74ls138
Yes plusminus, echo47, Old Nick you were right. (sorry for post lenght)
plusminus said:
4.1 volt and 1.1 volt out is not good. When nothing is connected to the outsuts or only 1 logic port the output voltages should be almost 5 volt and almost zero.
What is connected on the outputs?
I checked again only demux(decoder) and manualy changed all states.
I got this:
Vcc = 5.05V
I (all) = 17mA (7segment LED + 74ls138)
I (Yo) = 11.5mA same for other ports
Y0 0.15V(0.42V) - 3.99V(3.99V)
Y1 0.15V(0.42V) - 3.99V(3.99V)
Y2 0.15V(0.42V) - 3.99V(3.98V)
Y3 0.15V(0.41V) - 4.00V(3.99V)
Y4 0.15V(0.41V) - 4.00V(4.00V)
Y5 0.15V(0.40V) - 4.00V(4.00V)
Y6 0.15V(0.39V) - 3.99V(3.99V)
Y7 0.15V(0.42V) - 3.99V(3.99V)
Y0-Y7 are outputs, in brackets is voltage under load. Every output is connected to one cathode of LED display.
echo47 said:
Is the 74LS138 connected to other devices? Maybe they are causing the problem. Try disconnecting the other devices, and see if the 74LS138 now behaves normally.
It sounds like you are measuring the signals with a voltmeter. If the signals have pulses, then a voltmeter can give you weird readings. Use an oscilloscope instead.
74LS138 is connected to PIC 16F84A and it seems that there is the error.
RB0-A0
RB1-A1
RB2-A2
RB3-Enable inverted (data input)
RB4-Enable
RB5-nothing
RB6-nothing
RB7-nothing
I'm measuring with a voltmeter, but I slowed port change to every 3sec, so I can keep track.
Old Nick said:
Usually, by the time those to things have been done, the problem has presented itself. If not, then things become a bit annoying, you have to start looking for opens/shorts on the rest of your board.
I'm using breadboard, so it's easier.
I detected the problem, but what now, I have to use it with PIC16F84A!
Thanks!