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My project is to create a device that will read temperatures and transmit data to the computer. My plans are to transmit the data through the serial port using the MAX232 and ATMEGA88 microchip. I do not think these are the problems but the serial port does not supply enough power for the application I am using. I want to power up the device using USB. Is it possible to just connect the pins 1 & 4 as V+ and GND to my circuit? If I am not mistaken it is 100mA and 5V supply from PC through USB.

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Why don't you use the USB port for trasmiting the data instead of the RS232 port, here is a PDF doc. that describes how to do that:

URL http://www.atmel.com/dyn/resources/prod_documents/doc2556.pdf

The source code is even there:


I hope it works.

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You can actually tap the voltage you need from the serial port unless you need a lot of current. AtMega chips only require micro-amps. It is the additional circuitry that requires additional.
Here is a schematic of a 5 volt tap from the serial port.



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Well, if you have to use the serial port for data transmit, then you can use the USB port to power supply your circuit, and 100mA is enough since a wireless receiver does not consume much power as a transmitter.

If you run your Atmel with low frequency (1~4 MHz) and low voltage (2.7~3.3v) it will consume current in nAmps.

More over new USB ports are high power load this means they can supply 500mA.

But trying the circuit given by "MP" won't be useless, try to get rid of any un-necessary elements. Then, maybe 5~7 mA is enough.

Good luck

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...of course, the answer is yes. He can certainly use the USB power pins as a power supply. There is no association between the power pins and the data transmission lines. They are just on the same connector so that they can power the device connected. Current limiting with a resistor is a good idea since you are connecting to the Computer internal electronics.


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As additional information for those who were interested in the serial port information:
The Lynx technology transmit and receive chips use 3 Ma typically and actually 1.5 mA on average. Since I would never take up two ports on the computer to run one device, I would look for a low current consumption device such as this.



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