8U2/16U2. I guess it only "wastes energy" if you have no use for what it does smiley-wink
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Chip wasting energy? If it is, how can I disable it?

Yes, the USB serial converter chip is consuming power straight from the Vcc bus, no matter what the source of this voltage is. Probably around 10-20ma or so, the datasheet for the chip will give better details. No easy way to disable it as it has it's own firmware burned into it and is not simple to write your own version and have some kind of sleep option installed, but it's not impossible I guess.

 I looked at Atmel datasheet, and don't understand all the clk_io abbrevations, and how to manipulate them. Also, what does the USB chip do, anyway?

The purpose of the chip is to allow serial communication to the PC via it's USB bus. It's used by the Arduino PC IDE program to upload new sketches and optionally to allow serial communications inside your sketch code to send and receive to a PC application program (or the arduino IDE's serial monitor) if you so desire.

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I guess it only "wastes energy" if you have no use for what it does
Precisely.

The USB chip is used for downloading sketches and for communications with the PC for the serial monitor.  Once you have downloaded your sketch and if you are not using the serial monitor then the power consumed by the USB chip is indeed wasted.

Most modern microprocessor chips have the capability of being put to sleep (but not euthanized) when not in use and it is possible that this is being done in the Arduino implementation.  In that case the wasted power would be insignificant.


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4xAA cells is a bit much, absolute maximum voltage for the ATmega328P is 5.5V.
4 NiMH rechargable batteries are 1.2V each. Combining them would make 4.75V (some voltage is lost through connecting batteries in series.) Is that okay? Mixing NiMH and non-rechargeable batteries will damage them, right?
Code:
power_spi_disable();
power_twi_disable();
Will this disable the serial chip from using battery power? I need to use my multimeter when I have time.
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Will this disable the serial chip from using battery power?
No.

The best way to not waste power is to build a stand alone system and do not include this chip. Program it up with a separate USB to TTL serial interface.
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Quote
4xAA cells is a bit much, absolute maximum voltage for the ATmega328P is 5.5V.
4 NiMH rechargable batteries are 1.2V each. Combining them would make 4.75V (some voltage is lost through connecting batteries in series.) Is that okay? Mixing NiMH and non-rechargeable batteries will damage them, right?

Sure, that works. I'm not aware how a series connection causes any voltage to be "lost", though.

Quote
Code:
power_spi_disable();
power_twi_disable();
Will this disable the serial chip from using battery power? I need to use my multimeter when I have time.

That'll only disable two peripherals. Others are still running, as is the CPU and the flash clock. But just putting those statements in a regular Arduino sketch and uploading it as usual programs the ATmega328P, not the 8U2 ("serial chip"). See retrolefty's reply above.
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I'll answer the easy question that you didn't really ask. smiley-wink   
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...or 9V battery into DC power jack), is the USB Interface Chip wasting energy?
With 9V, almost half of the total power/energy is consumed (wasted)  by the regulator. 

Power is calculated as voltage x current.  In this (series) circuit, the same current flows the regulator as everything else in series with the power supply.  The voltage gets divided between the regulator and the load.   So...  4/9ths of the power is consumed by the regulator and 5/9ths of the power is consumed by the "active" circuit.

A switching regulator is different.   Switching regulators are more complicated and more costly, but they can be nearly 100% efficient.     A switching regulator performs a kind of power-conversion with high-voltage, low-current in, and low-voltage high-current out.     

But a switching regulator does take some power to "run" (as does a standard linear regulator), and may  not be efficient at a few-milliamps. 
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A -perhaps silly- question:

What's the purpose of the ATmega8U2 or ATmega16U2 (USB) chip if it's not connected via a USB cable ?
Does it really drain the battery for nothing ???

I learned that if you really go for low power, you should run the programmed ATmega328P chip in a standalone environment at 8MHz clock speed and 3.3V power, and let it sleep whenever possible.
« Last Edit: May 25, 2012, 07:51:59 am by michael_x » Logged

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http://arduino.cc/it/Main/ArduinoBoardPro

This is exactly why the Arduino PRO exists.  It has no USB components and is designed to even run from a 3V Lipo battery
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Quote
4xAA cells is a bit much, absolute maximum voltage for the ATmega328P is 5.5V.
4 NiMH rechargable batteries are 1.2V each. Combining them would make 4.75V (some voltage is lost through connecting batteries in series.) Is that okay? Mixing NiMH and non-rechargeable batteries will damage them, right?

NiMH are 1.3V or so (1.25 to 1.35 is typically seen), expect upto 5.4V from 4 of them.  A lot of people seem to think NiMH are the same voltage as NiCd whereas they use completely different chemistry on the positive electrode.

The absolute max voltage for the Arduino is actually 6.0V, 5.5V is the maximum recommended operating voltage, so 4 NiMH are just inside that and won't risk getting to the abs max at all.
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The absolute max voltage for the Arduino is actually 6.0V, 5.5V is the maximum recommended operating voltage, so 4 NiMH are just inside that and won't risk getting to the abs max at all.

That'll learn me to quote the datasheet from memory smiley-red
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