Power with Vin or Vdd

Blake

Hi,
I'm designing a pcb for the first time and I have a really simple question about powering the microcontroller. I am using an external 7805 regulator to provide power. Do I want to route this to the Vin of the chip "just in case" or to the Vdd of the chip since it's already regulated?

If something were to happen to the external regulator, would the chip fry too?

What is the right way to do this?

Thanks

Mike Green

The power source for the Stamp is an either / or situation. You can supply power to the Stamp by way of Vin. In that case, the Stamp's Vdd is the output of the module's regulator. As an alternative, you can supply regulated +5V to Vdd and leave Vin unconnected. There are advantages and disadvantages to each choice so it's not like there's a "right" way. You can't use the output of a +5V regulator for Vin since the Stamp's built-in regulator requires an input voltage above the regulated output voltage. Typically, the Stamp's regulator needs a minimum of 5.5V for Vin or the regulator doesn't regulate.

If you use the 7805 as the Stamp's power source and something goes wrong with the 7805 in such a way that Vdd goes over 7.5V, the Stamp will likely be damaged. Even if you use the Stamp's built-in regulator instead of the 7805's output, the Stamp is connected to other circuitry powered from the 7805 and, if the supply voltage goes over 7.5V, the Stamp can be damaged through its I/O pins.

If you're concerned about the 7805 failing, you'd be better served by using a "crowbar" circuit (see here).

I'd probably use the 7805 as the power source for the Stamp (into Vdd) unless I'd want to be able to switch the 7805 on and off from the Stamp (so the Stamp has to be able to run when the +5V regulated supply is off).