tristate output leaving input floating?

Thread Starter

Robin Mitchell

Joined Oct 25, 2009
819
Hi everyone

I have multiple things feeding into a common bus. There outputs go to tristate buffers which then drive that to the bus. The bus connects to CMOS inputs. will the inputs of the CMOS float when all tristate buffers are discconected because there will be no physical connection to the inputs?

Thanks
 

tom66

Joined May 9, 2009
2,595
What will happen is the input to the CMOS chip will not work properly, and will probably pick up random 1's and 0's. To solve this problem, just include a 1k or 10k pull down to ground or pull up to supply, that way, the lines will not float and cause issues. If you have no space for resistors, then you can also enable weak pull-downs or pull-ups on certain microcontrollers.
 

Papabravo

Joined Feb 24, 2006
19,287
The actual behavior of a real bus will depend on many things. Adding pullup or pulldown resistors to a bus may cause you other problems on very high speed buses. If the system has short periods where the buss is not being driven and the rest of the system does not care then there may be enough capacitance on the bus to keep it from going wild during those short periods.
 
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