High - Low level input voltage in 74194

[nithinsarath]

Hi ,
This is my first post at edaboard, and i hope this is the right place to ask this.I`m using a 74194 ( 4 bit PIPO bidirectional shift register) as a part of my circuit. My problem is that with just the Vcc and GND pins connected , i have a logic low level voltage of 1.2V at the input pins , and most importantly at the CLOCK pin. So a low to high transition I am providing at its CLOCK pin is not being recognised as a transition at all.. Any ideas why the low level input voltage is so high ?

The above mentioned pins were not connected to any other point on the circuit while i measured these low level voltage values.

Thanks in advance

 

[IanP]

First, have a look at the chart showing threshold voltages for different logic IC families ..
http://www.interfacebus.com/voltage_threshold.html
and identify V[IL] for the TTL-family .. you see, it must be below 0.8V ..

Next, letting a TTL input 'float' (left unconnected) will usually make it go to logic '1', but such a state is vulnerable to stray signals, which is why it is good practice to connect TTL inputs to Vcc using pull-up resistors (1k to 10k) ..

From here, if you pull a pin down by connecting it directly to GND you will have transition from “H” to “L”, when you release the connection to GND (because of the pullup resistor) you will have transition from “L” to “H” ..

Rgds,
IanP

 

[nithinsarath]

Thanks a lot for the reply .
I did look up the TTL logic levels and found that the voltage i had at the floating inputs was way higher than what was permissible as a logic low.
I will try out your suggestion right now.. Lets hope this works.

 

[nithinsarath]

Solved

Hi ,
Finally figured out that the problem was due to a silly mistake on my part. The 74194 i mentioned was connected to a mega8, which i was using to feed in the data and the clock.While programming the mega8, my friend accidentally forgot !! to set the Data direction for the ports used. Hence the occurrence of the "high" voltages even while we were trying to output a logic low at any pin of that port.without the data direction set,the internal pull downs were disabled.
Corrected that part of the code and it was fixed.

Thanks everyone.