• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

High impedence inputs

Status
Not open for further replies.

ikalogic

Member
Hello,

I'll be using SRAM ICs from microchip

Here is the datasheet :
http://www.electro-tech-online.com/custompdfs/2009/08/22126B.pdf

In page 12, it is said that if i pull the HOLD line to 0, the I/O pins go to high Impedance. I am more concerned about the SI pin (or MOSI):

I want to be able to apply a voltage that swings from 0 to 25 volts on that pin, while the device is tri-stated. is this OK? Is there still a risk of damaging the IC?

Would it be better to add a 10K resistor in series to protect the input? if yes, will that affect the normal operation of the input pin?

I tried to contact microchip about that, but still no answer

thanks a lot for your advices :)
 

ericgibbs

Well-Known Member
Most Helpful Member
Hello,

I'll be using SRAM ICs from microchip

Here is the datasheet :
http://www.electro-tech-online.com/custompdfs/2009/08/22126B-1.pdf

In page 12, it is said that if i pull the HOLD line to 0, the I/O pins go to high Impedance. I am more concerned about the SI pin (or MOSI):

I want to be able to apply a voltage that swings from 0 to 25 volts on that pin, while the device is tri-stated. is this OK? Is there still a risk of damaging the IC?

Would it be better to add a 10K resistor in series to protect the input? if yes, will that affect the normal operation of the input pin?

I tried to contact microchip about that, but still no answer

thanks a lot for your advices :)
hi,
I would say the 25V will blow that input.

You could use a 10K or 4K7 resistor in series and connect the pin to 0V via a
4V7 or 5V2 zener. Im assuming that your logic is 5V.
 

Electronworks

New Member
HI

I cannot work out why you would want to apply 25V to a logic pin, but here goes.

Generally it is bad design practice to apply high voltages to low level inputs even if you do use a current limiting resistor. Looking into the input of most electronic devices (analogue and digital) you have a diode pointing from the input to the Vcc pin and one pointing from Ground to the input. So if the input goes above Vcc, the top diode conducts. If it goes negative, the bottom diode conducts. This is where your absolute maximum voltages in any datasheet comes from (normally Vcc + 0.6V and Vee - 0.6V).

If you apply a high voltage, even via a resistor, you inject current into the substrate of the chip and it can latch, power up or all manner of other exciting things.

A zener is a good way of limiting the voltage, or a plain simple resistive divider to take the voltage down to 5V.

Alternatively, if the 25V is permanently available to you (and not just a 25V pulse coming in from the outside world), you could power an analogue switch and switch out the 25V signal when you expect it to appear

Hope this helps
:D
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top