What is max. voltage for external ARef

[jrdoner]

I understand that the Moteino involves a 3.3 v. version of an Atmega 328, but is 5 v. tolerant (just keep the 5 v. off the pins used by the transceiver).

What about declaring an external ARef?  Can that be above 3.3v. , assuming I don't use more than 5 v.?

Thanks in advance for any help?

[TomWS]

I understand that the Moteino involves a 3.3 v. version of an Atmega 328, but is 5 v. tolerant (just keep the 5 v. off the pins used by the transceiver).

What about declaring an external ARef?  Can that be above 3.3v. , assuming I don't use more than 5 v.?

Thanks in advance for any help?

Actually, the ATMega328P will operate between 1.8V and 5.5V (with clocks adjusted accordingly).  There isn't a special 3.3V version, so the Moteino COULD run on a 5V supply if you remove the VR, Flash, and Radio as these are all 3.3V parts (3.6V max on flash and radio).

All pins (except XTAL1 & Reset) must not exceed VCC+0.5V (Table 29-1, ATMega spec).

Tom

[jrdoner]

OK, thanks.  Should be no problem with external ARef at 4 v. then.

[TomWS]

OK, thanks.  Should be no problem with external ARef at 4 v. then.

How do you figure that?  VCC is 3.3V, +0.5V is 3.8V where I come from.

[emjay]

@jrdoner,

Should be no problem with external ARef at 4 v. then

Tom is correct about the max Vref input of 3.8v before the ESD protection diode cuts in and possible damage if the current into that pin is not limited.  For an input level higher than the chip Vcc it is unreasonable to expect that the comparator logic behind that pin will work as desired.

[jrdoner]

I am supplying vcc = 5 v.

[emjay]

@jrdoner,

I am supplying vcc = 5 v.

Hmm - some confusion perhaps.  You can supply 5v (or greater) to the Moteino since it has a regulator cutting this down to 3.3V for the MCU, flash, radio module etc.  This becomes the Vcc for the chip - hence the cautions about the maximum voltage on Vref. - it is w.r.t. the chip supply.