Continue with the demultiplexer...circuit attached

[overgift]

Hi all,

Yesterday I asked my question here and thanks for many replies. The most frustrate thing is I'm afraid I destroyed my demux, three of them...

The thing is the demuxs only switch channel 0, even I change the code in MCU, no changing at all.

I attached my circuit, can anyone give me some hints? It's a project and actually the deadline will come next Monday, so...

 

[overgift]

yes to make it simple i just ignore them, there are 16 out channels on the demux. -E is demux enable, active low.

 

[dknguyen]

yes to make it simple i just ignore them, there are 16 out channels on the demux. -E is demux enable, active low.

Yeah, I figured out that you just omitted the output pins for simplicity from reading your previous posts so I deleted my question (but not before you answered I guess).

 

[dknguyen]

A lot of chips nowadays have have digital inputs that will work with 3V or 5V logic threshold voltages, even if the chip itself is being powered by a voltage higher than 3V or 5V. Your demux chip is NOT one of them.

Page 6 of your datasheet has the following table. As you can see, if you provide it with higher supply voltages, the logic threshold for HI and LO signals also rises and above 12V it is at around the limits of what microcontrollers can output.

 

[overgift]

Your MCU is not providing high enough voltages for the digital input thresholds of the demux.

Not really, before I tried on breadboard it was totally okay, just now I soldered the DEMUX on the MCU development board

 

[ericgibbs]

Not really, before I tried on breadboard it was totally okay, just now I soldered the DEMUX on the MCU development board

hi,
The dwg shows 12V on the MCU, AT89C5131, is that correct.?

 

[overgift]

Your MCU is not providing high enough voltages for the digital input thresholds of the demux. A lot of chips nowadays have have digital inputs that will work with 3V or 5V logic threshold voltages, even if the chip itself is being powered by a voltage higher than 3V or 5V. Your demux chip is NOT one of them.

Page 6 of your datasheet has the following table. As you can see, if you provide it with higher supply voltages, the logic threshold for HI and LO signals also rises and above 12V it is at around the limits of what microcontrollers can output.

I got it, maybe u r right, but then how can it switch channel 0? Or channel 0 is open by default? But I need i/o output of MCU, in this case what kind of component should I add on the i/o port of MCU.

 

[overgift]

hi,
The dwg shows 12V on the MCU, AT89C5131, is that correct.?

Yes I use a USB development board from USB Development Board

 

[ericgibbs]

Yes I use a USB development board from USB Development Board

So the AT has 3.3V and the 4067 has 12V.. ????

If yes, thats why the 4067 is not selecting.!

 

[Mikebits]

That 12v is just into the board, but the Atmel part runs lower voltage.

Oops late post...

 

[ericgibbs]

That 12v is just into the board, but the Atmel part runs lower voltage.

hi Mike,
That makes the posted dwg, useless.

 

[overgift]

yes the 12v is into the board, not MCU..Sorry I didnt draw it clear in the pic. or describe it clear.

 

[ericgibbs]

yes the 12v is into the board, not MCU

OK, so the MCU and 4067 are operating from 3.3V.

What voltage level exactly are the MCU to 4067 connections pulling up to when high.?

 

[dknguyen]

I got it, maybe u r right, but then how can it switch channel 0? Or channel 0 is open by default?

Any voltage below the logic LO input threshold voltage will be recognized as a LO by the demux. As you increase demux supply voltage, this threshold just keeps getting higher. So any LO signal output by the MCU will still be below the threshold and that means your MCU can still send zeroes to the demux.

Any voltage above the logic HI input threshold will be recognized as a HI by the demux. BUt the HI voltage threshold for the demux increases as you you increase demux supply voltage too. Your MCU cannot output a HI signal to exceed this voltage threshold. Even when the MCU tries to send HI signal to demux, the voltage is so low that it is less than the logic LO threshold so your demux will see it as a LO signal.

So your MCU can still send LO to the demux, but not HI. If your MCU sends HI to the demux, demux will see LO. So if switching channel 0 only requires all LO signals, your MCU can do it. But it can't do anything that requires a HI signal.

But I need i/o output of MCU, in this case what kind of component should I add on the i/o port of MCU.

You could use a pull-up resistor to 12V on the input pins of the demux and use a pull-down logic-level MOSFET that has it's gate driven by the MCU. SO when the transistor was off, but the resistor would apply a HI to the demux input, but when the transistor is on it would pull the line LO.

Do you understand what I mean when I say pull-up resistor and pull-down transistor?

Not really, before I tried on breadboard it was totally okay, just now I soldered the DEMUX on the MCU development board

I'm curious...how did you get it to work on the breadboard? Was the demux also being powered by 12V on the breadboard? Or by the same voltage as the MCU?

 

[overgift]

Any voltage below the logic LO input threshold voltage will be recognized as a LO by the demux. As you increase demux supply voltage, this threshold just keeps getting higher. So any LO signal output by the MCU will still be below the threshold and that means your MCU can still send zeroes to the demux.

BUt the HI voltage threshold for the demux increases too, and your MCU cannot output a HI signal to exceed this voltage threshold. Even when the MCU tries to send HI signal to demux, the voltage is so low that it is less than the logic LO threshold so your demux will see it as a LO signal.

So your MCU can still send LO to the demux, but not HI. So if switching channel 0 only requires all LO signals, your MCU can do it. But it can't do anything that requires a HI signal.


You could use a pull-up resistor to 12V on the input pins of the demux and use a pull-down logic-level MOSFET that has it's gate driven by the MCU. SO when the transistor was off, but the resistor would apply a HI to the demux input, but when the transistor is on it would pull the line LO.

Do you understand what I mean when I say pull-up resistor and pull-down transistor?


I'm curious...how did you get it to work on the breadboard? Was the demux also being powered by 12V on the breadboard? Or by the same voltage as the MCU?

Thanks for the explanation, I got u but not very clear. e.g. how can I calculate the value of pull up...etc. And sorry for the silly problems I'm not a professional EE but I'm trying my best learning.

 

[dknguyen]

Oh, sorry. I guess I forgot to put that in. Just use a pull-up resistor large enough so current flowing through it is not going to overheat resistor or the transistor. If MOSFET resistance or voltage drop is small enough to ignore, yOu can just use:

R = V/I
I = current flowing through resistor and transistor (you don't want I to burn up transistor or overheat resistor)
V = 12V

Any value from 1K to 12K should be fine for almost any transistor. You probably could even use 500 ohms.

Example: If you ignoring transistor voltage drop or resistance because it is so small...
1K = 12mA flowing
12K = 1mA flowing
500 ohm = 24mA flowing

Any transistor you can find will not even get hot running 24mA. But 24mA will overheat a 250mW resistor so I wouldn't use it.

 

[ericgibbs]

hi,

I'm curious...how did you get it to work on the breadboard? Was the demux also being powered by 12V on the breadboard? Or by the same voltage as the MCU?

I was told yesterday they are powered from the same power supply, but like you I'm beginning to wonder.

EDIT:
What is the voltage on pin 24 of the 4067.????

 

[overgift]

hi,


I was told yesterday they are powered from the same power supply, but like you I'm beginning to wonder.

EDIT:
What is the voltage on pin 24 of the 4067.????

yesterday I forgot to specify that I use a board. I'm not very sensitive to the voltage level I think that my problem. like the driving voltage for DEMUX.

 

[ericgibbs]

yesterday I forgot to specify that I use a board. I'm not very sensitive to the voltage level I think that my problem. like the driving voltage for DEMUX.

Whats the voltage on pin24 of the 4067.?

 

[overgift]

Oh, sorry. I guess I forgot to put that in. Just use a pull-up resistor large enough so current flowing through it is not going to overheat resistor or the transistor. If MOSFET resistance or voltage drop is small enough to ignore, yOu can just use:

R = V/I
I = current flowing through resistor and transistor (you don't want I to burn up transistor or overheat resistor)
V = 12V

Any value from 1K to 12K should be fine for almost any transistor. You could probably even use 500 ohms if you wanted.

Thanks for the explain. And what kind of pull-down-transistor should I use?