Author Topic: Low power serial interface  (Read 2089 times)

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Offline Pedro

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Low power serial interface
« on: March 01, 2011, 10:16:19 pm »
Hello everyone!

I am designing a low power device and it needs to have a full 232 DCE port. The device has a power consumption of 36mW,  but when I plug the 232 port to a Pc it goes up to 100mW!!!
My best guess right now is that the 232 inputs have a 3k input resistance,  which combined with the efficiency of the driver for all four outputs, make such a power loss to happen.

What seems funny for me is that all drivers have such input impedance being low power,  I would expect to have some high impedance drivers in the market.

Anyway, I would like to hear from you, which is your favorite comm interface for low power devices??

See ya!!

Offline mikeselectricstuff

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Re: Low power serial interface
« Reply #1 on: March 02, 2011, 12:33:44 am »
The trick is to turn off your transceiver when not in use if possible. If not, and low power draw is more important than maximum compatibility then use 0v/5v levels. The power draw is coming from driving -v into the PC's terminating resistors in idle.
I've never come across a serial port that didn't work fine with 0v/5v levels.
Maxim and probably others do some RS232 transceivers with various smart power-saving features - probably worth a look at these.

Another approach, if you have control of the handshake lines of the thing you're connecting to, is to use these to provide the signal levels, e.g. using analogue switches or transistors.
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Offline scrat

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Re: Low power serial interface
« Reply #2 on: March 02, 2011, 02:14:45 am »
Voltage levels are the (maybe unavoidable) cause.
I was writing this simple calculation, in the middle Mike posted his one, which I agree.
For 4 bits, considering that the output level is always + or - V, power dissipated into 3k loads will be Pdiss = V^2/3k * 4. For classical V=12V, 12^2/3k * 4 = 192mW. For V=5V, 5^2/3k * 4 = 33mW.

If it has to be a fully compliant with RS-232, there is no way other than turning off the driver when not needed.
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