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I have a project where I want to interface with a keyboard matrix / scanner (this is a PCB from a consumer electronics device). There's already an implementation that uses some analog muxes for this, but I'd like to change this to an MT8809 8x8 crosspoint switch: https://www.microsemi.com/product-directory/analog-cross-point-switches/4916-mt8809

That is, the MT8809 will be used to connect/disconnect the rows and columns of the matrix scanner to simulate keypresses.

The MT8809 runs at 4.5-12v, so I'd connect it to a 5v rail, but the device I'm interfacing to runs at about 3v. Are there any concerns here? Do I need to clip the voltage?

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    \$\begingroup\$ Really need to know what it is connected to ( like with a datasheet and model #) or know the I/O levels of the matrix scanner \$\endgroup\$ – Voltage Spike Oct 15 '20 at 17:30
  • \$\begingroup\$ It's remote controls, several different ones from different manufacturers. And we may expand this to keypads for things like security devices. All low cost consumer stuff. It's for some test automation efforts. I was hoping that there'd be a universal yes this is generally fine to do, or no this is not good to do. \$\endgroup\$ – BobIsNotMyName Oct 15 '20 at 17:34
  • \$\begingroup\$ What are the logic levels of the device you are interfacing to? \$\endgroup\$ – Voltage Spike Oct 15 '20 at 17:34
  • \$\begingroup\$ I'll have to double check. The current implementation uses (4) SN74LV4051ADR analog muxes running at 3.3v to interface to the remote control PCBs. But I'll check the actual logic levels. \$\endgroup\$ – BobIsNotMyName Oct 15 '20 at 17:38
  • \$\begingroup\$ Ok, I just looked... The remote control's logic - well for the one I looked at - is the same as the voltage input to the board, which is about 3.3v. However, in normal operation this would be under 3v as it runs off of (2) AA batteries. And we want to simulate draining batteries so eventually we'll probably add a digitally controlled variable voltage regulator in here to adjust it down. \$\endgroup\$ – BobIsNotMyName Oct 15 '20 at 17:54
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You can connect 3V signals to inputs of 5V devices and it should work. But you can't input more than 3V into a 3V powered device even if the said device can run on more than 3V. As a general rule (except if otherwise explicitly mentioned in the datasheet) input voltages should never exceed the supply voltage of the device.

In the case you have a 5V signal which should be used as input into a 3V powered ic, use a resistor and a 3V zener diode to limit the voltage, or two resistors to create a voltage divider.

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  • \$\begingroup\$ I think it should still be a 3V logic signal into a 3V device. It's just that that 3V signal happens to be switched by an analog device powered with 5V. But I'm not 100% that the 3V in (to the M8809 analog device) means no more than 3V out. \$\endgroup\$ – BobIsNotMyName Oct 19 '20 at 14:46
  • \$\begingroup\$ @BobIsNotMyName "But I'm not 100% that the 3V in (to the M8809 analog device) means no more than 3V out." Definitely not! Generally, output is equal to Vdd (or Vcc) and as it must be powered by at least 4.5V, which mean 5V in practice,, you will get 5V outputs even if the inputs are 3V, (Unless, of course, it's a device especially designed to transmit the same voltage and the same current, unmodified) \$\endgroup\$ – Fredled Oct 19 '20 at 21:53

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