Problem with demux 74ls138

Hello,

I'm using demux 74ls138, and I'm having big problems with it.

For settings:
ENABLE = 1..................|=>now 74ls138 is working in decoder mode
Both inverted ENABLE=0 |

for any adress exepct A2=0, A1=0, A0=0 I have correct values.

When I have logical 1 on expected output I see 4.10V which is good.
When I have logical 0 on expected output I see 1.41V which is also good.

But, when I set adress A2=0, A1=0, A0=0 on output Y0 I see this:

When I have logical 1 on Y0 I see 2.84V which is not good.
When I have logical 0 on Y0 I see 0.15V which is good, but diffrent from other output values.


First I thought that I burned the first one, so I bought another one, but this 74ls138 shows same behavior.
I tried first with no load, then with load(I=~14mA), same results.
So I'm doing something wrong, but what?!

Please help it's somehow urgent, thanks.

First you have to keep
E1 & E2 LOW
and E3 HIGH

then as you change the data on A0, A1 &A2, the corresponding pin will become low while other pins remain high.
thhis is as per truth table.

How do you see logic 1? don't load the pin and have a buffer transistor to drive an LED if needed for testing
and with a diode in seires to base and a termination of sat 10K from bas to ground.

jhanus said:

Hello,

When I have logical 1 on expected output I see 4.10V which is good.
When I have logical 0 on expected output I see 1.41V which is also good.

But, when I set adress A2=0, A1=0, A0=0 on output Y0 I see this:

When I have logical 1 on Y0 I see 2.84V which is not good.
When I have logical 0 on Y0 I see 0.15V which is good, but diffrent from other output values.

Click to expand...

hi,
The output of LS138 are active LOW, that is when E1 and E2 are low and E3 is high AND 'A,B,C' are all LOW,,,
Y0 should be LOW and Y1 thru Y7 HIGH.

If 'A' is HIGH and 'B and C' are LOW , then Y1 will be LOW and the rest of the Y's will be HIGH. Look at the Table on the datasheet

Dont expect the HIGH and LOW output voltage to be +5V and 0V, most likely they will about 0.2V and +3V to +4V.

Do you have anything connected on the 'Y' outputs???

Is the Vsupply +5V?

mvs sarma said:

How do you see logic 1? don't load the pin and have a buffer transistor to drive an LED if needed for testing
and with a diode in seires to base and a termination of sat 10K from bas to ground.

Click to expand...

I checked 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.

Here is how I connected it:
Click


Voltages on PIC are:

If I have logical 1, on output I see 3.49V( it stays same with and without 74ls138)
If I have logical 0, on output I see 0.00V( it stays same with and without 74ls138)

Now I know why I have 3.49V on output of PIC, I'm constantly , every 10us changing value of the port to 0V. Because I don't have oscilloscope I measure it with voltmeter, so my measured voltage is not accurate and I forgot that.
I will try to eliminate switching and post again.

The output of an HCT device is not strong enough to pull to ground through a LED and a resistor. In your original post you said you had an LS part. So which is it? Also, a CMOS output stage is a very different animal than a TTL totem pole output.

I have SN74LS138N, in hurry I set the wrong demux in picture.

Well now it's working fine, that is when I disable port switching, but my initial idea was to fast switch demux so that human eye can't see the switching of 7seg diodes.

What now? If I fast switch then I get again those side effects, is this normal?

jhanus said:

Well now it's working fine, that is when I disable port switching, but my initial idea was to fast switch demux so that human eye can't see the switching of 7seg diodes.

What now? If I fast switch then I get again those side effects, is this normal?

Click to expand...

How fast are you switching, can you post your program??

I'm switching every 10us (4MHz crystal).

BUT

It is alive, it's working, I don't know what I done diffrent, but I reconnected everything and in steps bit for bit tested, in the end I fast switched.

So thank you all for helping me!

Expect more(problems) from me!