input resistance attiny45

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Hello everybody!

Do anybody know the input resistance of a common avr input pin! I'm developing a kind of a bus system with an own protocol handled by software!
So I'd like to connect several slave avr's to one master avr!
How many can I connect/drive directly?

best regards

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The quick answer is probably lots!

CMOS inputs don't really have any meaningful resistance: they have leakage currents through the protection diodes which per the datasheet are probably 1uA worst case at maximum supply voltage and temperature. You are much more likely to be limited by signal line capacitance limiting the maximum transmission rate, or by whatever measures you need to take to control ringing.

- John

Last Edited: Thu. Aug 3, 2006 - 06:19 PM
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It is not a simple resistor, but you could say 5V/1uA = 5 megaohms.

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Whether or not direct connection of lots of AVR inputs makes sense depends a lot on the distance over which this "network" has to function. If it is a meter or two, then probably no difficulty, with almost any number. The limiting factor will be capacitance, not resistance and this will limit your maximum data rate.

If the distance is greater than a meter or two, you need to start considering things like RS422 (1 fixed master, many slaves) or RS485 (multi-master) or CAN or such.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net