Lpc2000

redsp@... wrote:

>I have two observations for you.  
>
>1) If you use a pullup, your output should be configured as open
>collector or open drain (depending on your nomenclature preference). 
>This is done by writing a '0' to the output data and controlling the
>output drive (on/off or input/output).  
>
>2) The data sheet for the UDN2981 says it is TTL compatible.  That
>means the input switching level is between 0.8 and 2.0 volts, so you
>don't need a pullup to 5 volts.  
>  
>
Correct answer redsp ....
Hmmm, funny that onceinfour didn't work out that a low-impedance cmos 
output wouldn't pull-up to +5V with a resistor, unless of course the 
resistor was ridiculously low :)

some further thoughts for onceinfour;
Of course when the pin is not outputing a low it becomes an input which 
are designed to be +5V tolerant. The normal method of making an output 
appear to be open collector is to set the output port low initially and 
then simply set/reset the direction register (as redsp pointed out) to 
make the pin flip from low to high-impedance, thus mimicking an 
open-collector output. Unlike a true open-collector output, the pin 
cannot handle breakdown voltages greater than the chip is designed for 
though.
But why use pull-ups and open-collector drives for this application when 
they are redundant anyway?


Peter Jakacki