Mux Problem with 8051 micro.

[mak.goel]

Dear All,

I am using 5 muxs with 8051 micro for 40 digital inputs. Schemetc has been attached for your ready reference. Normally eaverthing is ok. But mux are being faulty in some systems after continuous running (10 Days or more) of the systems.

Kindlly help.

Regards
Mak

 

[keith1200rs]

I think you need to look at the signals going in to the MUX. I would guess you are exceeding the supply voltage or going below ground, possibly with spikes.

Keith.

 

[mak.goel]

Dear Mr. Keith,

Thanks for your reply. Systems are installed at site. I'll check all signals by CRO & revert to you.

Meanwhile please give your views on below also..

1.0 Output of all muxs is short. Is it ok?

2.0 11.0592 Mhz crystol has been used with micro 8051. I am not giving any delay between chanel selection. Is it ok?

Regards
Mak

 

[keith1200rs]

I guess you could theoretically blow them up by not allowing a switching delay. The maximum current is 10mA and the switch resistance could be 115 ohms so a path could exist which puts 2*115 ohms across 5V (assuming one input is at 5V and another at 0V) so 22mA maximum. I would be surprised if that was really the cause, but it would be a good idea to make sure it isn't the case by adding a delay.

Connecting all the outputs together is fine (subject to the potential problem above).

Keith.